for Loop in C

A for loop repeatedly executes a block of statements while its condition remains true.

for (initialization; condition; update)
{
    // statements
}

The loop is commonly used for counting, processing arrays, generating sequences, and repeating tasks.

for (initialization; condition; update)
{
    statement;
}

The three expressions are separated by semicolons.

• Initialization: sets the starting value.
• Condition: determines whether the loop continues.
• Update: changes the loop variable.

for (int i = 1; i <= 5; i++)
{
    printf("%d\n", i);
}

Output:

1
2
3
4
5

The loop starts at 1 and continues until i becomes greater than 5.

Initialization usually sets the starting value of the loop variable.

for (int i = 1; i <= 5; i++)
{
    printf("%d ", i);
}

Here, int i = 1 initializes the loop variable to 1.

The condition determines whether the loop body should execute.

for (int i = 1; i <= 5; i++)
{
    printf("%d ", i);
}

The body executes while i <= 5 is true.

The update expression changes the loop variable after each iteration.

for (int i = 1; i <= 5; i++)
{
    printf("%d ", i);
}

The i++ expression increases i by one after each iteration.

The basic execution order is:

1. Initialization executes once.
2. The condition is checked.
3. If the condition is true, the loop body executes.
4. The update expression executes.
5. The condition is checked again.
6. The process repeats until the condition becomes false.

for (int i = 1; i <= 10; i++)
{
    printf("%d\n", i);
}

This loop prints the numbers from 1 through 10.

for (int i = 10; i >= 1; i--)
{
    printf("%d\n", i);
}

The decrement operator -- decreases the value by one after each iteration.

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

Output:

2 4 6 8 10

The update expression increases the value by 2 each time.

for (int i = 1; i <= 10; i += 2)
{
    printf("%d ", i);
}

Output:

1 3 5 7 9

int sum = 0;

for (int i = 1; i <= 5; i++)
{
    sum = sum + i;
}

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

Output:

Sum = 15

The variable sum stores the running total.

int number = 5;

for (int i = 1; i <= 10; i++)
{
    printf("%d x %d = %d\n", number, i, number * i);
}

This program generates the multiplication table of 5.

#include <stdio.h>

int main()
{
    int n;

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

    for (int i = 1; i <= n; i++)
    {
        printf("%d ", i);
    }

    return 0;
}

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

int number = 5;
int factorial = 1;

for (int i = 1; i <= number; i++)
{
    factorial = factorial * i;
}

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

Output:

Factorial = 120

for (int i = 0; i <= 20; i += 5)
{
    printf("%d ", i);
}

Output:

0 5 10 15 20

The update expression can increase the loop variable by any suitable amount.

for (int i = 10; i >= 0; i--)
{
    printf("%d ", i);
}

Output:

10 9 8 7 6 5 4 3 2 1 0

If a loop contains only one statement, braces may technically be omitted.

for (int i = 1; i <= 5; i++)
    printf("%d ", i);

However, braces are recommended when a loop contains multiple statements or when future modifications are likely.

for (int i = 1; i <= 3; i++)
{
    printf("Number = %d\n", i);
    printf("Hello\n");
}

Both statements execute during every iteration.

for (int i = 1; i <= 10; i++)
{
    if (i % 2 == 0)
    {
        printf("%d ", i);
    }
}

An if statement can be used inside a loop to process only selected values.

A for loop can be placed inside another for loop. This is called a nested loop.

for (int i = 1; i <= 3; i++)
{
    for (int j = 1; j <= 3; j++)
    {
        printf("%d %d\n", i, j);
    }
}

The inner loop completes its iterations for each iteration of the outer loop.

A for loop can be written without a condition:

for (;;)
{
    printf("Running...\n");
}

This creates an infinite loop because there is no condition that becomes false. Such a loop should normally contain a controlled way to terminate, such as a break statement.

The initialization, condition, and update expressions can be omitted, but the semicolons remain necessary.

int i = 1;

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

This works, although the normal three-part form is usually easier to read for simple counting loops.

A wrong condition can make a loop execute too many or too few times.

for (int i = 1; i < 5; i++)
{
    printf("%d ", i);
}

Output:

1 2 3 4

If you want to include 5, use i <= 5.

If a loop variable never changes and the condition remains true, the loop can become infinite.

int i = 1;

for (;;)
{
    printf("%d ", i);
}

Here, i never changes and there is no stopping condition.

A normal counting loop should update its control variable:

for (int i = 1; i <= 5; i++)
{
    printf("%d ", i);
}

An unnecessary semicolon after the for statement creates an empty loop body.

for (int i = 1; i <= 5; i++);

printf("Hello");

The loop above does not repeat the printf() statement. The semicolon is the loop body.

Correct:

for (int i = 1; i <= 5; i++)
{
    printf("Hello\n");
}

#include <stdio.h>

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

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

    for (int i = 1; i <= n; i++)
    {
        sum += 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;

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

    for (int i = 1; i <= 10; i++)
    {
        printf("%d x %d = %d\n",
               number, i, number * i);
    }

    return 0;
}

#include <stdio.h>

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

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

    if (number < 0)
    {
        printf("Factorial is not defined for negative integers.");
    }
    else
    {
        for (int i = 1; i <= number; i++)
        {
            factorial *= 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 all even numbers from 1 to 100.
4. Print all 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. Count the numbers between two limits.
9. Print multiples of 5.
10. Create a pattern using nested for loops.

Practice changing the initialization, condition, and update parts to understand how each part controls the loop.