for / while / do-while

// Repeats a specified number of times using a counter.
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;

#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;
}

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. Pay particular attention to array indices — 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.