Go is a fast, fun and productive open source language built by Google.
package main
import "fmt"
func main() {
fmt.Printf("Hello, world!")
}Here, 🔝 the program is in the package named “main” We import the package named “fmt” via it’s path Finally, we declare a function named “main”
All go programs start in package “main” and the function “main”
A type implements a interface by defining the required methods. So, printf we saw earlier can be used like so:
fmt.Printf(“Hello, %s”, “world”)
The function Printf takes an argument that need not be a string. Printf defines that %s can be used to define any value that has a method named “String” with no arguments that returns a string
So:
package main
import "fmt"
type World struct{}
func (w *World) String() string {
return "world"
}
fmt.Printf("Hello, %s", new(World))How does this work? Package fmt defines an interface called Stringer. The interface says that any value that has a “String” method without any arguments, would cause %s to call that method to obtain the value to print on the screen.
The name suffix “er” is a convention - used when you have simple interfaces with one method, you get the interface name by taking the method name and adding an “er”
Eg: reader, marshaler etc
Another example:
type Office int
const (
Boston Office = iota
NewYork
)
func (o Office) String() string {
return "Google, " + officePlace[o]
}
func main() {
fmt.Printf("Hello, %s", Boston)
}Here, Office is of type int. We define new constants called Boston, NewYork with values 0, 1 etc We also have an array officePlace which has the office names. Something like: [“Boston”, “New York”, ]
We can print weekdays like so:
package main
import (
"fmt"
"time"
)
func main() {
day := time.Now().Weekday()
fmt.Printf("Hello, %s (%d)", day, day)
}
// Hello, Saturday (6)Here, day is an instance (?) Of Weekday which implements the “String” method
The actual code for Weekday:
package time
type Weekday int
const (
Sunday Weekday = iota
Monday
Tuesday
Wednesday
Thursday
Friday
Saturday
)
var days = [...]string {
"Sunday",
"Monday",
"Tuesday",
"Wednesday",
"Thursday",
"Friday",
"Saturday",
}
func (d Weekday) String() string { return days[d] }So, we have constants “Sunday” (which translates to 0) etc
Another example: time.Duration
See how nice it to write these constants since Duration has a String method
package time
type Duration int64
const (
Nanosecond Duration = 1
Microsecond = 1000 * Nanosecond
Milisecond = 1000*Milisecond
)
// let's see it in action
package main
func main() {
start = time.Now()
fetch("http://google.com")
fmt.Println(time.Since(start))
}Here, since time.Since returns Duration which implements the String method, we can print it. Also, the String method of Duration has logic in it to return the unit of time as ms, ns, s, m, h (whatever is appropriate)
Note, these types (Office, Weekday, Duration etc) don’t include the “implements” declarations - nowhere they say that they implement the fmt.Stringer interface - they just do it - they might not even know it exists!
This also have the advantage that you can notice patterns and then write interfaces to describe them without having to go back in the package and annotate their types (like in Java, you have to include the “implements” keyword)
This is how many of the interfaces in the Go standard libarary - eg: io.Writer This interface, we can say from the convention is the interface with only one method - the Write method
// here, we declare the interface
type Writer interface {
Write(p []byte) (n int, err error)
}
// example usage
fmt.Fprintf(os.Stdout, "Hello World!")
// You can also do this
h := crc32.NewIEEE()
fmt.Fprintf(h, "Hello World")
fmt.Printf(h.Sum32())Here, the return value of crc32.NewIEEE() implements Write method and hence, we can write to it using Fprintf method
We can also implement writers that write writers Example: io.MultiWriter
package io
// MultiWriter creates a writer that duplicates its writes to all the provided writers, similar to Unix tee command
// here, the function name is MultiWriter, it takes many "writers" (0 or more) which are of type Writer and it returns a single Writer
func MultiWriter(writers ...Writer) Writer {
return &multiWriter{writers}
}
type multiWriter struct {
writers []Writer
}
// now, we have to implement MultiWriter's Write method so that it loops over the many writers in it's list and calls their individual Write method
func (t *multiWriter) Write(p []byte) (n int, err error) {
for _, w := range t.writers {
n, err = w.Write(p)
if err != nil {
return
}
if n != len(p) {
err = ErrShortWrite
return
}
}
return len(p), nil
}
// example usage
h := crc32.NewIEEE()
w := io.MultiWriter(h, os.Stdout)
fmt.Fprintf(w, "Hello World!")One more example of Writer.
We have package hex which provides a function Dumper - if you call Dumper with a Writer w, it returns a hex dump of all written data to w. The format of the dump matches the output of `hexdump -C` on the command line
func Dumper(w io.Writer) io.WriteCloser {
The function Dumper returns WriteCloser which writes output in hex to w. We also need the Closer method to indicate all data has been written
package main
import (
"encoding/hex"
"fmt"
"os"
)
func main() {
h := hex.Dumper(os.Stdout)
// here, we queue a h.Close call to run when main returns
defer h.Close()
fmt.Printf(h, "Hello World")
}
// <prints the hex dump>We also have a Reader interface. Folks who implement it - os.File, bufio.Reader, net.Conn, compress/gzip, crypto/tls, bytes.Buffer etc
We can also use “%v” to print “values” without implementing String explicitly
type Lang struct {
Name string
Year int
URL string
}
func main() {
lang := Lang{"Go", 2009, "http://golang.org"}
fmt.Printf("%v", lang)
}
// {"Go", 2009, "http://golang.org"}
// note the braces to indicate the structHow did this work? We did not implement the String method for the Lang struct type. This takes to our second topic - Reflection
Go has support for reflection on types and values
Reflection means the go runtime gives you any type’s basic information and basic operations on those types available at runtime
Look at this example:
func main() {
fmt.Print("Hello", 42, "\n")
// Print is just like Printf but without the format string. It just uses %v for printing each argument
// Hello 42
}Here, “Print”, how does it know that the first argument is a string and has to be printed as such and that the second argument is an int and has to be converted to string and printed - using Reflection!
We can write our own implementation of Print called myPrint
func main() {
fmt.myPrint("Hello", 42, "\n")
}
func myPrint(args ...interfaces{}) {
for _, arg := range args {
switch v := reflect.ValueOf(arg); v.Kind() {
case reflect.String:
os.Stdout.WriteString(v.String())
case reflect.Int:
os.Stdout.WriteString(strconv.FormatInt(v.Int(), 10))
}
}
}Here, we simply use reflect.ValueOf to get the type information and then do use the appropriate way to print the value of that variable
All this uses reflection. We can use reflection to print valid json as well: json.Marshal function
type Lang struct {
Name string
Year int
URL string
}
func main() {
lang := Lang{"Go", 2009, "http://golang.org"}
data, err := json.Marshal(lang)
if err != nil {
log.Fatal(err)
}
fmt.Printf("%s", data)
}json.Marshal is essentially Printf, but it prints json notation and uses reflection to get the correct json encoding
Similarly, we also have xml.MarshalIndent
This makes the standard library code very nice - and so your programs can also be designed like that
We can also do JSON to Struct
func main() {
input, err := os.Open("/tmp/foo.json")
if err != nil {
log.Fatal(err)
}
io.Copy(os.Stdout, input)
}
// io.Copy takes an io.Writer and an io.Reader and copies the bytes. It is like memcopy, but for ioSo, we basically have “cat” right now We can convert this to struct as well
func main() {
input, err := os.Open("/tmp/foo.json")
if err != nil {
log.Fatal(err)
}
dec := json.NewDecoder(input)
for {
var lang Lang
err := dec.Decode(&lang) // this asks the decoder to parse the json and put it in the format of the Lang struct
if err != nil {
if err == io.EOF {
break
}
log.Fatal(err)
}
fmt.Printf("%v", lang)
}Here, we get ourselves a new json decoder (which takes in json and outputs the format we want) which loops over each line and decodes it as Lang struct
We can also go from json to struct to XML
func main() {
input, err = os.Open('/tmp/lang.json')
if err != nil {
log.Fatal(err)
}
dec := json.NewDecoder(input)
for {
var lang Lang,
err := dec.Decode(&lang)
if err != nil {
if err == io.EOF {
break
}
log.Fatal(err)
}
}
// now from Lang, we can go to XML
xml_data, err := xml.MarshalIndent(lang, "", " ")
if err != nil {
log.Fatal(err)
}
fmt.Printf("%s", data)
}In golang, you can pass functions as arguments to another functions
Another example:
func main() {
start := time.Now()
do(func(lang Lang) { //here, the "do" function is something that we defined and which times the time it takes to load the page and the bytes loaded
count(lang.Name, lang.URL)
})
fmt.Printf("%.2fs total\n", time.Since(start).Seconds())
}
// the count function
func count(name, url string) {
start := time.Now()
r, err := http.Get(url)
if err != nil {
fmt.Printf("%s: %s", name, err)
return
}
n, _ := io.Copy(ioutil.Discard, r.Body) //we copy the body of the response to ioutil.Discard which is /dev/null of writers. But it returns the number of bytes copied, so we get it
r.Body.Close()
fmt.Printf("%s %d [%.2fs]\n", name, n, time.Since(start).Seconds())
}
// results:
// Python 19190 [0.52s]
// Ruby 19190 [2.22s]
// Scala 19190 [2.49s]
// Go 19190 [0.49s]
// 5.50s totalHere, we get a very high total time because we are synchronous (each individual page took 2s but total time is 5s)
We can do it in parallel, using concurrency
Parallelism - running multiple things simultaneously Concurrency - way to deal with multiple things simultaneously
Concurrency is the coordination of parallel computations
Go has goroutines to let you run multiple computations run simultaneously And goroutines create parallelism Go also provides channels to let you coordinate these parallel computations by explicit communication
To introduce parallelism, you need to add the word go before the “count” function call
func main() {
start := time.Now()
do(func(lang Lang) { //here, the "do" function is something that we defined and which times the time it takes to load the page and the bytes loaded
go count(lang.Name, lang.URL) // now the call runs in it's own go routine
})
time.Sleep(10*time.Second)
fmt.Printf("%.2fs total\n", time.Since(start).Seconds())
}Here, we don’t know when the goroutines finish - we need to coordinate that - using channels
func main() {
start := time.Now()
c := make(chan string)
n := 0 // we initialize the counter to 0
do(func(lang Lang) {
n++ // we increment the counter here for each language
go count(lang.Name, lang.URL, c)
})
for i := 0; i < n; i++ { // we count upto the counter n to retreive that many messages
fmt.Print(<-c)
} // after this loop is done, we know that all the goroutines have completed
fmt.Printf("%.2fs total\n", time.Since(start).Seconds())
}
func count(name, url string, c chan<- string) {
start := time.Now()
r, err := http.Get(url)
if err != nil {
c <- fmt.Sprintf("%s: %s", name, err)
return
}
n, _ := io.Copy(ioutil.Discard, r.Body) //we copy the body of the response to ioutil.Discard which is /dev/null of writers. But it returns the number of bytes copied, so we get it
r.Body.Close()
c <- fmt.Sprintf("%s %d [%.2fs]\n", name, n, time.Since(start).Seconds())
}
// results:
// Python 19190 [0.52s]
// Ruby 19190 [2.22s]
// Scala 19190 [2.49s]
// Go 19190 [0.49s]
// 2.49s totalHere, we use channels in the count func Here, c is a parameter is a channel of type string The left arrow means that the only allowed operation is sending to the channel We also have Sprintf which returns a string, and we send it to the channel
If we want to give up after 1s and not continue the fetch, we can do use time.After function that takes in a Duration and gives a channel. It also promises that after the Duration has elapsed, it will send the current time to that channel.
