Loops in C

In C programming, loops are used to execute a block of code repeatedly until a specified condition becomes false.

Without loops, we would have to write the same statement again and again.

For example, to print a message three times without a loop:

#include <stdio.h>

int main()
{
    printf("Hello GfG\n");
    printf("Hello GfG\n");
    printf("Hello GfG\n");

    return 0;
}

Output

Hello GfG
Hello GfG
Hello GfG

Writing the same statement multiple times is not practical. Loops solve this problem by allowing us to repeat code efficiently.

Why Do We Need Loops?

Loops help us:

Reduce code duplication.
Improve readability.
Save development time.
Execute a block of code multiple times.
Process collections of data efficiently.

Example of Loop

#include <stdio.h>

int main()
{
    for(int i = 0; i < 3; i++)
    {
        printf("Hello GfG\n");
    }

    return 0;
}

Output

Hello GfG
Hello GfG
Hello GfG

Using a loop, the same result is achieved with much less code.

Types of Loops in C

C provides three types of loops:

for Loop
while Loop
do-while Loop

for Loop

The for loop is an entry-controlled loop. This means the condition is checked before executing the loop body.

It is commonly used when the number of iterations is known in advance.

Syntax

for(initialization; condition; update)
{
    // loop body
}

Components of for Loop

Initialization

Initializes the loop variable.

int i = 0;

Condition

Checks whether the loop should continue.

i < 5

Update

Updates the loop variable after each iteration.

i++

Example of for Loop

#include <stdio.h>

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

    return 0;
}

Output

1 2 3 4 5

while Loop

The while loop is also an entry-controlled loop.

The condition is checked before executing the loop body.

It is generally used when the number of iterations is not known beforehand.

Syntax

while(condition)
{
    // loop body
}

Example of while Loop

#include <stdio.h>

int main()
{
    int i = 1;

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

    return 0;
}

Output

1 2 3 4 5

How while Loop Works

Initialize the loop variable.
Check the condition.
Execute the loop body if the condition is true.
Update the loop variable.
Repeat until the condition becomes false.

do-while Loop

The do-while loop is an exit-controlled loop.

Unlike for and while, the condition is checked after executing the loop body.

Therefore, the loop body executes at least one time, even if the condition is false.

Syntax

do
{
    // loop body
}
while(condition);

Example of do-while Loop

#include <stdio.h>

int main()
{
    int i = 1;

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

    return 0;
}

Output

1 2 3 4 5

Difference Between while and do-while

In a while loop, the condition is checked before execution.

In a do-while loop, the condition is checked after execution.

Because of this, a do-while loop always executes at least once.

Infinite Loop

An infinite loop is a loop that never terminates because its condition never becomes false.

Such loops continue running forever until the program is manually stopped.

Example Using for Loop

for(;;)
{
    printf("Infinite Loop");
}

Example Using while Loop

while(1)
{
    printf("Infinite Loop");
}

Example Using do-while Loop

do
{
    printf("Infinite Loop");
}
while(1);

Nested Loops

A nested loop is a loop inside another loop.

The inner loop executes completely for every iteration of the outer loop.

Nested loops are commonly used for:

Pattern printing
Matrix operations
Working with two-dimensional arrays

Example of Nested Loop

#include <stdio.h>

int main()
{
    for(int i = 0; i < 3; i++)
    {
        for(int j = 0; j < 2; j++)
        {
            printf("i = %d, j = %d\n", i, j);
        }
    }

    return 0;
}

Output

textCopy

i = 0, j = 0
i = 0, j = 1
i = 1, j = 0
i = 1, j = 1
i = 2, j = 0
i = 2, j = 1

Loop Control Statements

Loop control statements are used to alter the normal execution of loops.

C provides three loop control statements:

break
continue
goto

break Statement

The break statement immediately terminates the loop.

Example

#include <stdio.h>

int main()
{
    for(int i = 0; i < 5; i++)
    {
        if(i == 3)
        {
            break;
        }

        printf("%d ", i);
    }

    return 0;
}

Output

0 1 2

continue Statement

The continue statement skips the current iteration and moves to the next iteration of the loop.

Example

#include <stdio.h>

int main()
{
    for(int i = 0; i < 5; i++)
    {
        if(i == 3)
        {
            continue;
        }

        printf("%d ", i);
    }

    return 0;
}

Output

0 1 2 4

goto Statement

The goto statement transfers program control to a labeled statement.

Although supported in C, it should be used carefully because excessive use can make code difficult to understand.

Example

#include <stdio.h>

int main()
{
    int i;

    for(i = 0; i < 5; i++)
    {
        if(i == 3)
        {
            goto skip;
        }

        printf("%d ", i);
    }

skip:
    printf("\nJumped to label");

    return 0;
}

Output

0 1 2
Jumped to label

Choosing the Right Loop

Use a for loop when the number of iterations is known.

Use a while loop when the number of iterations is unknown and depends on a condition.

Use a do-while loop when the loop body must execute at least once.

Summary

Loops are used to execute a block of code repeatedly.
C provides three loops: for, while, and do-while.
for and while are entry-controlled loops.
do-while is an exit-controlled loop.
Infinite loops run forever because their condition never becomes false.
Nested loops are loops placed inside other loops.
Loop control statements such as break, continue, and goto modify the normal flow of loops.
Choosing the correct loop improves program readability and efficiency.