The while Loop

Imagine you need to print the numbers from 1 to 100. You could write printf a hundred times, but it would take ages, and changing anything later would be even worse. Programs repeat work like this all the time: they go through every student in a class, check every product in a shop, or redraw a game screen 60 times a second. That is what loops are for. A loop is a piece of code that runs again and again.

The simplest loop is while. It repeats a block of code while its condition is true:

int number = 1;
while (number <= 5) {
    printf("%d\n", number);
    number++;
}

This prints:

1
2
3
4
5

A while loop looks like an if: the keyword, a condition in parentheses and a block of code in curly braces. The difference is that if runs its block at most once, but while keeps coming back to the condition. Step by step:

1. The program creates number with the value 1.
2. It checks the condition number <= 5. It is true, so the program runs the block: it prints 1, and number++ increases number to 2.
3. It goes back to the condition. 2 <= 5 is still true, so it prints 2 and increases number to 3.
4. And so on, until number is 6. Then the condition is false, the loop ends, and the program continues with the code after the loop's closing brace }.

Each run of the block is called an iteration. This loop has five iterations. To print the numbers up to 100, you would only change 5 to 100.

Indent the block of a loop by 4 spaces, just like the block of an if. Then you can see at a glance what repeats. The code after the loop runs only once, when the loop has finished:

int number = 1;
while (number <= 3) {
    printf("%d\n", number);
    number++;
}
printf("Done\n");

1
2
3
Done

Notice the line number++. Something in the block must change the variable in the condition, so that the condition becomes false at some point. Without it, the loop would never end. You'll see what happens then in the next exercise.