So:
func main() {
start := time.Now()
c := make(chan string)
n := 0 // we initialize the counter to 0
do(func(lang Lang) {
n++ // we increment the counter here for each language
go count(lang.Name, lang.URL, c)
})
for i := 0; i < n; i++ { // we count upto the counter n to retreive that many messages
select { // select is like the switch statement but the cases are communication operators
case result := <-c: // the first case is that we might receive the result from the channel c, in that case we print it
fmt.Print(result)
case <- timeout: // the second case is that we might receive the current time from channel timeout, in that case we print Timed out
fmt.Print("Timed out\n")
return // we return from main which will exit the program
}
}// each time we run the select statement, it blocks until one of the cases can proceed and then it runs that case
fmt.Printf("%.2fs total\n", time.Since(start).Seconds())
}
func count(name, url string, c chan<- string) {
start := time.Now()
r, err := http.Get(url)
if err != nil {
c <- fmt.Sprintf("%s: %s", name, err)
return
}
n, _ := io.Copy(ioutil.Discard, r.Body) //we copy the body of the response to ioutil.Discard which is /dev/null of writers. But it returns the number of bytes copied, so we get it
r.Body.Close()
c <- fmt.Sprintf("%s %d [%.2fs]\n", name, n, time.Since(start).Seconds())
}
// results:
// Python 19190 [0.52s]
// Ruby 19190 [2.22s]
// Scala 19190 [2.49s]
// Go 19190 [0.49s]
// 2.49s totalNote how there are no conditional variables, no select loops, no epolls, no kqueues, no callback functions, no semaphores, and no mutexes. They exist in the run time but the abstractions that go provides means that you can focus on the job at hand and not these low level details
The Go project:
- Go 1 released in March 2012
- 2 separate compilers, coordinated with language spec
- 1st implementation is called GCcompilers
- derived from the Plan 9 compiler toolchain
- 2nd implementation is basically a frontend for the GCC compiler
- called GCC Go
- 1st implementation is called GCcompilers
- Runs on FreeBSD, Linux, NetBSD, OpenBSD, OS X, Plan 9, Windows
package main
import (
"fmt"
"time"
)
func main () {
fmt.Println("Hello World")
fmt.Println("The time is: ", time.Now())
}
Every go program is a part of a package Programs start running in the main package, from the main method
package main
import (
"fmt"
"math/rand"
"time"
)
func main() {
rand.Seed(1122)
fmt.Println("My favorite number is", rand.Intn(100))
}- A name is exported if it begins with a capital letter
- eg: Pizza is exported, pizza is not
- When importing a package, only the exported names can be accessed from outside
A function can take 0 or more arguments
func <fn_name>(<arg 1> <type of arg 1>, ..., <arg n> <type of arg n>) <return type> {
// function body
}
// eg:
func add(x int, y int) int { // since x and y share the same type, we can write this as: func add(x, y int) int {
return x + y
}
func main() {
fmt.Println(add(2, 3))
}Functions can return more than 1 value
func add(str1, str2 string) (string, string) {
return (str2, str1)
}
func main() {
a, b := swap("hello", "world")
fmt.Println(a, b)
}Return values can be named - at the function declaration step
func split(sum int) (x, y int) {
x = sum * 4 / 9
y = sum - x
return // this is a naked return - it will return x and y as they have been named in the function declaration line
}Var is used to declare a list of variables - like function argument lists - the type is last A variable can be at the package or at the function level
var a, b, c int
You can also initialize during declaration
var a, b int = 1, 2
If the initialization is present, we can skip type var a, b = 1, “str1”
You can also use := to declare variables - this is a shorthand for using var and type
:= <- this cannot be used outside a function, there we have to use var
Go’s basic types are:
- bool
- string
- int, int8, int16, int32, int64
- uint, uint8, uint16, uint32, uint64, uintptr
- byte // alias for uint8
- rune // alias for int32, represents a Unicode code point
- float32, float64
- complex64, complex128
int/uint is 32 bit on a 32 bit system, 64 bit on a 64 bit system
You can factor var declarations into a block (just like with import)
package main
import (
"fmt"
"math/cmplx"
)
var (
ToBe bool = false
MaxInt uint64 = 1<<64 - 1
z complex128 = cmplx.Sqrt(-5 + 12i)
)
func main() {
fmt.Printf("Type: %T Value: %v\n", ToBe, ToBe) // T is for type, %v uses refelection to get type at runtime
fmt.Printf("Type: %T Value: %v\n", MaxInt, MaxInt)
fmt.Printf("Type: %T Value: %v\n", z, z)
// Output
// Type: bool Value: false
// Type: uint64 Value: 18446744073709551615
// Type: complex128 Value: (2+3i)
}Variables declared without an explicit initial value are given their zero value
- 0 for numeric type
- false for boolean type
- ”” for strings
Type conversions
- `T(v)` converts value v to type T
var a int = 2
var b float64 = float64(a)
var c uint = uint(b)
// or equivalently
a := 42
b := float64(a)
c := uint(b)Go is strongly typed - this means each variable has a predefined type and it cannot refer to something else. So, to make b refer to an int, the int needs to be type casted to float64 or else it won’t work
Type inference When declaring a variable without specifying an explicit type (either := or var = syntax), the variable’s type is inferred from the value on the rhs
var i int
j := i
i := 42 // int
f := 3.142 // float64
g := 0.867 + 0.5i // complex128Constants are declared like variables, with the `const` keyword They cannot use the := syntax
package main
import "fmt"
const Pi = 3.14
func main() {
const World = "世界"
fmt.Println("Hello", World)
fmt.Println("Happy", Pi, "Day")
const Truth = true
fmt.Println("Go rules?", Truth)
}Numeric constants
- they are high precision values
- an untyped constant takes the type needed by its context
package main
import "fmt"
const (
// Create a huge number by shifting a 1 bit left 100 places.
// In other words, the binary number that is 1 followed by 100 zeroes.
Big = 1 << 100
// Shift it right again 99 places, so we end up with 1<<1, or 2.
Small = Big >> 99
)
func needInt(x int) int { return x*10 + 1 }
func needFloat(x float64) float64 {
return x * 0.1
}
func main() {
fmt.Println(needInt(Small))
fmt.Println(needFloat(Small))
fmt.Println(needFloat(Big))
}
// output
// 21
// 0.2
// 1.2676506002282295e+29Here, we see that they are printed so as to display their values correctly - this is due to their following the Stringer interface - their string method has logic with switch/case statements displaying the correct format
Go has only 1 looping construct - the `for` loop It has the usual 3 components:
- init statement - executed before the first iteration
- the variables declared here will be only visible in the scope of the `for` statement
- this is optional
- condition expression - evaluated before every iteration
- post statement - executed at the end of every iteration
- this is optional
No () needed around for, but for the for code block, {} mandatory
package main
func main() {
for a := 0; a < 10; a ++ {
fmt.Printf(a)
}
for ; sum < 1000; { // if you make both the init and post statement optional, you can drop the semicolons to get the C's while
sum += sum
}
for sum < 1000 { // this is equivalent to :top:
sum += sum
}
}C’s while is spelled for in Go.
Omitting the conditional expression as well gives you an infinite loop for { // code }
If statements are like the `for` statements - no () on if statement, {} mandatory for code block
package main
import (
"fmt"
"math"
)
func sqrt(x float64) string {
if x < 0 { // see ma, no braces!
return sqrt(-x) + "i"
}
return fmt.Sprint(math.Sqrt(x))
}You can also include a tiny “declaration” statement with an if - just like the init in “for” - the variable declared here will only be accessible in the if block - and also inside any of the `else` blocks
package main
import (
"fmt"
"math"
)
func pow(x, y, lim int) int {
if v := math.pow(x, y); v < lim {
return v
}
return lim
}
func main() {
fmt.Println(
pow(3, 2, 10),
pow(3, 3, 20),
)
}
Switch A `switch` statement is a shorter way to write a sequence of `if-else` It runs the first case whose conditional expression evaluates to true
Go’s switch runs only the selected cases, not all cases that follow - so no break needed at end of every case’s code expression Also, the cases need not be constants, and the values involved need not be integers
package main
import (
"fmt"
"time"
)
func main() {
today = time.Now().Weekday()
switch time.Saturday { // here, we are looking for time.Saturday -> it gives us a time.Weekday
case today + 0:
fmt.Println("Today")
case today + 1:
fmt.Println("Tomorrow")
default today + 1:
fmt.Println("Too far away")
}
}We can also use switch without any condition. This is same as `switch true` -> it will execute the first block that evaluate to true This is a clean way to write long-if-then-else chains
package main
import (
"fmt"
"time"
)
func main() {
t = time.Now()
switch {
case t.Hour() < 12:
fmt.Println("Good morning")
case t.Hour() < 17:
fmt.Println("Good afternoon")
default:
fmt.Println("Good evening")
}
}Defer A `defer` statement defers the execution of a function until the surrounding function returns The deferred call’s arguments are evaluated immediately, but the function itself is not executed until the surrounding function returns
package main
import "fmt"
func main() {
defer fmt.Println("world")
fmt.Println("hello")
}We can have multiple defer calls, they are put pushed onto the stack - so they execute LIFO
func main() {
for a:=0;a<10;a++ {
defer fmt.Println(a)
}
fmt.Println("end")
}
// output
// end
// 9
// 8
// 7
// 6
// 5
// 4
// 3
// 2
// 1Go has pointers. A pointer holds the memory address of a value (a value can be an integer, data structure, anything)
`*T` is a pointer to a `T` value It’s zero value is `nil` - the value it gets after declaration, before initialization
var p *int
To get the pointer to a certain value, use `&T`
a := 42 p = &a
The `*` operator denotes the pointer’s underlying value
fmt.Println(*p) // we get the value pointed to by the pointer p. *<some pointer> gives the value pointed to by the pointer // this is also called dereferencing or indirecting *p = 21 // here we set the value of the memory location pointed to by the pointer to 21
package main
import "fmt"
func main() {
i, j := 42, 2701
p := &i // point to i
fmt.Println(*p) // read i through the pointer
*p = 21 // set i through the pointer
fmt.Println(i) // see the new value of i
p = &j // point to j
*p = *p / 37 // divide j through the pointer
fmt.Println(j) // see the new value of j
// output
// 42
// 21
// 2701/37 = 73
}A struct is a collection of fields
package main
import "fmt"
// note the syntax: type <variable name> <variable type>
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1, 2}
fmt.Println(v)
fmt.Println(v.X) // You can access the struct fields using the dot notation
v.X = 5
fmt.Println(v)
}
// {1 2}
// 1
// {5 2}We can have pointers to structs as well. In the example above,
package main
import "fmt"
// note the syntax: type <variable name> <variable type>
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1, 2}
ptr_v := &v
fmt.Println(*ptr_v) // you can print it using
fmt.Println((*ptr_v).X) // you can access the attributes like so
fmt.Println(ptr_v.X) // this is also allowed to make it simple to access - recall C has ptr_v->X
}You can allocate a new struct value by listing the values of its fields
type Vertex struct {
X, Y int
}
var (
v1 = Vertex{1, 2}
v2 = Vertex{X: 1} // Y:0 is implicit
v3 = Vertex{} // X:0, Y:0 is implicit
v3 = &Vertex{1, 2} // v3 is a pointer to Vertex (type *Vertex)
)The type `[n]T` is an array of `n` values of type `T`
var foobar[10]int
Here, foobar is an array of 10 integers
Arrays cannot be resized.
