Lecture 7.md

Lecture 7 - Oct. 6, 2015

Basic I/O

In C++ there is an implicit conversion from cin (istream) to (void*)

cin evaluates to true if !(cin.fail())

cin evaluates to false if cin.fail()

We can use this fact to do things like:

int main() {
    int i;
    while (cin >> i) {
        count << i << endl;
    }
}

Side note: bit shifting

In C: 21 >> 3; shift 3 bits to the right;

10101 (21) >> 3 -> 00010 (as if we pushed 3 bits off a "cliff")

This is the same in C++ if we use it on integers.

We use cin >> x where >> is an input operator.

This is an example of operator overloading

Once a read fails, we must call cin.clear() to lower the fail flag, then cin.ignore() if we want to ignore that bad input.

#include <iostream>

int main() {
    int i;
    while(true) {
        if(cin >> i) { //read successful
            cout << i << endl;
            continue;
        }
        if(cin.eof()) { //EOF
            break;
        }
        //invalid input
        cin.clear();
        cin.ignore();
    }
}

Note that cin stops at the first occurrence of a whitespace character.

To get a full line, we can use getline(cin, s)

This reads into s from the first non whitespace character until \n.

You can use combinations of cin and getline.

I/O Manipulators

To format output we can use I/O manipulators

cout << hex <-- changes the internal state of cout to print integers in hexadecimal

cout << dec <-- changes back to decimals

noskipws -- doesn't skip on whitespace

boolalpha -- prints true/false instead of 1/0

Look up <iomanip> for others.

Other Sources of Data

The stream abstraction can be applied to other sources of data.

e.g.) files

#include <fstream>

ifstream - read from a file

ofstream - write to a file

#include <iostream>
#include <fstream>

int main() {
    std::ifstream file("suite.txt");
    std::string s;
    while(file >> s) {
        std::cout << s << std::endl;
    }
}

The file is closed when the stack frame of main is popped from memory.

The default destructor of file is called.