do-while-loops.md

Do while loops

Do while loops are used to guarantee to run the loop body at least once. With the while loop there is no guarantee the loop body will run.

Simple do while loop example

In this example i is assigned the number 25. The while loop will only execute, if i is less than 5. Using the do while loop, means that the value of i will be always be printed and the loop terminates because 25 is not less than 5.

#include <stdio.h>

int main(void)
{

    int i = 25;

    do 
    {
        printf("i: %d\n", i);
        i++;
    } while (i < 5); // ; after condition, for do while loops

    return 0;

}

Output:

i: 25

Do while loop used to validate user input

Do while loops are useful when needing to validate a user input. Only when the correct value is entered should the program proceed.

In this example the user is requested to enter a number greater than 0. If a negative number or zero is entered the user is prompted again. The program only exits the loop when a number greater than 0 is entered.

#include <stdio.h>

int main(void)
{

    int i = 0;

    do 
    {
        printf("Enter number (>0): ");
        scanf("%d" , &i);
    } while (i <= 0); // while i less than 0, continue loop

    return 0;

Output:

Enter number (>0): -12
Number must be >0!
Enter number (>0): 0
Number must be >0!
Enter number (>0): -12312
Number must be >0!
Enter number (>0): 2