package main
import "fmt"
func main() {
var a [2]string
a[0] = "Hello"
a[1] = "World"
fmt.Println(a[0], a[1])
fmt.Println(a)
primes := [6]int{2, 3, 5, 7, 11, 13} // you can declare and initialize in one go (no pun intended)
fmt.Println(primes)
}Arrays are fixed length but slices are dynamically sized - they offer a flexible view into the elements of an array.
A slice is formed by specifying the 2 indices - a low and high bound
a[low:high]
Low is included, high is not
package main
import "fmt"
func main() {
primes := [6]int{2, 3, 5, 7, 11, 13}
var s []int = primes[1:4] // slices have simlar syntax as arrays
fmt.Println(s)
}Slices are like references to arrays - they are just a dynamic resizable sliding window to peek at (and modify) the contents of the array
Slice literals is like an array literal without a length
[3]bool{true, true, true} // this is an array
[]bool{true, true, true} // this is a slice - it creates the array first and then builds a slice to reference it
// examples:
[]int{1, 2, 3, 4, 5}
[]struct {
X int,
Y string
}{
{1, "one"},
{2, "two"},
{3, "three"}
}
// slices have a default value for low and high index as well
// default for low: 0
// default for high: len()The slice has length -> the number of elements it contains -> len(s) The slice has capacity -> the number of elements in the underlying array -> cap(s)
So, we can do this:
package main
func main () {
s := []int {1, 2, 3, 4, 5}
s1 := s[:0] // s1 has size 0
s1 := s[:4] // s1's size has been increased to 4
}The zero value of a slice is `nil` A `nil` slice has length and capacity of 0 and has no underlying array
package main
import "fmt"
func main() {
var s []int // empty array, so the slice is nil
fmt.Println(s, len(s), cap(s))
if s == nil {
fmt.Println("nil!")
}
}Slices can be made with `make` - it’s a built in function `make` allocates a 0ed array and returns a slice that refers to that array
Syntax: make([]int, <int for len>, <int for capacity>)
Matrix:
board := [][]string{ []string{””, ””, ””}, []string{””, ””, ””}, []string{””, ””, “_”}, }
Appending to a slice. We have the append function described in the docs as:
func append(s []T, vs …T) []T
It takes as arguments “s” which is a slice of T type. And it takes in a variable number of arguments of type T and it returns a slice of type T
If the backing array of “s” is too small to fit all the given values, a bigger array will be allocated and a pointer to that new array will be returned
// create a slice
var s []int // this has 0 capacity right now
s = append(s, 0) // this increases the capacity of s, returns a pointer to new arrayThe range form of the “for” loop iterates over a slice or map When ranging over a slice, the default behavior is that of Python’s enumerate(iterable) The 2 items returned are the index, the copy of the element at that index
a := []int {11, 22, 33, 44}
for i, v := range a {
fmt.Println(i, v)
}
// you can skip the index by using _
for _, v := range a {
fmt.Println(_, v) // this will throw an error
}A map maps keys to values A zero value of a map is `nil` A `nil` has no keys, nor can keys be added
type Vertex struct {
Lat, Lon float64
}
var m map[string]Vertex
func main() {
m = make(map[string]Vertex)
m["Bell Labs"] = Vertex{
1.11, 2.22
}
fmt.Println(m)
}
var mymap = map[string]Vertex{
"foo" : Vertex{1.1, 2.2},
"bar" : Vertex{1.2, 2.3},
}
// we can skip the type name here
var mymap = map[string]Vertex{
"foo" : {1.1, 2.2},
"bar" : {1.2, 2.3},
}
// we can insert an element in map m
m[key] = elem
// we can get an element in map m
elem = m[key]
// we can delete an element in map m
delete(m, key)
// test if a key is there
elem, ok = m[key]
// ok is true/false. If ok is false, elem is zero value of the map's element type
///////////////////////////
// testing the word count
///////////////////////////
package main
import (
"golang.org/x/tour/wc"
"strings"
)
func WordCount(s string) map[string]int {
res := make(map[string]int) // make is used to create a new dict
words := strings.Fields(s)
for _, v := range words { // _ because we don't want the index
res[v] = res[v] + 1
}
return res
}
func main() {
wc.Test(WordCount)
}
///////////////////////////
// testing the slices
///////////////////////////
package main
import "golang.org/x/tour/pic"
func Pic(dx, dy int) [][]uint8 {
res := make([][]uint8, dx) // here, we make the doubly index, (but currently empty) slice
for i := 0; i < dx; i++ {
res[i] = make([]uint8, dy) // here, we make the inner slice
for j := 0; j < dy; j++ {
res[i][j] = uint8(i^j) // here, we populate the inner slice
}
}
return res
}
func main() {
pic.Show(Pic)
}Functions are values too. They can be passed around just like other values - they can be used as function arguments and return values
package main
import (
"fmt"
"math"
)
func compute(fn func(float64, float64) float64) float64 { // this line is extremely readable - we take in an argument called fn, which is of type function, taking in 2 floats and returning a single float
return fn(3, 4) // we always call fn with 3, 4
}
func main() {
hypot := func(x, y float64) float64 { // here, we create a function and assign it to variable called hypot
return math.Sqrt(x*x + y*y)
}
fmt.Println(hypot(5, 12)) // we can call the hypot variable
fmt.Println(compute(hypot)) // we can call the compute function
fmt.Println(compute(math.Pow)) // we can call the compute function
}Functions in Go can be closures as well. Closures are just functions that reference variables from outside its body.
If that function has a access to the variables, it can assign and reference them
func main() {
pos, neg := adder(), adder()
for i := 0; i < 10; i++ {
fmt.Println(
pos(i),
neg(-2*i),
)
}
}
func adder() func(int) int { // adder returns a function that takes an int and returns an int
sum := 0
return func(x int) int { // this function that we return has access to the "sum" variable and is a function closure. Each returned function has it's own "sum" variable
sum += int
return sum
}
}
//////////////////////////////////////
// we can have a fibonacci closure ///
//////////////////////////////////////
package main
import "fmt"
// fibonacci is a function that returns
// a function that returns an int.
func fibonacci() func() int {
last := 0
now := 1
return func() int {
res := last + now
last = now
now = res
return res
}
}
func main() {
f := fibonacci()
for i := 0; i < 10; i++ {
fmt.Println(f())
}
}Go doesn’t have classes. But we can define methods on types
So, we can define a struct type and define methods on that struct. Now, you can use the struct instantiation to call that method If we want to associate it with a type, we need to use a special receiver argument - it appears in its own argument list between the `func` keyword and the method name
type Vertex struct{ // a normal struct with 2 attrs, both float
X, Y float4
}
func (v Vertex) Abs() float64 { // note the syntax, we have: func <type to associate it with> <function name> (<function args>) <function return values>
return math.Sqrt(v.X*v.X + v.*v.Y)
}
func main() {
v := Vertex{3, 4}
fmt.Println(v.Abs()) // we can call the function on the type as if it were the type's attribute
}The terminology: the Abs method has a receiver of type Vertex named v
So, functions are normal “unassociated with type” functions Methods are functions which are associated with type
Remember: a method is just a function with a receiver argument.
If someone gives you a method, you can ask who is the receiver of this method? Or more explicitly, what type is the receiver of this method?
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func Abs(v Vertex) float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := Vertex{3, 4}
fmt.Println(Abs(v))
}You can declare methods on all types, not just structs.
type MyFloat float64 // we declare a new type called MyaFloat which is just a float
func (f MyFloat) Abs() float64 { // the correct way to read this is to say, this function Abs can be applied on variables of type MyFloat
if f < 0 {
return float64(-f)
}
return float64(f)
}
func main() {
f := MyFloat(-math.Sqrt2)
fmt.Println(f.Abs())
}We can also create methods on pointer receivers In that case, the receiver argument part of the function declaration will have (t *T) which is to say that this function can be called on variables of type *T, that is, this function can be called on pointers to T (where T is some type)
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func (v *Vertex) Scale(f float64) { // this function can be called on pointers of Vertex struct
// here, the function Scale is associated with the type "pointer of struct Vertex". So, it modifies the parent struct
v.X = v.X * f // recall, this should have been (*v).X, C makes this easy by v->X, Go makes it easier by v.X
v.Y = v.Y * f
}
func main() {
v := Vertex{3, 4}
v.Scale(10)
fmt.Println(v.Abs()) // here, we can call the method on "v", and not on &v (which would have looked like: (&v).Abs()) which represents a pointer to Vertex, or *Vertex, which is what the method is associated to. This is because Go compiler executes this as *v for us.
