for, while, and do-while

for: counted repetition

for (int i = 0; i < 5; i++) {
    printf("%d ", i);        // 0 1 2 3 4
}

init; condition; step — run init once, test before every pass, step after. Loop variable visible only inside the loop (declare it there).

while: condition first

int n = 40;
while (n > 0) {
    n = n / 2;
}

Use when the count is unknown in advance. The condition is tested BEFORE the first pass — a false condition skips the body entirely.

do-while: body first

int choice;
do {
    choice = read_menu();     // runs at least once
} while (!valid(choice));

Tested AFTER each pass. Rare but right for "ask, then check" flows.

Termination is your job

Every loop needs something inside the body to move the condition toward false. while (1) with no exit is an infinite loop — the sandbox will kill it at the timeout and the challenge fails.