C Loops

What You’ll Learn

Loops let you repeat code without copy-pasting it dozens of times. C gives you three tools: for for counted work, while for open-ended repetition, and do-while when the body must run at least once. This hub explains when to use each and links to full tutorials with five examples each.

Definition and Usage

A loop executes a block of code repeatedly until a condition becomes false or you exit with break. Without loops, programs could not walk arrays, print patterns, validate input, or run menus—every repeated task would need duplicate lines.

C standardizes three loop forms. They are interchangeable in theory (any loop can be rewritten as another), but each shape makes certain programs clearer. This page compares them; each tutorial below goes deep with syntax, examples, and FAQs.

📝 Syntax

All three C loop forms side by side:

/* for — init, test, update in header */
for (int i = 0; i < n; i++) {
    /* body */
}

/* while — test before body */
while (condition) {
    /* body */
}

/* do-while — body first, then test; note semicolon */
do {
    /* body */
} while (condition);

Loop control

• break; — leave the innermost loop immediately
• continue; — skip to the next iteration
• Nested loops — inner loop completes for each outer step

Compile

gcc main.c -std=c11 -o main

Examples Gallery

Five small programs showing each loop type in action. Compile with gcc file.c -std=c11 -o out.

#include <stdio.h>

int main(void) {
    int sum = 0;

    for (int i = 1; i <= 10; i++) {
        sum += i;
    }

    printf("Sum 1..10 = %d\n", sum);
    return 0;
}

Sum 1..10 = 55

#include <stdio.h>

int main(void) {
    int n = 5;

    while (n > 0) {
        printf("%d...\n", n);
        n--;
    }

    printf("Go!\n");
    return 0;
}

5...
4...
3...
2...
1...
Go!

#include <stdio.h>

int main(void) {
    int choice;

    do {
        printf("Pick 1, 2, or 3: ");
        if (scanf("%d", &choice) != 1) {
            return 1;
        }
    } while (choice < 1 || choice > 3);

    printf("You picked %d\n", choice);
    return 0;
}

Pick 1, 2, or 3: 9
Pick 1, 2, or 3: 2
You picked 2

#include <stdio.h>

int main(void) {
    int row, col;

    for (row = 1; row <= 3; row++) {
        for (col = 1; col <= 3; col++) {
            printf("%d ", row * col);
        }
        printf("\n");
    }

    return 0;
}

1 2 3
2 4 6
3 6 9

#include <stdio.h>

int main(void) {
    int i;

    printf("for:       ");
    for (i = 0; i < 3; i++) {
        printf("%d ", i);
    }
    printf("\n");

    printf("while:     ");
    i = 0;
    while (i < 3) {
        printf("%d ", i);
        i++;
    }
    printf("\n");

    printf("do-while:  ");
    i = 0;
    do {
        printf("%d ", i);
        i++;
    } while (i < 3);
    printf("\n");

    return 0;
}

for:       0 1 2
while:     0 1 2
do-while:  0 1 2

🚀 Common Use Cases

• Array processing — walk every element by index.
• Pattern printing — nested loops for triangles and grids.
• User menus — while or do-while until exit.
• Input validation — do-while retry until valid.
• File I/O — while until EOF.
• Search — loop with break when found.

📝 Notes

• Semicolon after do { } while (cond); is required.
• Avoid while (cond); with a stray semicolon after the header.
• Always ensure the condition can become false to avoid accidental infinite loops.
• for loop variables declared in the header are scoped to the loop (C99+).
• Nested loops increase total iterations multiplicatively.
• break only exits one level of nesting; use flags or goto for complex exits.

🚀 Usage Tips

• Learn for first — most array and counting code uses it.
• Read each tutorial — five examples and FAQs per loop type.
• Match loop to problem — do not force everything into for.
• Trace by hand — write i values on paper for the first few iterations.
• Follow sidebar order — for → while → do-while.

💡 Best Practices