}Since the methods often need to modify their receiver (the type they are associated with), pointer receivers are more common than value receivers
Go has “pass by value” like C. You can “pass by reference” with pointers
func Scale(v *Vertex, f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
Scale(&v, 10) // we now need to pass a pointer to the functionHere, if try to pass just “v” and not the pointer to v, it will throw an error - the function Scale expects a pointer to Vertex, it must be give a pointer to Vertex
So:func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
v := Vertex{3, 4}
// we should be doing
fmt.Println(v.Abs())
p := &v
// we can do
fmt.Println(p.Abs())
// here, this will be interpreted as
fmt.Println((*p).Abs())
///////////////////////////////////////
/// we can have a fibonacci closure ///
///////////////////////////////////////
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func AbsFunc(v Vertex) float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := Vertex{3, 4}
fmt.Println((&v).Abs())
fmt.Println(AbsFunc(v))
p := &Vertex{4, 3}
fmt.Println(p.Abs())
fmt.Println(AbsFunc(*p))
}Bottomline: Functions only accept what they explicitly say they will accept Methods can accept either values or their pointers -> they will get the right one and work with it So, use value receiver if you don’t want to modify receiver, use pointer receiver if you want to modify the receiver Also, you might want to use a pointer receiver when you want to avoid copying the value on each method call
Example, look at Abs below ⬇️
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}Here, 🔝 we used pointer receiver even when we didn’t want to change the Vertex struct
There is a special type - the “interface” type It is defined as a set of method signatures - it just has the signatures, not the body
When you declare a variable of this interface type, it can hold any value (of any type that implements this interface.
So, to make things concrete: Let’s say we have:
- An interace “x” with 3 method signatures
- A type “y” which implements all the 3 methods
- Now, you can do var x_1 x = y
- Note, we were able to store y in variable x
package main
import (
"fmt"
"math"
)
type MyFloat float64
type Abser interface {
Abs() float64
}
func (f MyFloat) Abs() float64 {
if f < 0 {
return float64(-f)
}
return float64(f)
}
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
var a Abser
f := MyFloat(-math.Sqrt(2))
v := Vertex{3, 4}
a = f // this is allowed since MyFloat implements the Abser interface
// so how this works is that we can have an interface and folks who implemented that interface can be assigned to a variable of that type
a = &v // this is allowed since the method is implemented by pointer to Vertex, *V. a = v is not allowed
}A type implements an interface by implementing its methods. There is no explicit declaration of intent, no “implements” keyword This means that you can declare a new interface with some method and old types that implemented that interface automatically implement that interface
“Implicit interaces decouple the definition of an interface from its implementation”
package main
import "fmt"
type I interface {
M()
}
type T struct {
S string
}
func (t T) M() {
fmt.Println(t.S)
}
func main() {
var i I = T{"hello"} // defining a variable of type I interface and assigning to it struct of type T - this is allowed since type T implements (implicitly) the interface I
i.M() // technically, this interface should be called Mer and not I - recall this is the Go standard
}We can also consider One db for qa and prod but using only a readonly user in qa This will allow us to test out the gateway app with the prod data but we won’t be able to test the pipeline in qa - only dev for that.
Interfaces values (the variables of type interfaces) can be thought of as a tuple of a value and a concrete type –> (value, type) The interface value holds a value of a specific underlying concrete type
Calling a method on the interface type executes the method of the same name on its underlying type (we know that the type would have that method defined as it implements the interface)
package main
import "fmt"
// define an interface I
type I interface {
M()
}
// define a struct type
type T struct {
S string
}
// define a float64 type
type F float64
// make the struct type implement the interface
func (t T) M() {
fmt.Println(t.S)
}
// make the float64 type implement the interface
func (f F) M() {
fmt.Println(f)
}
// in main, create a var of type interface and assign it to new struct and new float64 type and call that method
func main() {
var i I
i = F(math.Pi)
i.M()
describe(i) // we can pass i to a function that accepts interface I
i = T("hello")
i.M()
}
func describe(i I) {
fmt.Printf("(%v, %T)\n", i, i)
}If the concrete value that is associated to the interface value (the interface variable) then the method will be called with a nil receiver
This would have given a null pointer exception in other languages, since we passed a null pointer as the receiver to the method - but in go, it is common to write methods that gracefully handle being called with a nil receiver
package main
import "fmt"
// define an interface I
type I interface {
M()
}
// define a struct type
type T struct {
S string
}
// make the struct type implement the interface
func (t *T) M() {
if t == nil {
fmt.Println("<<nil>>")
return
}
fmt.Println(t.S)
}
func main() {
var i I
var t *T
i = t
describe(i) // will print (<nil>, *main.T) -> <nil> is the internal representation of the struct. Note this is: (value, type)
i.M() // here, we are passing a nil (null) receiver to the method M
i = &T{"hello"}
describe(i) // will print (&{hello}, *main.T) --> here, the value is not nil
i.M() // this will print hello
}A nil interface value holds neither a value nor concrete type So, this is a interface value (interface variable) that is not associated with an instance of a type that implements this interface
var i I
describe(i) // (<nil>, <nil>) --> nil for both value and type
i.M() // this is a run time error because there is no type inside the interface tuple to indicate which concrete method to call
// this :top: will give "panic: runtime error: invalid memory address or nil pointer dereference"The interface type that specifies zero methods is known as the empty interface
interface{}An empty interface can hold values of any type This is used by code that handles of unknown type - like fmt.Println
This is a way of sidestepping the type system in go
func main() {
var a interface{}
describe(a) // (<nil>, <nil>)
a = 42 // this is okay
describe(a) // (42, int)
a = "hello" // this is okay as well
describe(a) // (hello, string)
}A type assertion provides access to an interface value’s underlying concrete value
t := i.(T) // here we assert that the interface value i holds the concrete type T and assigns the underlying T value to the variable t
// this will trigger a panic if i doesn't hold any type T
// to test if the interface value holds a specific type, a type assertion can return 2 values - the underlying value and a boolean value that reports wheather the assertion succeeded
t, ok := i.(T)
// if i holds a value, t will hold it and ok will be true
// if not, t will be the zero value of type T and ok will be false --> no panic occurs
var i interface{} = "hello" // the catch all interface type instance i given a string (here, the interface has no name, it is an anonymous interface)
s := i.(string) // s is hello
s, ok := i.(string) // s is hello, ok is true
s, ok := i.(float64) // s is 0, ok is false
s := i.(float64) // panicNote we talked about reflections that allow us to get the type of the varialbe, type assertions might be using it under the hood to get the type information
They are a construct that permits several type assertions in series
switch v := i.(type) {
case T:
// here, the type is T
case S:
// here, the type is S
default:
// no match, here v has the same type as i
}
func do(i interface{}) {
switch v := i.(type) {
case int:
fmt.Printf("here, the type is int")
case string:
fmt.Printf("here, the type is string")
default:
fmt.Printf("None of the above, it is %T", v)
}
}
func main() {
do(21)
do("hello")
do(true) // None of the above, it is bool. Here, i is "bool", so v also gets the bool type
}One of the most ubiquitous interfaces is the Stringer interface, defined by the fmt package
It has a single method called String -> as the name suggests
type Stringer interface { // we have the Stringer interface
String () string
}
func (p Person) String() string { // the struct Person implements the Stringer interface
return fmt.Sprintf("%v (%v years)", p.Name, p.Age) // Sprintf prints the string
}
type Person struct {
Name string,
Age int
}
func main() {
p = Person{23, "Darshan"}
fmt.Printf(p) // Darshan (23 years) // the fmt.Printf just calls the String() method on the input value
}A simple Example:
package main
import "fmt"
type IPAddr [4]byte
// TODO: Add a "String() string" method to IPAddr.
func (ip IPAddr) String() string {
return fmt.Sprintf("%d.%d.%d.%d", ip[0], ip[1], ip[2], ip[3])
}
func main() {
hosts := map[string]IPAddr{
"loopback": {127, 0, 0, 1},
"googleDNS": {8, 8, 8, 8},
}
for name, ip := range hosts {
fmt.Printf("%v: %v\n", name, ip) // loopback: 127.0.0.1
}
}Go expresses error state with “error” values The `error` type is a builtin interface similar to fmt.Stringer
type error interface {
Error() string
}The fmt package looks for the error interface when printing the values
Functions often return `error` value and the calling code should handle errors by testing wheather the error equals `nil`
i, err := strconv.Atoi("42")
if err != nil {
fmt.Printf("Could not convert to integer")
return
}
Fmt.Printf("got the integer: ", i)Another example:
package main
import (
"fmt"
"time"
)
// new struct MyError
type MyError struct {
When time.Time
What string
}
// implement the Error method required by the error interface
func (m *MyError) Error() string {
return fmt.Sprintf("Did not work at %v and %v", m.When, m.What)
}
// write a function that returns an error
func run() error { // this method returns type error, which is to say, anything that implements the error interface, that is to say, anything type implementes the Error() method
return &MyError{time.Now(), "it did not work"}
}
// in main, call the function :top: returning an error and see how the Error method is invoked
func main() {
if err := run(); err != nil {
fmt.Println("err message: ", err) // when you try to print the error, it looks for the Error() method
}
}So, all the functions that can fail return “error”
We can change our sqrt function from earlier to return an error on getting negative values as input
package main
import (
"fmt"
"math"
)
type ErrNegativeSqrt float64 // we define a custom error type called ErrNegativeSqrt
func (e ErrNegativeSqrt) Error() string { // the custom error type ErrNegativeSqrt implements the error interface
return fmt.Sprintf("cannot Sqrt negative number: %f", float64(e)) // here, the type casting to float64 is necessary else, we'll call the Error method of this e, which calls itself - infinite loop. By type casting as e, we call the String method of e which is the String method of float64 since we don't define it explicitly
}
func Sqrt(x float64) (float64, error) { // the Sqrt function returns a float64 and an error (which may be nil)
z := 1.0
prev_z := 0.0
eps := 2.220446049250313e-16
if x < 0 {
return 0, ErrNegativeSqrt(x) // if the input value is <0, return an error - any value that implements the error interface
}
for n := 0; n < 10; n++ {
z -= (z*z - x) / (2 * z)
// fmt.Printf("Iteration %v: %v (%v)\n", n, z, prev_z-z)
if math.Abs(prev_z-z) < eps {
// fmt.Printf("eps has been reached")
break
}
prev_z = z
}
return z, nil
}
func main() {
fmt.Println(Sqrt(2))
fmt.Println(Sqrt(-2)) // here, the Sqrt function will return an error, which will be printed using Error() method above
}The `io` package specifies the `io.Reader` interface, which represents the read end of a stream of data Any type that implements the `Read` method implements this interface
The `Read` method of the `io.Reader` interface:
func (T) Read(b []byte) (n int, err error) // the method ought to return num of bytes read (len(b)) and error if anyRead populates the given byte slice with data. It also returns an error, or io.EOF when the stream ends
package main
import (
"fmt"
"io"
"strings"
)
func main() {
r := strings.NewReader("Hello World!") // strings.NewReader returns a reader, a value that implements the Reader interface, so it has the Read method. Here, we give the NewReader the string data to read
b := make([]byte, 8) // we make a byte slice
for { // infinite loop
n, err := r.Read(b) // this fills the byte array with the data
fmt.Printf("n = %v err = %v b = %v\n", n, err, b) // this prints the number of bytes read, error if any and the values filled in the bytes slice
fmt.Printf("b[:n] = %q\n", b[:n]) // we can print the bytes slice
if err == io.EOF { // if we come across eof, we break the infinite loop
break
}
}
}Exercise: Implement a custom Reader type that emits an infinite series of ‘A’
So, basically, we will define a new type (say a struct) called MyReader that will have a Read method associated with it That Read method should fill the input byte slice with ‘A’s
package main
import "golang.org/x/tour/reader"
type MyReader struct{}
// TODO: Add a Read([]byte) (int, error) method to MyReader.
