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Nested Loops in C with Examples

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  • Difficulty Level : Medium
  • Last Updated : 25 Oct, 2022
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A nested loop means a loop statement inside another loop statement. That is why nested loops are also called “loop inside loops“. We can define any number of loops inside another loop.

1. Nested for Loop

Nested for loop refers to any type of loop that is defined inside a ‘for’ loop. Below is the equivalent flow diagram for nested ‘for’ loops:

Nested for loop in C

Nested for loop in C

Syntax:

for ( initialization; condition; increment ) {

   for ( initialization; condition; increment ) {
      
      // statement of inside loop
   }

   // statement of outer loop
}

Example: Below program uses a nested for loop to print a 3D matrix of 2x3x2.

C




// C program to print elements of Three-Dimensional Array
// with the help of nested for loop
#include <stdio.h>
  
int main()
{
    // initializing the 3-D array
    int arr[2][3][2]
        = { { { 0, 6 }, { 1, 7 }, { 2, 8 } },
            { { 3, 9 }, { 4, 10 }, { 5, 11 } } };
  
    // Printing values of 3-D array
    for (int i = 0; i < 2; ++i) {
        for (int j = 0; j < 3; ++j) {
            for (int k = 0; k < 2; ++k) {
                printf("Element at arr[%i][%i][%i] = %d\n",
                       i, j, k, arr[i][j][k]);
            }
        }
    }
    return 0;
}

Output

Element at arr[0][0][0] = 0
Element at arr[0][0][1] = 6
Element at arr[0][1][0] = 1
Element at arr[0][1][1] = 7
Element at arr[0][2][0] = 2
Element at arr[0][2][1] = 8
Element at arr[1][0][0] = 3
Element at arr[1][0][1] = 9
Element at arr[1][1][0] = 4
Element at arr[1][1][1] = 10
Element at arr[1][2][0] = 5
Element at arr[1][2][1] = 11

2. Nested while Loop

A nested while loop refers to any type of loop that is defined inside a ‘while’ loop. Below is the equivalent flow diagram for nested ‘while’ loops:

Nested while loop in C

Nested while loop in C

Syntax:

while(condition) {

   while(condition) {
      
      // statement of inside loop
   }

   // statement of outer loop
}

Example: Print Pattern using nested while loops

C




// C program to print pattern using nested while loops
#include <stdio.h>
  
int main()
{
    int end = 5;
  
    printf("Pattern Printing using Nested While loop");
  
    int i = 1;
  
    while (i <= end) {
        printf("\n");
        int j = 1;
        while (j <= i) {
            printf("%d ", j);
            j = j + 1;
        }
        i = i + 1;
    }
    return 0;
}

Output

Pattern Printing using Nested While loop
1 
1 2 
1 2 3 
1 2 3 4 
1 2 3 4 5 

3. Nested do-while Loop

A nested do-while loop refers to any type of loop that is defined inside a do-while loop. Below is the equivalent flow diagram for nested ‘do-while’ loops:

nested do while loop in C

do while loop in C

Syntax:

do{

   do{
      
      // statement of inside loop
   }while(condition);

   // statement of outer loop
}while(condition);

Note: There is no rule that a loop must be nested inside its own type. In fact, there can be any type of loop nested inside any type and to any level.

Syntax:

do{

   while(condition) {
      
      for ( initialization; condition; increment ) {
      
         // statement of inside for loop
      }

      // statement of inside while loop
   }

   // statement of outer do-while loop
}while(condition);

Example: Below program uses a nested for loop to print all prime factors of a number. 

C




// C Program to print all prime factors
// of a number using nested loop
  
#include <math.h>
#include <stdio.h>
  
// A function to print all prime factors of a given number n
void primeFactors(int n)
{
    // Print the number of 2s that divide n
    while (n % 2 == 0) {
        printf("%d ", 2);
        n = n / 2;
    }
  
    // n must be odd at this point. So we can skip
    // one element (Note i = i +2)
    for (int i = 3; i <= sqrt(n); i = i + 2) {
        // While i divides n, print i and divide n
        while (n % i == 0) {
            printf("%d ", i);
            n = n / i;
        }
    }
  
    // This condition is to handle the case when n
    // is a prime number greater than 2
    if (n > 2)
        printf("%d ", n);
}
  
/* Driver program to test above function */
int main()
{
    int n = 315;
    primeFactors(n);
    return 0;
}

Output:

3 3 5 7

Break Inside Nested Loops

Whenever we use a break statement inside the nested loops it breaks the innermost loop and program control is inside the outer loop. 

Example: 

C




// C program to show working of break statement
// inside nested for loops
#include <stdio.h>
  
int main()
{
  
    int i = 0;
    for (int i = 0; i < 5; i++) {
        for (int j = 0; j < 3; j++) {
            // This inner loop will break when i==3
            if (i == 3) {
                break;
            }
            printf("* ");
        }
        printf("\n");
    }
    return 0;
}

Output

* * * 
* * * 
* * * 

* * * 

In the above program, the first loop will iterate from 0 to 5 but here if i will be equal to 3 it will break and will not print the * as shown in the output. 

Continue Inside Nested loops

Whenever we use a continue statement inside the nested loops it skips the iteration of the innermost loop only. The outer loop remains unaffected.

Example:

C




// C program to show working of continue statement
// inside nested for loops
#include <stdio.h>
  
int main()
{
  
    int i = 0;
    for (int i = 0; i < 5; i++) {
        for (int j = 0; j < 3; j++) {
            // This inner loop will skip when j==2
            if (j==2) {
                continue;
            }
            printf("%d ",j);
        }
        printf("\n");
    }
    return 0;
}

In the above program, the inner loop will be skipped when j will be equal to 2. The outer loop will remain unaffected.


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