while Loop in C

A while loop repeats a block of statements while its condition remains true.

while (condition)
{
    // statements
}

The condition is checked before every iteration.

while (condition)
{
    statement;
}

The condition must be written inside parentheses. The statements to be repeated are written inside the braces.

int i = 1;

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

Output:

1
2
3
4
5

The variable i is increased after every iteration.

Unlike the initialization section of a for loop, a while loop normally requires the loop variable to be initialized before the loop.

int i = 1;

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

Here, i starts with the value 1.

The condition controls whether the loop continues.

int i = 1;

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

The loop continues while i <= 5 is true. When i becomes 6, the condition becomes false and the loop stops.

The loop variable should normally be changed inside the loop so that the condition can eventually become false.

int i = 1;

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

The i++ statement increases the value of i by one.

The execution takes place in this order:

1. Initialize the loop variable.
2. Check the condition.
3. If the condition is true, execute the loop body.
4. Update the loop variable.
5. Check the condition again.
6. Repeat until the condition becomes false.

int i = 1;

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

This program prints numbers from 1 to 10.

int i = 10;

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

Output:

10 9 8 7 6 5 4 3 2 1

The decrement operator -- decreases the value by one.

int i = 2;

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

Output:

2 4 6 8 10

int i = 1;

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

Output:

1 3 5 7 9

int i = 1;
int sum = 0;

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

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

Output:

Sum = 15

int number = 5;
int i = 1;

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

This program prints the multiplication table of 5.

#include <stdio.h>

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

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

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

    return 0;
}

The value entered by the user determines how many numbers are printed.

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

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

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

Output:

Factorial = 120

int i = 0;

while (i <= 20)
{
    printf("%d ", i);
    i += 5;
}

Output:

0 5 10 15 20

The loop variable increases by 5 after each iteration.

int i = 1;

while (i <= 10)
{
    if (i % 2 == 0)
    {
        printf("%d ", i);
    }

    i++;
}

The if statement allows the program to print only even numbers.

int i = 1;

while (i <= 3)
{
    printf("Number = %d\n", i);
    printf("Hello\n");

    i++;
}

Both statements execute during every iteration of the loop.

char choice = 'y';

while (choice == 'y')
{
    printf("You selected yes.\n");

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

A while loop can also be controlled by a character condition.

int number = 1;

while (number != 0)
{
    printf("Enter a number (0 to stop): ");
    scanf("%d", &number);
}

The loop continues until the user enters 0.

int count = 5;

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

printf("Start!");

The loop counts downward and stops when count becomes 0.

int i = 1;

while (i <= 10 && i > 0)
{
    printf("%d ", i);
    i++;
}

Logical operators such as && and || can be used inside loop conditions.

If the condition is false before the first check, the loop body is skipped.

int i = 10;

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

Nothing is printed because 10 < 5 is false.

If the loop variable is not changed, the condition may remain true forever.

int i = 1;

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

Here, i always remains 1, so the condition remains true.

Correct:

int i = 1;

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

The update should move the loop variable toward making the condition false.

int i = 1;

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

Here, i moves away from the stopping condition i > 5, so the loop does not terminate normally.

Use comparison operators such as == when you want to compare values.

int i = 1;

while (i == 1)
{
    printf("Hello\n");
    i++;
}

Here, == compares the values. The assignment operator = has a different purpose and should not be confused with comparison.

#include <stdio.h>

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

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

    while (i <= n)
    {
        sum += i;
        i++;
    }

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

    return 0;
}

This program calculates the sum of all integers from 1 to the number entered by the user.

#include <stdio.h>

int main()
{
    int number;
    int i = 1;

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

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

    return 0;
}

#include <stdio.h>

int main()
{
    int number;
    int factorial = 1;
    int i = 1;

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

    if (number < 0)
    {
        printf("Factorial is not defined for negative integers.");
    }
    else
    {
        while (i <= number)
        {
            factorial *= i;
            i++;
        }

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

    return 0;
}

For larger values, an int can overflow, so appropriate data types and input limits should be considered.

Practice the following programs:

1. Print numbers from 1 to 100.
2. Print numbers from 100 to 1.
3. Print even numbers from 1 to 100.
4. Print odd numbers from 1 to 100.
5. Calculate the sum from 1 to N.
6. Calculate the factorial of a number.
7. Print a multiplication table.
8. Print multiples of 5 up to 100.
9. Read numbers until the user enters 0.
10. Create a countdown program using a while loop.

Practice changing the condition and update statement to understand how the while loop works.