func (mr MyReader) Read(b []byte) (n int, err error) {
for i := 0; i < len(b); i++ { // we iterate thru the slice and fill it with As
b[i] = 'A'
}
return len(b), nil
}
func main() {
reader.Validate(MyReader{}) // the Validate method will give it some empty byte slice and see if MyReader fills it with 'A's
}A common pattern is to take an io.Reader and wrap it with another io.Reader that modifies the stream in some way An example: the gzip.NewReader function takes an io.Reader (a stream of compressed data) and returns a *gzip.Reader that also implements io.Reader (a stream of decompressed data)
Exercise: Implement a reader that takes in a stream of data and replaces each character with it’s 13th character
package main
import (
"io"
"os"
"strings"
)
type rot13Reader struct {
r io.Reader
}
func (mr *rot13Reader) Read(b []byte) (n int, err error) {
// we read the data using the rot13Reader's inner Reader and modify the stream and return that
for {
n, err := mr.r.Read(b) // we read the data using the original inner reader
for i := 0; i < n; i++ {
b[i] = 'a' // we modify the data stream
}
return n, err // we can just do "return" here as we have already named the return variables above
}
}
func main() {
s := strings.NewReader("Lbh penpxrq gur pbqr!") // this is the input to the normal NewReader
r := rot13Reader{s} // here, we wrap it with out own reader
io.Copy(os.Stdout, &r) // this basically calls the Read method on r and copies the bytes to stdout
}The io.Copy handles calling the Read method muliple times till EOF is returned etc
The standard library has a package called “Images”. It defines the `Image` interface
package image
type Image interface {
ColorModel() color.Model
Bounds() Rectangle // Rectangle is in images package, but since Image interface is also inside images package, we don't need to write images.Rectangle
At(x, y int) color.Color // these are also interfaces, but here we will use predefined implementations color.RGBA, color.RGBAModel (implemented in images/color package)
}
// to get a new image
m := image.NewRGBA(image.Rect(0, 0, 100, 100)) // this returns an implemnetation of the Image interfaceWe can create our own image:
package main
import (
"image"
"image/color"
"golang.org/x/tour/pic"
)
// this is our implementation of the standard Image interface
type MyImage struct {
i image.Image
}
// here the receiver is our custom image
func (i MyImage) ColorModel() color.Model {
return color.RGBAModel
}
// here the receiver is our custom image
func (i MyImage) Bounds() image.Rectangle {
return image.Rect(0, 0, 300, 300)
}
// this is interesting
// At is supposed to return color.Color
// From the documentation, we see that color.Color is also an interface with only 1 method RGBA() (r, g, b, a, uint32)
// Now, the standard library implements the RGBA method. The std lib has a struct RGBA and a method RBGA() which has this struct as the receiver.
// It looks like this:
//type RGBA struct {
// R, G, B, A uint8
//}
func (i MyImage) At(x, y int) color.Color {
return color.RGBA{uint8(y), uint8(x), uint8(x+y), 255} // when we do this, we create a new struct. Now this struct implements the Color interface since the standard library has the RGBA() method defined with this struct as the receiver. So, we ca return this struct as color.Color return type
}
func main() {
m := MyImage{}
pic.ShowImage(m)
}This gives the result:
They are lightweight threads managed by the Go runtime. They give you useful abstractions to do concurrent programming. You don’t have to deal with mutexes, locks, semaphores etc - the runtime handles that for you. You get a set of very clean abstractions to work with.
go f(x, y, z)This stars a new goroutine running f(x, y, z). The evaluation of f, x, y, z happens in the current goroutine (execution happens in a new goroutine)
Goroutines run in the same address space - so access to shared memory must be synchronized - use channels.
package main
import (
"fmt"
"time"
)
func say(s string) {
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Millisecond)
fmt.Println(s)
}
}
func main() {
go say("world")
say("hello")
}
// this prints
// world
// hello
// hello
// world
// world
// hello
// hello
// world
// world
// helloWhy does it print “hello” so many times? 🤔
They are a “typed” conduit through which you can send and receive values with the channel operator <-
ch <- v // send v to channel ch
v := <-ch // receive from ch and assign to vChannels must be created before use: ch:= make(chan int)
By default, sends and receives block until the other side is ready - this allows goroutines to synchronize without explicit locks/conditional variables
Look at this snippet:
package main
import "fmt"
func sum(s []int, c chan int) {
sum := 0
for _, v := range s {
sum += v
}
c <- sum // send sum to c
}
func main() {
s := []int{7, 2, 8, -9, 4, 0}
c := make(chan int)
go sum(s[:len(s)/2], c) // we give the slice to the function
go sum(s[len(s)/2:], c)
x, y := <-c, <-c // receive from c
fmt.Println(x, y, x+y)
}We can make buffered channels as well Just provide the second argument to make(chan int)
ch := make(chan int, 100)Sends block only when the buffer is full. Receives block when buffer is empty
package main
import "fmt"
func main() {
ch := make(chan int, 2) // here, the channel has capacity of 2
ch <- 1
ch <- 2
// ch <- 3 // this line will cause a deadlock, because the buffer is full and no one is reading from the channel
fmt.Println(<-ch)
fmt.Println(<-ch)
}A sender can close a channel to indicate that no more values will be sent The receiver can test for this by accepting a second argument when receiving
v, ok := <- chHere, `ok` is `false` if there are no more values to receive - that is, `ok` is true if the channel is closed.
for a := range c // will receive values repeatedly till the channel closesOnly the send ought to close the channel, not the receiver. Sending on a closed channel will cause a panic
Closing channels isn’t mandatory, it should be done to tell the receiver that no more values are to be expected
package main
import (
"fmt"
)
func fibonacci(n int, c chan int) {
x, y := 0, 1
for i := 0; i < n; i++ {
c <- x
x, y = y, x+y
}
close(c) // this closes the channel with "c" as the final value being sent
}
func main() {
c := make(chan int, 10)
go fibonacci(cap(c), c) // cap(c) gives the capacity of channel c
for i := range c { // you can use range only if the sender actually closes the chan. If he never does, you won't know when to stop listening and this will cause a deadlock if the writer is finished writing and you are still blocked on listening
fmt.Println(i)
}
}We can use `select` statement to let a goroutine wait on multiple communication operations
`select` blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready
package main
import "fmt"
func fibonacci(c chan int, quit chan int) {
x, y := 0, 1
for {
select {
case c <- x:
x, y = y, x+y
case <-quit:
fmt.Println("quit")
return
}
}
}
func main() {
c := make(chan int) // here, we make 2 channels.
quit := make(chan int)
go func() { // this anonymous function just pulls 10 values from chan c and then puts a 0 in quit chan
for i := 0; i < 10; i++ {
fmt.Println(<-c)
}
quit <- 0
}()
fibonacci(c, quit) // in this function, we just see if we put a value on c chan, or if there is a value on quit chan - we'll be able to put a value on c chan if someone is listening on it
}We can have a default case as well, which will run if no other case is ready
Consider this:
package main
import "fmt"
import "time"
func main() {
tick := time.Tick(100 * time.Millisecond) // this will return a chan with Time type. The channel will receive the time every tick interval - every 100ms here
boom := time.After(500 * time.Millisecond) // here too we get a chan with Time as the type. This chan receives the current time after the mentioned time lapse.
for {
select {
case <-tick:
fmt.Println("tick")
case <-boom:
fmt.Println("BOOM!")
return
default:
fmt.Println(" .")
time.Sleep(50 * time.Millisecond)
}
}
}To store the same data, we can have many binary trees. We have to find if 2 given trees have the same data
type Tree struct {
Left *Tree
Value int
Right *Tree
}package main
import "golang.org/x/tour/tree"
import "fmt"
// Walk walks the tree t sending all values
// from the tree to the channel ch.
func Walk(t *tree.Tree, ch chan int) {
if t.Left != nil {
Walk(t.Left, ch)
}
ch <- t.Value
if t.Right != nil {
Walk(t.Right, ch)
}
}
// Same determines whether the trees
// t1 and t2 contain the same values.
func Same(t1, t2 *tree.Tree) bool {
ch1 := make(chan int)
go Walk(t1, ch1) // the Walk method just does an inorder traversal of the BST and gives the values
ch2 := make(chan int)
go Walk(t2, ch2)
var v1, v2 int
for c := 0; c < 10; c ++ {
v1, v2 = <-ch1, <-ch2 // we compare each value and if even one is not same, we conclude the trees are not the same
if v1 != v2 {
return false
}
}
return true
}
func main() {
fmt.Println(Same(tree.New(10), tree.New(10)))
}Channels are great for communication among goroutines, but what if we don’t need communication, but just want only one goroutine to access a variable at a time to avoid conflicts
This is called mutual exclusion and the datastructure that provides it is called mutex
Go’s standard library has sync.Mutex with these 2 methods:
- Lock
- Unlock
Look at this example, starting from main
package main
import (
"fmt"
"sync"
"time"
)
// SafeCounter is safe to use concurrently.
type SafeCounter struct {
v map[string]int
mux sync.Mutex
}
// Inc increments the counter for the given key.
func (c *SafeCounter) Inc(key string) {
c.mux.Lock()
// Lock so only one goroutine at a time can access the map c.v.
c.v[key]++
c.mux.Unlock()
}
// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Value(key string) int {
c.mux.Lock()
// Lock so only one goroutine at a time can access the map c.v.
