while and do-while Loops in C with Examples

Use while when you do not know in advance how many repetitions you need: repeat while a condition holds. do-while is the same, except it always runs the body at least once.

while#

int savings = 1000, months = 0;
while (savings < 2000) {
    savings = savings * 110 / 100;   /* grows 10% a month, in whole rupees */
    months++;
}
printf("%d months, savings %d\n", months, savings);

8 months, savings 2142

The condition is checked before each repetition, so if it is false at the start, the body never runs.

Working with digits#

Taking digits off a number with % 10 and / 10 is a classic while loop:

int n = 12345, reversed = 0, digit_sum = 0;
while (n > 0) {
    int digit = n % 10;
    reversed = reversed * 10 + digit;
    digit_sum += digit;
    n /= 10;
}
printf("%d %d\n", reversed, digit_sum);

54321 15

do-while#

do-while checks the condition after the body, so the body runs at least once. It suits input that must be repeated until it is valid:

#include <stdio.h>

int main(void) {
    int age;
    do {
        printf("Enter your age (0-110): ");
        if (scanf("%d", &age) != 1) return 1;
    } while (age < 0 || age > 110);
    printf("Thank you, age %d recorded\n", age);
    return 0;
}

Enter your age (0-110): -5
Enter your age (0-110): 120
Enter your age (0-110): 42
Thank you, age 42 recorded

Note the semicolon after while (...) at the end of a do-while.

Reading until the end of input#

getchar() returns the next character, or the special value EOF when there is no more input. Its result must be stored in an int, not a char, so that EOF can be told apart from real characters:

#include <stdio.h>
#include <ctype.h>

int main(void) {
    int c, letters = 0, others = 0;
    while ((c = getchar()) != EOF) {
        if (isalpha(c)) letters++;
        else if (c != '\n') others++;
    }
    printf("letters: %d, other characters: %d\n", letters, others);
    return 0;
}

Hello C
letters: 6, other characters: 1

When typing at the keyboard, end the input with Ctrl+D (Linux and macOS) or Ctrl+Z then Enter (Windows).

while or for?#

These two loops are equivalent; choose the one that reads more naturally:

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

0 1 2 0 1 2 

Counting loops read better as for; "until something happens" loops read better as while.

Practice#

#include <stdio.h>

int main(void) {
    int n = 1000, steps = 0;
    while (n > 1) {
        n /= 2;
        steps++;
        printf("%d ", n);
    }
    printf("\n%d steps\n", steps);
    return 0;
}

500 250 125 62 31 15 7 3 1 
9 steps

Key takeaway#