do-while Loop in C

A do-while loop executes a block of statements and then checks whether the condition is true.

do
{
    // statements
}
while (condition);

If the condition is true, the loop repeats.

do
{
    statement;
}
while (condition);

Notice the semicolon ; after the while condition. It is required in a do-while loop.

int i = 1;

do
{
    printf("%d\n", i);
    i++;
}
while (i <= 5);

Output:

1
2
3
4
5

The execution order of a do-while loop is:

1. Initialize required variables.
2. Execute the loop body.
3. Update values if required.
4. Check the condition.
5. If true, execute the body again.
6. If false, exit the loop.

The main difference between while and do-while is that the do-while loop executes its body before checking the condition.

int i = 10;

do
{
    printf("Hello");
}
while (i < 5);

Even though i < 5 is false, Hello is printed once.

int i = 1;

do
{
    printf("%d ", i);
    i++;
}
while (i <= 5);

Output:

1 2 3 4 5

int i = 5;

do
{
    printf("%d ", i);
    i--;
}
while (i >= 1);

Output:

5 4 3 2 1

int i = 2;

do
{
    printf("%d ", i);
    i += 2;
}
while (i <= 10);

Output:

2 4 6 8 10

int i = 1;

do
{
    printf("%d ", i);
    i += 2;
}
while (i <= 10);

Output:

1 3 5 7 9

int i = 1;
int sum = 0;

do
{
    sum = sum + i;
    i++;
}
while (i <= 5);

printf("Sum = %d", sum);

Output:

Sum = 15

int number = 5;
int i = 1;

do
{
    printf("%d x %d = %d\n",
           number, i, number * i);
    i++;
}
while (i <= 10);

This program prints the multiplication table of 5.

int number = 5;
int factorial = 1;
int i = 1;

do
{
    factorial = factorial * i;
    i++;
}
while (i <= number);

printf("Factorial = %d", factorial);

Output:

Factorial = 120

#include <stdio.h>

int main()
{
    int number;

    do
    {
        printf("Enter a positive number: ");
        scanf("%d", &number);
    }
    while (number <= 0);

    printf("You entered: %d", number);

    return 0;
}

The program keeps asking for input until the user enters a positive number.

int choice;

do
{
    printf("\n1. Add");
    printf("\n2. Subtract");
    printf("\n3. Exit");
    printf("\nEnter choice: ");
    scanf("%d", &choice);
}
while (choice != 3);

A do-while loop is useful for menus because the menu should normally be displayed at least once.

int i = 1;

do
{
    if (i % 2 == 0)
    {
        printf("%d ", i);
    }

    i++;
}
while (i <= 10);

The if statement checks whether each number is even.

char choice;

do
{
    printf("Hello\n");

    printf("Continue? (y/n): ");
    scanf(" %c", &choice);
}
while (choice == 'y');

The loop repeats when the user enters y.

int number;

do
{
    printf("Enter a number: ");
    scanf("%d", &number);

    printf("You entered %d\n", number);
}
while (number != 0);

The program continues until the user enters 0.

int i = 1;

do
{
    printf("Number = %d\n", i);
    printf("Welcome to C programming\n");

    i++;
}
while (i <= 3);

All statements inside the braces execute during every iteration.

int count = 5;

do
{
    printf("%d\n", count);
    count--;
}
while (count > 0);

printf("Start!");

The loop prints a countdown from 5 to 1.

int number;

do
{
    printf("Enter number between 1 and 10: ");
    scanf("%d", &number);
}
while (number < 1 || number > 10);

The loop continues while the entered number is outside the valid range.

The semicolon after the while condition is required.

do
{
    printf("Hello");
}
while (condition);

Without the semicolon, the program will produce a syntax error.

int i = 10;

do
{
    printf("%d", i);
    i++;
}
while (i < 5);

Output:

10

The condition is false initially, but the body still executes once.

The loop variable should normally change so that the condition can eventually become false.

int i = 1;

do
{
    printf("%d ", i);
}
while (i <= 5);

Here, i never changes, so the condition remains true and the loop continues indefinitely.

Correct:

int i = 1;

do
{
    printf("%d ", i);
    i++;
}
while (i <= 5);

The update should move the variable toward the condition becoming false.

int i = 1;

do
{
    printf("%d ", i);
    i--;
}
while (i <= 5);

Here, i becomes smaller while the condition i <= 5 remains true, so the loop does not terminate normally.

#include <stdio.h>

int main()
{
    int age;

    do
    {
        printf("Enter your age between 1 and 100: ");
        scanf("%d", &age);
    }
    while (age < 1 || age > 100);

    printf("Valid age: %d", age);

    return 0;
}

This program repeatedly asks for an age until the user enters a value between 1 and 100.

#include <stdio.h>

int main()
{
    int choice;

    do
    {
        printf("\n--- MENU ---\n");
        printf("1. Say Hello\n");
        printf("2. Show Message\n");
        printf("3. Exit\n");
        printf("Enter choice: ");
        scanf("%d", &choice);

        if (choice == 1)
        {
            printf("Hello!\n");
        }
        else if (choice == 2)
        {
            printf("Welcome to C programming!\n");
        }
        else if (choice != 3)
        {
            printf("Invalid choice.\n");
        }

    }
    while (choice != 3);

    printf("Program ended.");

    return 0;
}

#include <stdio.h>

int main()
{
    int n;
    int i = 1;
    int sum = 0;

    printf("Enter a positive number: ");
    scanf("%d", &n);

    if (n > 0)
    {
        do
        {
            sum += i;
            i++;
        }
        while (i <= n);

        printf("Sum = %d", sum);
    }
    else
    {
        printf("Please enter a positive number.");
    }

    return 0;
}

Practice the following programs:

1. Print numbers from 1 to 10.
2. Print numbers from 10 to 1.
3. Print even numbers from 1 to 20.
4. Print odd numbers from 1 to 20.
5. Calculate the sum from 1 to N.
6. Calculate the factorial of a number.
7. Print a multiplication table.
8. Ask for a positive number until the user enters one.
9. Create a menu-driven program.
10. Ask the user whether to continue using y/n.

Practice these programs to understand why a do-while loop is useful when the statements should execute at least once.