Loops in C: while, for, and do-while Statements

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Loops let a program repeat a block of code without writing it out multiple times. C provides three looping constructs — while, for, and do-while — each suited to slightly different situations. This is Post 1 of the Unit II series, following Unit I’s coverage of decision making.

The while Loop

Checks its condition before each iteration — if the condition is false from the start, the body never runs even once.

while (condition) {
    // body
}

#include <stdio.h>

int main() {
    int i = 1;
    while (i <= 5) {
        printf("%d ", i);
        i++;
    }
    printf("\n");
    return 0;
}

Sample Output

1 2 3 4 5
The infinite loop trap: If you forget i++ (or otherwise never change the condition’s variables), the loop condition stays true forever and the program hangs. Every while loop needs a clear path toward making its condition eventually false.

The for Loop

Bundles initialization, condition, and update into one line — ideal when you know in advance how many times you want to repeat something.

for (initialization; condition; update) {
    // body
}

#include <stdio.h>

int main() {
    for (int i = 1; i <= 5; i++) {
        printf("%d ", i);
    }
    printf("\n");
    return 0;
}

Sample Output

1 2 3 4 5
What actually happens, in order: 1) initialization runs ONCE at the very start. 2) condition is checked — if false, the loop ends immediately. 3) if true, the body runs. 4) update runs. 5) back to step 2. This is exactly equivalent to a while loop with the initialization before it and the update as the last line inside the body — for is just a more compact way to write that same pattern.

The do-while Loop

Checks its condition after the body runs — guaranteeing the body executes at least once, even if the condition is false from the start.

do {
    // body
} while (condition);

#include <stdio.h>

int main() {
    int choice;

    do {
        printf("\nMenu: 1) Add  2) Subtract  3) Exit\n");
        printf("Enter choice: ");
        scanf("%d", &choice);

        if (choice == 1) printf("Result: addition selected\n");
        else if (choice == 2) printf("Result: subtraction selected\n");
        else if (choice != 3) printf("Invalid choice, try again.\n");

    } while (choice != 3);

    printf("Goodbye!\n");
    return 0;
}
Why menus commonly use do-while: A menu needs to be shown at least once regardless of any condition — you can’t check “should I show the menu” before the user has even seen it to make a choice. This “run at least once” guarantee is exactly what do-while provides and while/for don’t.

Choosing Between Them

Loop Condition checked Best for
while Before each iteration Unknown iteration count, condition-driven (e.g. read until EOF)
for Before each iteration Known iteration count (e.g. loop exactly n times)
do-while After each iteration Body must run at least once (menus, input validation)

Nested Loops: Printing a Pattern

#include <stdio.h>

int main() {
    int rows = 5;
    for (int i = 1; i <= rows; i++) {
        for (int j = 1; j <= i; j++) {
            printf("* ");
        }
        printf("\n");
    }
    return 0;
}

Sample Output

*
* *
* * *
* * * *
* * * * *

The outer loop controls the row number; the inner loop, which restarts fresh on every outer iteration, prints exactly as many stars as the current row number.

Summary

  • while: checks first, may run zero times.
  • for: compact form for known-count loops, checks first.
  • do-while: checks last, always runs at least once.
  • Nested loops let an inner loop repeat completely for every single iteration of an outer loop — the basis of most pattern-printing and matrix-processing programs.

Further Reading

Next in this series: break, continue, and goto statements.

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