defer c.mux.Unlock()
return c.v[key]
}
func main() {
c := SafeCounter{v: make(map[string]int)} // initialize the struct, it has an internal mutex
for i := 0; i < 1000; i++ {
go c.Inc("somekey") // the Inc method increase the key's value after Lock()
}
time.Sleep(time.Second)
fmt.Println(c.Value("somekey")) // Value locks to read the value and then unlocks before method exit
}In structs, you can have anonymous fields. When looking at the source for github.com/PuerkitoBio/fetchbot (a slim web crawler), I came across the `BasicAuthProvider` interface
type BasicAuthProvider interface {
BasicAuth() (user string, pwd string) // return the username pass to perform request with basic auth
}There is a “Cmd” struct that must implement this if it needs to set the auth headers
type Cmd struct {
U *url.URL
M string
}Now, normally you would implement it as:
func (c *Cmd) BasicAuth() (string, string) {
return "id", "password"
}However, in the tests for the package, I saw this:
type basicAuthCmd struct {
*Cmd
user, pwd string
}
func (ba *basicAuthCmd) BasicAuth() (string, string) {
return ba.user, ba.pwd
}Note here, the receiver is not *Cmd, but *basicAuthCmd. However, even then, the Cmd is said to have implemented the interface and would pass the check
// here, cmd is "cmd Command"
// Command is an interface that mandates URL() and Method() methods, which Cmd struct does - simply return the U and M string attrs
if ba, ok := cmd.(BasicAuthProvider); ok {
req.SetBasicAuth(ba.BasicAuth())
}This is because in the `basicAuthCmd` struct, the `Cmd` is an anon attribute. So, you can refer to the attributes of Cmd (which are U, M) directly via the `basicAuthCmd` - so the compiler doesn’t complain if you pass basicAuthCmd struct where Cmd struct is expected - because the Cmd struct is an anon attribute whose attrs can be directly accessed
Link: http://golangtutorials.blogspot.in/2011/06/anonymous-fields-in-structs-like-object.html
Consider this:
package main
import "fmt"
type Kitchen struct {
numOfPlates int
}
type House struct {
Kitchen
numOfRooms int
}
func main() {
h := House{Kitchen{4}, 40}
fmt.Printf("Num of rooms: %d, Num of plates: %d\n", h.numOfRooms, h.numOfPlates) // this is allowed
}Also, we can access the Kitchen struct like so `h.Kitchen`, so this is allowed: `h.Kitchen.numOfPlates`
Naming conflicts are okay if they are not at the same level - so, we can have numOfPlates in House struct as well - when accessed, this will be presented and it will overshadow the value of Kitchen.numOfPlates
But, at the same level, we cannot have a naming conflict
type Kitchen struct {
numOfLamps int
}
type Bedroom struct {
numOfLamps int
}
type House struct {
Kitchen
Bedroom
}
func main() {
h := House{Kitchen{2}, Bedroom{3}} //kitchen has 2 lamps, Bedroom has 3 lamps
fmt.Println("Ambiguous number of lamps:", h.numOfLamps) //this is an error due to ambiguousness - is it Kitchen.numOfLamps or Bedroom.numOfLamps
}In a problem, I had to give a rotated slice of ints.
For eg, given 1 2 3 4 5, and rotation of 4, I had to output: 5 1 2 3 4
I created a custom type and attached a method to that type, (alternatively I could have written a function)
package main
import (
"bufio"
"fmt"
"os"
"strconv"
"strings"
)
func readSlice(reader *bufio.Reader) []int {
inp, _ := reader.ReadString('\n')
inp = strings.TrimSuffix(inp, "\n")
nums := strings.Split(inp, " ")
var a []int
for _, j := range nums {
i, _ := strconv.Atoi(string(j))
a = append(a, i)
}
return a
}
type mySlice []int
func (m *mySlice) rotatedValue(i, n int) int {
l := len(*m)
if (i - n) >= 0 {
return i - n
}
return l + (i - n)
}
func main() {
reader := bufio.NewReader(os.Stdin)
t := readSlice(reader)
s, n := t[0], t[1]
var inp mySlice
inp = readSlice(reader)
res := make(mySlice, s)
for index, v := range inp {
res[inp.rotatedValue(index, n)] = v
}
fmt.Println(strings.Trim(fmt.Sprint(res), "[]"))
}Note, here, the readSlice outputs []int. But, we are assigning output of readSlice to var inp mySlice. This is okay.
However, if you had something like this:
// Foo is a normal struct
type Foo struct {
a string
b string
}
// Foo2 is a superstruct for Foo, it has more fields
type Foo2 struct {
Foo
c string
}
func returnFoo(s Foo) Foo {
return s
}
func main() {
f := Foo{a: "a", b: "b"}
var b Foo2
b = returnFoo(f) // error! You cannot assign Foo to Foo2
fmt.Println(returnFoo(f))
fmt.Println(b)
}If we make make the functions return Foo2, then this happens:
func returnFoo2(s Foo2) Foo2 {
return s
}
func main() {
// f := Foo2{a: "a", b: "b"} --> Cannot use this to create Foo2
f := Foo2{Foo: Foo{a: "a", b: "b"}, c: "c"}
// f := Foo2{Foo{a: "a", b: "b"},"c"} --> also legal
// f := Foo2{Foo{"a","b"},"c"} --> also legal
var b Foo
// b = returnFoo2(f) --> error, cannot assign Foo2 to Foo
fmt.Println(returnFoo2(f))
fmt.Println(b)
}However, if the return type of returnFoo was an interface, we could assign to anything that implements that interface.
This is also not allowed:
type Foo3 Foo2
func main() {
// f := Foo2{a: "a", b: "b"} --> Cannot use this to create Foo2
f := Foo2{Foo{"a", "b"}, "c"}
var b Foo3 // cannot assign Foo2 to Foo3 variable
b = returnFoo2(f)
fmt.Println(returnFoo2(f))
fmt.Println(b)
}We can’t even use it for type Foo3 string
func returnFoo2(s Foo2) string {
return s.c
}
type Foo3 string
func main() {
// f := Foo2{a: "a", b: "b"} --> Cannot use this to create Foo2
f := Foo2{Foo{"a", "b"}, "c"}
var b Foo3 // error -> cannot assign string to Foo3
b = returnFoo2(f)
fmt.Println(returnFoo2(f))
fmt.Println(b)
}However, this works:
func returnFoo2(s Foo2) []int {
return []int{1}
}
type Foo3 []int
func main() {
// f := Foo2{a: "a", b: "b"} --> Cannot use this to create Foo2
f := Foo2{Foo{"a", "b"}, "c"}
var b Foo3
b = returnFoo2(f)
fmt.Println(returnFoo2(f))
fmt.Println(b)
}Here, we are able to assign []int to Foo3 Asked about this here: https://stackoverflow.com/questions/52308617/implicit-type-conversion-in-go
The answer is mostly that int[] is an unnamed type whereas int is a named type. And the spec says x is assignable to T if x's type V and T have identical underlying types and at least one of V or T is not a named type.
Good quote from Tim Heckman on Gophers slack regarding using methods vs function
Functions should generally be for things working on inputs, methods should be for things that use state and optionally inputs. So when I see things like `type Foo []int` I wonder if there is a better way.
Consider this:
package main
import "fmt"
func main() {
var a int
for i := 0; i < 10; i++ {
b := i
a = i
fmt.Println(b, a)
}
}Here, in each loop iteration, the variable i is shared. When the loop starts, the compiler created a variable i and gives it a memory location. On each iteration, it is overwritten. This is why you can’t use the index directly in a go routine etc.
However, in the example above 🔝
I use b:=i in each iteration, so I was confused as to why this is not erroring out, since b would be created in the first iteration and then next time, b would error out since we are using := which is to initialize new variables.
It is correct Go code since each iteration of a for loop is a new scope. So, when we use := in each iteration, it is similar to
package main
import (
"fmt"
)
func main() {
a := 0
{
a := 1
fmt.Println(a) // will print 1
}
fmt.Println(a) // will print 0
// here, if you want to promote the value of the inner scope to the outer scope, assign the value to a variable declared in the outer scope (use a = 1 in the inner scope for eg)
}However, if you aren’t using the inner scope variables outside the iteration scope, the compiler will reuse the memory for the next iteration scope.
In competitive program, this fact can be used to not have to reset variables used in the iteration’s scope. Note, this can only be used if you are using := in the iteration scope, and not if you are promoting the values from inside to outside.
When using maps, be careful to check val, ok := mymap[mykey]. ok will be true if the key was present, else it will be false. The statement will always execute and will return the zero type of the value of mymap. So, if it is map[int]int, it will return 0
This fails
// code
if k >= len(nums) {
acc := nums[:]
} else {
acc := nums[0:k]
}
sort.Slice(acc, func(i, j int) bool { return acc[i] > acc[j] })
// codeThis is because acc is defined only in the scope of the if statement. It doesn’t exist outside. To access it outside, do predeclare it with, var acc []int, and use it with acc = nums[:].
The entry point for any go software is main(). However, if that package has an init() (even the main package), it will be called before everything else. Actually, even before that package’s init(), the global vars in that package will be initialized first. SO:
var WhatIsThe = AnswerToLife()
func AnswerToLife() int {
return 42
}
func init() {
WhatIsThe = 0
}
func main() {
if WhatIsThe == 0 {
fmt.Println("It's all a lie.")
}
}Here, AnswerToLife will be called first. This is to initialize the WhatIsThe variable. Next, init() will be called. Finally, main().
Use make(map[int]int) for eg.
Since map is a reference, you can pass it directly to function and changes will show up.
Channels created using make are references too.
