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Loops in C

Repeat work with for, while and do-while loops.

C has three loops. They can all do the same job, but each one reads best in a different situation.

The for loop

A for loop packs three parts into its header: a starting step, a condition, and an update that runs after every round.

C
Open in playground
#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 5; i++) {
        printf("%d squared is %d\n", i, i * i);
    }
    return 0;
}
Output
1 squared is 1
2 squared is 4
3 squared is 9
4 squared is 16
5 squared is 25
Startint i = 1 (runs once)i <= 5?Yesprintf the squarei++ (after each round)NoEnd
The three parts of a for loop

i++ is shorthand for i = i + 1.

The while loop

A while loop is handy when you do not know in advance how many rounds you need.

C
Open in playground
#include <stdio.h>

int main(void) {
    int n = 1234;
    int digits = 0;
    while (n > 0) {
        n = n / 10;
        digits++;
    }
    printf("1234 has %d digits\n", digits);
    return 0;
}
Output
1234 has 4 digits

do-while

A do-while loop checks its condition at the end, so the body always runs at least once.

C
Open in playground
#include <stdio.h>

int main(void) {
    int count = 0;
    do {
        printf("This runs once even though count is %d\n", count);
    } while (count > 0);
    return 0;
}
Output
This runs once even though count is 0

Loops and arrays

Loops are often used to visit every item in an array, which is a fixed-size row of values of the same type.

C
Open in playground
#include <stdio.h>

int main(void) {
    int marks[] = {78, 92, 65, 88};
    int total = 0;
    for (int i = 0; i < 4; i++) {
        total += marks[i];
    }
    printf("Average: %.2f\n", total / 4.0);
    return 0;
}
Output
Average: 80.75

GATE-style practice questions

GATE-styleNAT · numerical answer

What does the following program print?

#include <stdio.h>
int main(void) {
    int count = 0;
    for (int i = 1; i <= 20; i *= 3)
        count++;
    printf("%d\n", count);
    return 0;
}
Show answer and explanationAnswer: 3

i takes the values 1, 3, 9 and then 27, which is greater than 20, so the body runs 3 times.

GATE-styleNAT · numerical answer

What does the following program print?

#include <stdio.h>
int main(void) {
    int i = 0, sum = 0;
    while (i < 10) {
        i++;
        if (i % 3 == 0) continue;
        if (i == 8) break;
        sum += i;
    }
    printf("%d\n", sum);
    return 0;
}
Show answer and explanationAnswer: 19

i is incremented first in every round. i = 3 and i = 6 hit continue and are skipped. At i = 8 the loop breaks before adding. So the values added are 1, 2, 4, 5 and 7, and their sum is 19.

GATE-styleNAT · numerical answer

What does the following program print?

#include <stdio.h>
int main(void) {
    int calls = 0;
    for (int i = 0; i < 5; i++)
        for (int j = i; j < 5; j++)
            calls++;
    printf("%d\n", calls);
    return 0;
}
Show answer and explanationAnswer: 15

For i = 0 the inner loop runs 5 times, for i = 1 four times, and so on down to 1 time for i = 4: 5 + 4 + 3 + 2 + 1 = 15.