Skip to content
Laviyx Learn

Try

C course menu

Loops in C: for, while and do-while Explained

An overview of loops in C: the three loop types, how each one works, when to use for, while or do-while, loop counters and accumulators, infinite loops, and nested loops, with examples.

Beginner7 min readUpdated
On this page

Loops repeat a block of code. C has three loop statements, and every repetition you will ever need can be written with them.

The three loops#

C
#include <stdio.h>

int main(void) {
    /* for: counting a known number of times */
    for (int i = 1; i <= 3; i++) {
        printf("for %d\n", i);
    }

    /* while: repeat while a condition is true */
    int n = 1;
    while (n <= 3) {
        printf("while %d\n", n);
        n++;
    }

    /* do-while: runs at least once, checks afterwards */
    int k = 1;
    do {
        printf("do-while %d\n", k);
        k++;
    } while (k <= 3);
    return 0;
}
Output
for 1
for 2
for 3
while 1
while 2
while 3
do-while 1
do-while 2
do-while 3
LoopChecks the conditionUse when
forBefore each repetitionYou know how many times, or you count through an array
whileBefore each repetitionYou repeat until something changes (input, a calculation)
do-whileAfter each repetitionThe body must run at least once (menus, input validation)

Accumulating a result#

Loops often build a result step by step:

C
int sum = 0;
for (int i = 1; i <= 100; i++) {
    sum += i;
}
printf("%d\n", sum);

long long factorial = 1;
for (int i = 2; i <= 15; i++) {
    factorial *= i;
}
printf("%lld\n", factorial);
Output
5050
1307674368000

long long is needed for the factorial: 15! does not fit in a 32-bit int.

Infinite loops#

A loop whose condition never becomes false runs forever. Sometimes that is intended, such as a server or an embedded device's main loop:

C
while (1) {
    read_sensor();
    update_display();
}

When it is not intended, it is a bug: check that something in the loop moves it towards its end. Press Ctrl+C in the terminal to stop a runaway program.

Nested loops#

A loop inside a loop runs completely for each repetition of the outer loop:

C
for (int row = 1; row <= 4; row++) {
    for (int col = 1; col <= row; col++) {
        printf("*");
    }
    printf("\n");
}
Output
*
**
***
****

The following lessons cover each loop in depth: for Loops, while and do-while and break, continue and goto.

Practice#

Practice exercise

Print the multiplication table of 9 from 9 x 1 = 9 to 9 x 10 = 90, then print the sum of all the results.

Show solution
C
#include <stdio.h>

int main(void) {
    int total = 0;
    for (int i = 1; i <= 10; i++) {
        printf("9 x %d = %d\n", i, 9 * i);
        total += 9 * i;
    }
    printf("Sum: %d\n", total);
    return 0;
}
Output
9 x 1 = 9
9 x 2 = 18
9 x 3 = 27
9 x 4 = 36
9 x 5 = 45
9 x 6 = 54
9 x 7 = 63
9 x 8 = 72
9 x 9 = 81
9 x 10 = 90
Sum: 495

Key takeaway#

Finished this lesson?

Related lessons

Cfor Loop in C: Syntax, Examples, Counting Down and Looping Over ArraysMaster the C for loop: initialisation, condition and update, counting up, down and in steps, looping over arrays with sizeof, several loop variables, off-by-one errors, and common for loop patterns such as sums, maximums and searches.Beginner5 min readCwhile and do-while Loops in C with ExamplesLearn the C while loop and do-while loop: syntax, loops that repeat until a condition changes, digit processing, input validation with do-while, reading until end of input with getchar and EOF, and the difference between while and do-while.Beginner6 min readCbreak, continue and goto in CLearn how to control loops in C with break and continue, break inside switch, leaving nested loops with a flag or a function, and the limited, legitimate use of goto for cleanup in error handling.Beginner7 min read