Strings are immutable, taking a substring mystr[a:b] returns a new string. You are not allowed to modify a string in place.
mystr := "hello world"
By default, arrays are zero values of their types
array literal myArray:=[...]{1, 2, 3}
Array type + length is a type, so [3]int and [4]int are different types
Arrays are fixed length
Slices are a pointer (a reference) to an array, so can be passed around and modified. Make liberal use of slices (they are cheap references to the same array)
Use make([]int, <len>, <cap>) to initialize it
Map is a reference to a hash table. To create it, either use a map literal
myMap := map[int]int{1:2}
or use make:
myMap := make(map[int]int)
- It contains Go source files
- It contains package objects
- They have `.a` extension. One directory for each arch (eg: darwin_amd64)
- It contains executable commands
- this path is under $PATH, so when you install, you can use the command directly from terminal
$GOPATH is used to specify the location of the workspace
- globally unique import path for your package
- When working on a new project, create the package in github.com/user dir, just in case you publish it later
- eg: let’s build our first program `hello`
`$ mkdir $GOPATH/src/github.com/user/hello`
package main
import "fmt"
func main() {
fmt.Printf("Hello, world.\n")
}Now we can build and install that program with the `go` tool
`go install github.com/user/hello`
You can run 🔝 from anywhere in the system, the `go` tool will use the $GOPATH If you are in the hello dir, you can do `go install`
The `install` command will produce an executable binary and places it in the workspace’s `bin` directory as `hello` (`hello.exe` under Windows) - eg: $GOPATH/bin/hello
This can be executed now with `$GOPATH/bin/hello`, or simply `hello` since `$GOPATH/bin` is in the `$PATH`
We’ll write a go library and use it from the `hello` program
We’ll create a dir for it in the right place
`$ mkdir $GOPATH/src/github.com/user/stringutil`
We can write `reverse.go` now
// Package stringutil contains utility functions for working with strings.
package stringutil
// Reverse returns its argument string reversed rune-wise left to right.
func Reverse(s string) string {
r := []rune(s)
for a, b := 0, len(r)-1, a < len(r/2), a, b = a+1, b-1 {
r[a], r[b] = r[b], r[a]
}
return string(s)
}Now we can build it: `$ go build github.com/user/stringutil` (just `go build` if in that dir) - this is just to make sure our program builds
Now, in `hello.go` we can do:
package main
import (
"fmt"
"github.com/user/stringutil" // we are using our new library here
)
func main() {
fmt.Printf(stringutil.Reverse("!oG ,olleH"))
}When we install `hello`, it will install `stringutil` as well since `stringutil` is a dependency for `hello`
Our workspace looks like this:
bin/
hello # command executable
pkg/
linux_amd64/ # this will reflect your OS and architecture
github.com/user/
stringutil.a # package object
src/
github.com/user/
hello/
hello.go # command source
stringutil/
reverse.go # package source
When we did `go install github.com/user/hello`, we got `stringutil.a` object in a directory inside `pkg/linux_amd64`. This is so that future invocations of `go` tool can find the package object there and not recompile the package again.
Go command executables are statically linked; the package objects need not be present to run Go programs - so, the binary that is generated is a single complete binary having all the components it needs to run (like `jq`)
The package name should be the last element of the import path; the package imported as “crypto/rot13” should be named “rot13”
The executable commands must always use “package main”.
Go has a lightweight testing framework in the std lib - the `testing` package, use it with `go test`
The tests can be in files ending with `_test.go` and have functions named `TestXXX` with the signature `t *testing.T`
If the function calls a failure function such as `t.Error` or `t.Fail` (both defined in the `testing` package), the test is considered to have failed We can add a test for our stringutil function
package stringutil
func TestReverse(t *Testing.T) {
cases := []struct {
in, want string
}{
{"Hello, world", "dlrow ,olleH"},
{"Hello, 世界", "界世 ,olleH"},
{"", ""},
}
for _, c := range cases {
got := Reverse(c.in)
if got != c.want {
t.Errorf("Reverse(%q) == %q, want %q", c.in, got, c.want)
}
}
}Run the test like so:
$ go test github.com/user/stringutil
ok github.com/user/stringutil 0.165sA straight translation of a program from Java to Go won’t produce satisfactory results, Go programs are to be written in Go perspective - this document will teach us about that.
Tip: The Go package sources (the standard library) is intended to serve as examples and ideas of how to use the language.
`gofmt` operates at the source file level, `go fmt` operates at the package level
Tabs are used by default, don’t change them unless you absolutely have to
Go has C-style block comments `/* */` and C++ style line comments `//`
Block comments are extracted by `godoc` to generate documentation hosted on godoc.org
Comments that appear before top-level declarations without any intervening newlines are extracted along with the declaration to serve as explanatory text for the item
Every package should have a package comment, a block comment preceding the package clause - in any file in the package works. It appears at the top in the godoc page for the package
Eg:
/*
Package regexp implements a simple library for regular expressions.
The syntax of the regular expressions accepted is:
regexp:
concatenation { '|' concatenation }
concatenation:
{ closure }
closure:
term [ '*' | '+' | '?' ]
term:
'^'
'$'
'.'
character
'[' [ '^' ] character-ranges ']'
'(' regexp ')'
*/
package regexp
The comments are unintrepreted plain text, so HTML, markdown etc won’t work
Any comment immediately preceding a top level declaration serves as a “doc comment” for that declaration. Every exported name in a package should have a doc comment - the doc comment should start with the name being declared
Capitalized names are exported in Go
Good package names are single words, no need of under_scores or mixCaps
The exported names will be prefixed by the package names, so use that to your advantage Example:
bufio.Reader and not bufio.BufReader
Also, we have io.Reader too, but since the package is different, it’s okay Similarly, we have a package ring - it has only 1 exported type: Ring So, to make new instances of Ring, we don’t have ring.NewRing, but ring.New
Long names don’t automatically make things more readable.
Go doesn’t have automatic support for getters and setters - but you can add it yourself It’s not idiomatic (godiomatic?) to put `Get` in the name
If you have an unexported field (say, owner), the getter method should be called Owner, and not GetOwner A setter function can be called SetOwner
This reads well:
owner := obj.Owner()
if owner != user {
obj.SetOwner(user)
}One method interfaces should have method + “-er” - eg: Reader, Writer, Formatter etc
Also, if you have a new type and it implements the same methods as a method on a well-known type, use the same name - eg, call the string converter method String and not ToString (String also happens to mandated by the Stringer interface)
Go likes MixedCaps and mixedCaps, under_scores are not welcome
Go’s formal grammar uses semicolons to terminate statements, but the lexer adds them automatically so they don’t need to appear in the source code
The rule is this: if the last token before a newline is an identifier (which includes words like `int` and `float64`), a basic literal such as a number or string constant, or one of these tokens - `break, continue, fallthrough, return, ++, –, ) }` the lexer inserts a semicolon after the token
Rephrasing 🔝
“If the newline comes after a token that could end a statement, insert a semicolon”
So, we won’t have semicolons inserted by the lexer
This has the consequence that you cannot put the opening brace of a control structure (if, for, switch or select) on the next line
if i < f() {
g()
}
// correct :top:
if i < f() // wrong!
{ // wrong!
g()
}
// incorrect :top:- Similar to C, but differ in important ways.
- There is no `do`, no `while`. `switch` is more flexible
- `if` and `switch` accept an optional initialization statement like that of `for`
- `break` and `continue` take an optional label to identify what to break or continue (like, `loop` from fetchbot)
- `select` can also used as a select for types (as a type switch)
- `select` can also be used as a multiway communications multiplexer
- the syntax is also a little different: no parantheses and the bodies must always be brace delimited
if x > 0 {
return y
}Recall `if` and `switch` accept an initialization statement, commonly used to setup a local variable
if err := file.Chmod(0664); err != nil {
log.Print(err)
return err
}Consider:
f, err := os.Open(name) // here, we declare f and err
d, err := f.Stat() // here, we re-assign err since it is already declared aboveThe `:=` declaration of variable `v` may appear even if it has already been declared, provided in these cases:
- this declaration is in the same scope as the existing declaration of `v`
- if v is already declared in an outer scope, the declaration will create a new variable (for that scope)
- the corresponding value in the initialization is assignable to v (that is, if it is the same type as v’s old type)
- there is at least one other variable in the declaration that is being declared anew (and the old one gets value of same type as last time)
Go’s `for` unifies C’s `for`, `while` and `do-while`
// this is C's for
for init; condition; post {}
// this is C's while
for condition {}
// this is C's for(;;)
for {}Creating an index right there in the for loop is convenient
sum := 0
for i := 0; i < 10; i++ {
sum += i
}To loop over an iterable (like array, slice, string, map, reading from a channel), use `range`
for key, value := range oldMap {
newMap[key] = value
}To use only the first item (the key of key, index), drop the second
for key := range m {
if key.expired() {
delete(m, key)
}
}To use only the 2nd, use _ for the first
sum := 0
for _, value := range array {
sum += value
}If you iterate thru a string, it is parsed as Unicode code points and each iteration gets a `rune` - which is one individual unicode code point (size varies of course, the ascii ones have the same encoding as ascii - 8 bits)
If the encoding is erroneous, the replacement rune is U+FFFD (size is one byte)
for pos, char := range "日本\x80語" { // \x80 is an illegal UTF-8 encoding
fmt.Printf("character %#U starts at byte position %d\n", char, pos)
}prints
character U+65E5 '日' starts at byte position 0 character U+672C '本' starts at byte position 3 character U+FFFD '�' starts at byte position 6 character U+8A9E '語' starts at byte position 7
Note the size varies for each rune
Go’s `switch` is more general than C’s The expressions need not be constants, or even integers The cases are tried from top to bottom
func unhex(c byte) byte {
switch {
case '0' <= c && c <= '9':
return c - '0'
case 'a' <= c && c <= 'f':
return c - 'a' + 10
}
return 0
}You can have multiple clauses for `case`
func shouldEscape(c byte) bool {
switch c {
case ' ', '?', '&', '=', '#', '+', '%':
return true
}
return false
}`break` can be used to terminate `switch` early. It can also be used to break out of a surrounding loop - you can put a label on the loop and `break` to that label
Loop:
for n := 0; n < len(src); n += size {
switch {
case src[n] < sizeOne:
if validateOnly {
break
}
size = 1
update(src[n])
case src[n] < sizeTwo:
if n+1 >= len(src) {
err = errShortInput
break Loop // we break out of Loop
}
if validateOnly {
break // here we only break out of the of the switch
}
size = 2
update(src[n] + src[n+1]<<shift)
}
}Example:
// Compare returns an integer comparing the two byte slices,
// lexicographically.
// The result will be 0 if a == b, -1 if a < b, and +1 if a > b
func Compare(a, b []byte) int {
for i := 0; i < len(a) && i < len(b); i++ {
switch {
case a[i] > b[i]:
return 1
case a[i] < b[i]:
return -1
}
}
switch {
case len(a) > len(b):
return 1
case len(a) < len(b):
return -1
}
return 0
}`switch` can also be used to discover the dynamic type of an interface variable. Such a type switch uses the syntax of a type assertion with the keyword `type` in parantheses. If a varialbe is declared in the expression, the variable will have the corresponding type in each clause
var t interface{}
t = functionOfSomeType() // this function returns some type
switch t := t.(type) { // use the same name here
default:
fmt.Printf("unexpected type %T\n", t) // %T prints whatever type t has
case bool:
fmt.Printf("boolean %t\n", t) // t has type bool
case int:
fmt.Printf("integer %d\n", t) // t has type int
case *bool:
fmt.Printf("pointer to boolean %t\n", *t) // t has type *bool
case *int:
fmt.Printf("pointer to integer %d\n", *t) // t has type *int
}In Go, functions and methods can return multiple values
Ex, look at the `Write` method on files from `os` package `func (file *File) Write(b []byte) (n int, err error)`
It returns the num of bytes written and a non-nil error when `n!=len(b)`
You can name the return variables of a function and they can be used as regular variables.
