for / while / do-while

for (initialization; condition; update) {
    process;
}

// Repeats while the condition is true (condition is evaluated first).
while (condition) {
    process;
}

// Executes the body first, then evaluates the condition (runs at least once).
do {
    process;
} while (condition);

// Exits the loop immediately.
break;

// Skips the rest of the current iteration and moves to the next.
continue;

sample_for_while.c

#include <stdio.h>

int main(void) {
    // Use a for loop to print 1 through 5.
    for (int i = 1; i <= 5; i++) {
        printf("%d ", i); // Outputs "1 2 3 4 5 ".
    }
    printf("\n");

    // Use nested for loops to print part of a multiplication table.
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            printf("%d*%d=%d ", i, j, i * j);
        }
        printf("\n");
    }

    // Use a while loop to repeat while the condition holds.
    int n = 1;
    while (n <= 100) {
        n *= 2;
    }
    printf("First power of 2 greater than 100: %d\n", n); // Outputs "128".

    // Use do while to execute at least once.
    int count = 0;
    do {
        count++;
        printf("count = %d\n", count);
    } while (count < 3); // Prints 3 times.

    // Use break to exit the loop once the target element is found.
    int nums[] = {3, 7, 2, 9, 4};
    for (int i = 0; i < 5; i++) {
        if (nums[i] == 9) {
            printf("9 is at index %d.\n", i); // Outputs "9 is at index 3.".
            break;
        }
    }

    // Use continue to print only even numbers.
    for (int i = 1; i <= 10; i++) {
        if (i % 2 != 0) continue; // Skip odd numbers.
        printf("%d ", i); // Outputs "2 4 6 8 10 ".
    }
    printf("\n");

    return 0;
}

Run the following command:

gcc for_while.c -o for_while
./for_while
1 2 3 4 5
1*1=1 1*2=2 1*3=3
2*1=2 2*2=4 2*3=6
3*1=3 3*2=6 3*3=9
First power of 2 greater than 100: 128
count = 1
count = 2
count = 3
9 is at index 3.
2 4 6 8 10

Common Mistake: Off-by-One Error

When the loop termination condition does not match the number of array elements, out-of-bounds access occurs.

#include <stdio.h>

int main(void) {
    int arr[5] = {10, 20, 30, 40, 50};
    /* NG: i <= 5 goes out of bounds (arr[5] does not exist) */
    for (int i = 0; i <= 5; i++) {
        printf("%d\n", arr[i]);
    }
    return 0;
}

arr[5] is out of bounds, producing an indeterminate value or a crash.

loop_oob_ok.c

#include <stdio.h>

int main(void) {
    int arr[5] = {10, 20, 30, 40, 50};
    int n = sizeof(arr) / sizeof(arr[0]);
    /* OK: use < n */
    for (int i = 0; i < n; i++) {
        printf("%d\n", arr[i]);
    }
    return 0;
}

Run the following command:

gcc loop_oob_ok.c -o loop_oob_ok
./loop_oob_ok
10
20
30
40
50

Since C99, you can declare the loop counter directly in the initialization part of a for statement (e.g., for (int i = 0; ...)). In this case, the variable's scope is limited to the loop body.

An incorrect loop termination condition can cause an infinite loop. Accessing beyond the array size results in undefined behavior. Use sizeof to check array sizes.

while (1) and for (;;) are idiomatic ways to write an intentional infinite loop, commonly used in event loops or server loops in combination with break. For combining loops with conditionals, see if / else.