While

So far you’ve seen how a program can execute actions in sequence and how it can choose between different paths.

Now another fundamental problem appears: repeating actions.

That’s where iteration statements come in. The first one you’ll study is while.

In this lesson you will learn:

• what iteration means
• how while works
• how to think about a loop with initial state, condition, and update
• what an infinite loop is
• how to use break and continue inside a loop

What is an iteration?

An iteration is each repetition of a set of instructions.

If a loop prints five numbers, then there were five iterations.

What does while do?

The while statement repeats a block of code as long as a condition is true.

Syntax

while (condition) {
    instructions;
}

Correct mental idea for reading a while

A while loop has three fundamental elements:

1. initial state
2. continuation condition
3. state update

If you forget one of these three, the loop is usually poorly thought out.

Basic example

#include <stdio.h>

int main() {
    int i = 1;

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

    return 0;
}

Step-by-step reading

• initial state: i = 1
• condition: i <= 5
• update: i = i + 1

As long as the condition is true, the block keeps executing.

What happens in each iteration?

Iteration 1

• i is 1
• 1 is printed
• i becomes 2

Iteration 2

• i is 2
• 2 is printed
• i becomes 3

And so on until i no longer satisfies the condition.

Infinite loop

An infinite loop occurs when the condition never becomes false.

Incorrect example

while (i <= 5) {
    printf("%d\n", i);
}

If i doesn’t change, the loop can repeat forever.

That’s why you always have to think:

what action within the loop brings the program closer to the end of the loop?

Using break in a while

break allows exiting the loop immediately.

while (i <= 10) {
    if (i == 5) {
        break;
    }
    printf("%d\n", i);
    i = i + 1;
}

When i is 5, the loop cuts off.

Using continue in a while

continue makes the loop jump directly to the next iteration.

while (i <= 5) {
    i = i + 1;

    if (i == 3) {
        continue;
    }

    printf("%d\n", i);
}

You don’t need to master it in detail yet, but understanding its general idea: it alters the normal flow of the loop.

When is while worth using?

while is great when the repetition depends on a condition and you don’t want to think about an exact number of repetitions first.

Summary

• while repeats a block as long as a condition is true
• to design a while well, you have to think about initial state, condition, and update
• if the condition never becomes false, an infinite loop appears
• break cuts off the loop
• continue jumps to the next iteration

Final idea

while teaches you one of the deepest ideas in programming: repeating isn’t copying code many times, but controlling how a state evolves while a condition continues to be met.