“When named, they are initialized to the zero values for their types when the function begins; if the function executes a return statement with no arguments, the current values of the result parameters are used as the returned values.”
The naming of the return variables can make the code shorter and cleaner - they serve as documentation if for example you are returning 2 ints etc
They are used nicely here:
func ReadFiull(r Reader, buf []byte) (n int, err error) {
for len(buf) > 0 && err == nil {
var nr int
nr, err = r.Read(buf)
n += nr
buf = buf[nr:]
}
return
}Go’s defer statement schedules a function call to be run immediately before the function executing `defer` returns
Canonical examples are unlocking a mutex, closing a file, response body etc
2 points about defer:
- multiple defers are allowed
- they are executed in a LIFO manner - like stacks
- so, defer fn_a() followed by defer fn_b() means, fn_b() runs first
- The arguments to the deffered function are evaluated when the defer executes, not when the call executes
- so, the argument is evaluated when you write the defer statement, not when the function being deffered is called
Example:
import "fmt"
func trace(s string) string {
fmt.Println("entering: ", s)
return s
}
func un(s string) {
fmt.Println("leaving: ", s)
}
func a() {
defer un(trace("a"))
fmt.Println("in a")
}
func b() {
defer un(trace("b"))
fmt.Println("in b")
a()
}
func main() {
b()
}
// prints
// entering: b
// in b
// entering: a
// in a
// leaving: a
// leaving: bGo has 2 allocation primitives - “new” and “make”
Both do different things:
- “new” allocates memory - it does not initialize it, it only zeros it
- so, new(T) allocates zeroed storage for a new item of type T and returns its address, a value of type *T
- In go terminology, it returns a pointer to a newly allocated zero value of type T
It is really nice if you can make the zero-value-useful. Since the memory returned by “new” is zeroed, this becomes particularly convenient
Eg: documentation of bytes.Buffers states that the zero value of Buffer is an empty buffer, ready to use
Similarly, sync.Mutex does not have explicit constructor or Init method, the zero value of a sync.Mutex is defined to be an unlocked mutex
This works well transitively - (if a = b, and b = c, then a = c) too Consider:
type SyncedBuffer struct {
lock sync.Mutex
buffer bytes.Buffer
}
// here, the struct is ready to be used on initialization directly
// both p and v will work without further arrangement
p := new(SyncedBuffer) // type *SyncedBuffer
var v SyncedBuffer // type SyncedBufferSometimes, the zero value isn’t good enough and you want an initializing constructor
Consider this derived from package `os`:
func NewFile(fd int, name string) *File {
if fb < 0 {
return nil
}
f := new(File)
f.fd = fd
f.name = name
f.dirinfo = nil
f.nepipe = 0
return f
}We can reduce the boilerplate by using a composite literal - an expression that creates a new instance each time it is evaluated:
func NewFile(fd int, name string) *File {
if fb < 0 {
return nil
}
f := File{fd, name, nil, 0}
return &f
}Here, we are returning the address of a local variable and that’s OK! Recall this is where we were making the mistake when writing the C program to reverse a linked list with Prabal
It’s okay in Go because taking the address of a composite literal allocates a fresh instance each time it is evaluated
If the fields of a composite literal are not named, they must be laid out in order and all must be present. If you name them however, you can skip some and the skipped ones get the zero values of their respect types
If no values are present, you get a zero value for all the struct’s fields - File{}
These are equivalent - new(File), &File{} - recall Pike says, you can initialize the values you care for and the rest will be given the type’s zero values
Composite literals can also be used for arrays, slices and maps eg:
a := [...]string {Encone: "no error", Eio: "Eio", Einval: "invalid argument"}
b := []string {Encone: "no error", Eio: "Eio", Einval: "invalid argument"}
c := map[int]string{Encone: "no error", Eio: "Eio", Einval: "invalid argument"}The other allocating construct in Go is “make” The `make(T, args)` serves a purpose different from new(T)
It creates ONLY - slices, maps and channels It returns an initialized (not zeroed) value of type T (not *T)
“The reason for the distinction is that these three types represent, under the covers, references to data structures that must be initialized before use” (not zeroed, but initialized)
The slice for example is a 3 item descriptor containing:
- a pointer to the data (inside an array)
- length
- capacity
All 3 are required and until those items are initialized, the slice is nil
For eg:
`make([]int, 10, 100)`
Allocates an array of 100 ints and then creates a slice structure with length 10 and a capacity of 100 pointing at the first 10 elements of an array
Slices are just an abstraction over Go’s arrays. Arrays are like in every other language, they are a sequences of typed data. To be defined, they need to have a length and element type. Example, [4]int is an array of size 4, with the data type being int. The size is fixed, so [4]int and [5]int are 2 different “types” (think of the array type as being a vector, like a vector has 2 components to be defined, magnitude and direction, the array has length and data type)
var a [4]int
a[0] = 1
b := a[0] // b == 1Array’s are initialized with the zero values of the data type. So, 🔝 a[3]==0 is true.
The in-memory representation of [4]int 🔝
Go’s arrays are values, in that the array variable is not a pointer to the first element of the array, but of the entire array. Since Go has pass by value, if you pass the array variable around, you create a copy of it, to avoid that, you have to pass the reference to the array (the pointer to the array)
“One way to think about arrays is as a sort of struct but with indexed rather than named fields: a fixed-size composite value.”
a := [2]string{"foo", "bar"}
b := [...]string{"foo", "baz"} // the compiler does the counting hereArrays are old, they are okay. Slices are awesome. The build on arrays to provide flexibility and convenience.
The type specification for a slice is `[]T`, note the absence of length declaration is exactly the same as arrays without the length
a := []string{"foo", "bar"}You can use `make` to create the slices, signature is:
func make([]T, len, cap) []TNote, the slice is returned, not the pointer to the slice
The capacity is optional. If omitted, cap = len
var s []byte // declaration
s = make([]byte, 5, 5) // initialization, equivalently, s = make([]byte, 5)
// we have s == []byte{0, 0, 0, 0, 0}You can create a slice by slicing an existing slice or array. So, b[1:4] will create a new slice with elements 1, 2, 3 from b
b := []byte{'g', 'o', 'l', 'a', 'n', 'g'}
// b[1:4] == []byte{'o', 'l', 'a'}, sharing the same storage as bLike in python, b[:] == b
How to create a slice given an array?
x := [3]string{"Лайка", "Белка", "Стрелка"}
s := x[:] // a slice referencing the storage of xA slice is a descriptor of an array segment. It consists of:
- a pointer to the array
- the length of the segment
- its capacity (the max length of the segment)
Recall we had a array byte from earlier, [5]byte
Our slice s = make([]byte, 5) looks like this:
The length (2nd block in slice, 5 here) is the number of elements referred to by the slice. The capacity is the number of elements in the underlying array beginning at the element referred to by the slice pointer.
Consider we slice s above s2 := s[2:4]
Now, we have another slice that points to the same underlying array that s was pointing to, but s2 has length 2 and capacity 3
“Slicing does not copy the slice’s data. It creates a new slice value that points to the original array.” - so if you modify a slice, the underlying array gets modified as well
To increase the capacity of a slice, you must create a new, larger slice and copy the contents of the original slice into it. (Just like how dynamic array implementations work in other languages)
Here, we have a slice “s”, we make a new slice “t” and copy the contents of s into t and then assign the value t to s
t := make([]byte, len(s), (cap(s)+1)*2)
for i := range(s) {
t[i] = s[i]
}
s = tWe can use the built-in copy function as well
`func copy(dst, src []T) int`
Here, int is the number of elements copied
t := make([]byte, len(s), (cap(s)+1)*2)
copy(t, s)
s = tOne common operation is appending data at the end of the slice
func AppendByte(slice []byte, data ...byte) []byte {
m := len(slice)
n := m + len(data)
if n > cap(slice) { // we need to reallocate, provision a larger slice
newSlice := make([]byte, (n+1)*2)
copy(newSlice, slice)
slice = newSlice
}
slice = slice[0:n]
copy(slice[m:n], data)
return slice
}Usage:
p := []byte{2, 3, 5}
p = AppendByte(p, 7, 11, 13)
// p == []byte{2, 3, 5, 7, 11, 13}This function can be used if you need fine grained control over how the slice grows, (2x each time or not etc) But if you don’t want that level of control, just fly with the in built `append` function
func append(s []T, x...T) []T
// example
a := make([]int, 1)
// a == []int{0}
a = append(a, 1, 2, 3)
// a == []int{0, 1, 2, 3}To append one slice to another, we can use … Syntax to expand the 2nd argument to a list of arguments (like python’s *list)
a := []string{"John", "Paul"}
b := []string{"George", "Ringo", "Pete"}
a = append(a, b...) // equivalent to "append(a, b[0], b[1], b[2])"
// a == []string{"John", "Paul", "George", "Ringo", "Pete"}Recall that the zero value of a slice (nil) acts like a 0 length slice, so you can declare a slice variable and append to it in a loop
// Filter returns a new slice holding only
// the elements of s that satisfy fn()
func Filter(s []int, fn func(int) bool) []int {
var p []int // == nil
for _, v := range s {
if fn(v) {
p = append(p, v)
}
}
return p
}Since a slice is a reference to the underlying array, image a scenario where you have a large file and you read the contents of that file into a []byte and then you do a regexp search for a few bytes. But since the slice that represents the few bytes references the array, the GC cannot claim the memory and the array stays in memory
To solve it, make a new slice and return that
func CopyDigits(filename string) []byte {
b, _ := ioutil.ReadFile(filename)
b = digitRegexp.Find(b)
c := make([]byte, len(b))
copy(c, b)
return c
}
// more concisely,
func CopyDigits(filename string) []byte {
b, _ := ioutil.ReadFile(filename)
b = digitRegexp.Find(b)
return append(make([]byte, len(b)), b)
}Arrays Slices Two-dimensional slices Maps Printing Append Initialization Constants Variables The init function Methods Pointers vs. Values Interfaces and other types Interfaces Conversions Interface conversions and type assertions Generality Interfaces and methods The blank identifier The blank identifier in multiple assignment Unused imports and variables Import for side effect Interface checks Embedding Concurrency Share by communicating Goroutines Channels Channels of channels Parallelization A leaky buffer Errors Panic Recover A web server