section1.md

CS50x Section 1 - C

Variables and Types

A variable is container paired with a symbolic name for some value that can change

int calls = 3;

The code above creates an integer names calls that gets the value 3.

Breaking it down:

• int is the integer type variable

• calls is the name of the variable

• 3 is the value of the variable

• = is the assignment operator

int country_code = 65;

the code above creates an integer names country_code that gets the value 65.

We specify the type of variable int. 65 is stored as binary value 01000001 by the computer.

char country_code = 65;

In this case the binary vale of 01000001 is stored as a char (character) equivalent to "A"

We can use syntactic sugar to make the code shorter in C :

int calls = 4;
calls = calls + 2;
calls = calls - 1;
calls = calls * 2;
calls = calls / 2;

When we run the code above, the result is 5.

int calls = 4;
calls += 2;
calls -= 1;
calls *= 2;
calls /= 2;

When we run the code, the result is 4 instead of 4.5. Any decimal result is "Truncated", not rounded.

Integers are whole numbers, they do not store decimal values.

Input and Printing

int calls = get_int("Calls: "); 

get_int("Calls: "); is a function call.

get_int is a function created by cs50.

When the user is prompted to enter a value, it is stored in the variable. If user enters "4":

int calls = 4

int calls = 4;
printf("calls is %i\n", calls);

The code above will return "calls is 4"

Breaking it down:

• The f in the printf function stands for formatted

• %i is the placeholder for the value of the variable calls

• The value of the variable that follows the coma , will be stored in the placeholder:

• printf("calls is %i\n", calls);
  
  

Types and format codes

• int %i

• float %f

• char %C

• string %s

#include <stdio.h>

int main(void)
{
    printf("Hello, world.\n");
}

Breaking it down:

• #include <stdio.h> Includes header file stdio C library

• the main function serves as the entry point of every C program.

• int specifies the return type of the "main" function.

• (void) This part specifies the parameter list of the main function. "void" means the function does not expect any command-line arguments.
  
  

Getting input from user

Using the CS50 library <cs50.h>, we can easily obtain user input:

#include <stdio.h>
#include <cs50.h>

int main(void)
{
    string name = get_string("What is your name? ");
    printf("Hello, %s\n", name);
}

#include <cs50.h> give us access to the cs50 library that includes The get_string function we are using.

• We declare a variable of type string called "name"

• We assign to it an input from user with function get_string("What is your name? ");

• The function prompts user for his name and stores it in the variable.

• printf("Hello, %s\n", name); function prints "Hello, user-input"

• %s is a string type placeholder for the string type variable prompted by user

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    string first_name = get_string("What is your first name? ");
    string last_name = get_string("What is your last name? ");
    printf("Hello, %s %s\n", first_name, last_name);
}

In this program we prompt the user for two inputs that we store in 2 different variables.

printf("Hello, %s %s\n", first_name, last_name); Function uses 2 placeholders and after the coma specifies which values should replace the placeholders in the result.

#include <cs50.h>
#include <stdio.h>

int main(void)
{
    string name = get_string("What is your name? ");
    int age = get_int("What is your age? ");
    string phone = get_string("What is your phone number? ");

    printf("Name: %s\n", name);
    printf("Age: %i\n", age);
    printf("Phone: %s\n", phone);
}

In the program above we prompt the user for 3 inputs (name, age, phone number) and we display contact information.

We chose string type for the phone number to include a variation of entries.

Functions, Loops and Conditionals

While Loops

These loops are useful when not sure when the condition will be true.

int j = 0;
While (j < 4)
{
    printf("#");
    j++;
}
printf("\n");

The code above will return "####" by repeatedly executing the loop until j is no longer less than 4.

• int j = 0; Creates variable called j and stores value of 0 in it

• While (j < 4) {} While the condition is True (value of j is less than 4)

• printf("#"); Print a single #

• j++; Increase value of j by 1

• The loop then goes back to the top with new value to reassess the condition, and runs the loop over and over until the condition returns false (j = 4).

• printf("\n"); Move to a new line

For Loops

These next loops are useful when wanting to specify how many times we want to iterate.

for (int j = 0; j < 4; j++)
{
    printf("#");
}
printf("\n");

for (int j = 0; j < 4; j++) achieves the same result as the while loop in a more compact form with the following order:

• Create a variable called j and assign value of "0" to it.

• Ask 0 < 4 ? Yes, so we move to j++ and becomes j = 1

• Print out "#"

• Goes back up to ask 1 < 4 ? Yes, so we move to j++ and becomes j = 2

• Print out "##"

• Goes back up to ask 2 < 4 ? Yes, so we move to j++ and becomes j = 3

• Print out "###"

• Goes back up to ask 3 < 4 ? Yes, so we move to j++ and becomes j = 4

• Print out "####"

• Goes back up to ask 4 < 4 ? No

• We move to printf("\n");

Nested Loops

We can create patterns using nested loops, such as a 4x4 grid of #

for (int i = 0; i < 4; i++)
{
    for (int j = 0; j < 4; j++)
    {
        printf("#");
    }
    printf("\n");
}

The code above will return:

####
####
####
####

We can create a 4 x 4 # grid (superposed 4 lines of 4 #) by nesting the same for loop in the original for loop.

Note that we need to make variables different in both loops

int get_int(string prompt)
{
    // Get int from user
}

get_int function is defined by cs50. It prompts user with a string to enter integer value in the example: int height = get_int("Height: ");

Defining and using functions is a key aspect of C programming. Here's an example:

void print_row(int length);

int main(void)
{
    int height = get_int("Height: ");
    for (int i = 0; i < height; i++)
    {
        print_row(i + 1);
    }
}

void print_row(int bricks)
{
    for (int i = 0; i < bricks; i++)
    {
        printf("#");
    }
    printf("\n");
}

This program prompts the user for the height of a pyramid and prints the corresponding rows.

• First loop sets what will be the height or how many rows will the code print using user's input.
• Second loop print_row defines how many # will be printed in each line.

Before calling a function we need to define it. If it's defined below where it is called, the program will not run. We need to include above the call a function prototype (its name, input, and return value) ex: void print_row(int length);

Parameters VS Arguments

#include <stdio.h>

// Function declaration with parameters
int addNumbers(int a, int b) {
    int sum = a + b;
    return sum;
}

int main() {
    // Function call with arguments
    int result = addNumbers(5, 7);

    // Print the result
    printf("Sum: %d\n", result);

    return 0;
}

In this example, a and b are parameters declared in the function addNumbers

5 and 7 are the arguments passed during the function call in main.