C for Loop

What You’ll Learn

The for loop is C’s go-to tool when you want to repeat code a known number of times—count from 0 to 9, walk every element of an array, or build a multiplication table with nested loops. This tutorial covers syntax, five worked examples, and pitfalls every beginner should avoid.

Definition and Usage

A for loop executes a block of code repeatedly while a condition is true. Unlike a while loop, which only has a condition, the for loop packs three steps into its header: set up a counter, test it, and update it after each pass.

Choose for when the number of iterations is clear before the loop runs—printing ten lines, summing an array, or drawing a pattern with row and column indices. The compact header keeps loop control visible in one place.

📝 Syntax

General form of the C for loop:

for (initialization; condition; update) {
    // statements to repeat
}

Components

• Initialization — runs once before the first test (e.g. int i = 0).
• Condition — checked before each iteration; if false, the loop ends (e.g. i < 10).
• Update — runs after each iteration (e.g. i++, i += 2).

Common patterns

• for (int i = 0; i < n; i++) — count from 0 up to n - 1.
• for (int i = n - 1; i >= 0; i--) — count down to 0.
• for (size_t i = 0; i < len; i++) — array index with size_t.

Compile

gcc program.c -std=c11 -o program

Examples Gallery

Compile with gcc example.c -std=c11 -o example. Each program demonstrates a core for loop pattern.

#include <stdio.h>

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

0
1
2
3
4

#include <stdio.h>

int main(void) {
    int n = 10;
    int sum = 0;

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

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

Sum 1..10 = 55

#include <stdio.h>

int main(void) {
    int arr[] = {10, 20, 30, 40, 50};
    size_t n = sizeof(arr) / sizeof(arr[0]);
    size_t i;

    for (i = 0; i < n; i++) {
        printf("arr[%zu] = %d\n", i, arr[i]);
    }

    return 0;
}

arr[0] = 10
arr[1] = 20
arr[2] = 30
arr[3] = 40
arr[4] = 50

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            printf("i = %d, j = %d\n", i, j);
        }
    }
    return 0;
}

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 2, j = 2
i = 2, j = 3
i = 3, j = 1
i = 3, j = 2
i = 3, j = 3

#include <stdio.h>

int main(void) {
    int nums[] = {4, 7, 2, 9, 5};
    int target = 9;
    size_t n = sizeof(nums) / sizeof(nums[0]);
    size_t i;
    int found_at = -1;

    for (i = 0; i < n; i++) {
        if (nums[i] == target) {
            found_at = (int)i;
            break;
        }
    }

    if (found_at >= 0) {
        printf("Found %d at index %d\n", target, found_at);
    } else {
        printf("Not found\n");
    }

    return 0;
}

Found 9 at index 3

🚀 Common Use Cases

• Counting — repeat an action N times.
• Array processing — sum, search, or transform elements by index.
• Pattern printing — nested loops for stars, numbers, and alphabet triangles.
• Tables and grids — rows and columns in 2D data.
• Buffered I/O — process fixed-size chunks of data.
• Retry limits — try an operation up to max attempts.

📝 Notes

• Semicolons separate the three header parts even when one is empty: for (;;).
• < vs <= changes whether you include the endpoint—watch off-by-one errors.
• Missing update (e.g. forgetting i++) causes an infinite loop.
• continue skips to the next iteration; break leaves the loop entirely.
• Declare loop variables with C99+ (-std=c99) for cleaner scope.
• Nested loops multiply work: 3 × 3 runs 9 inner iterations.

⚡ Optimization

Hoist invariant calculations out of the loop—compute sizeof(arr) / sizeof(arr[0]) once before the for, not on every iteration. Use size_t for indices when comparing with sizes. For very hot loops, avoid redundant function calls inside the body and prefer simple increment (i++) over heavier updates unless needed.

💡 Best Practices