15. C – Nested Loops

C supports nesting of loops in C. Nesting of loops is the feature in C that allows the looping of statements inside another loop.

Syntax of Nested loop

1. Outer_loop  
2. {  
3.     Inner_loop  
4.    {  
5.          // inner loop statements.  
6.    }  
7.        // outer loop statements.  
8. }  

Outer_loop and Inner_loop are the valid loops that can be a ‘for’ loop, ‘while’ loop or ‘do-while’ loop.

Nested for loop

The nested for loop means any type of loop which is defined inside the ‘for’ loop.

1. for (initialization; condition; update)   
2. {  
3.     for(initialization; condition; update)  
4.     {  
5.            // inner loop statements.  
6.     }  
7.     // outer loop statements.  
8. }  

Example of nested for loop

1. #include <stdio.h>  
2. int main()  
3. {  
4.    int n;// variable declaration  
5.    printf(“Enter the value of n :”);  
6.    // Displaying the n tables.  
7.    for(int i=1;i<=n;i++)  // outer loop  
8.    {  
9.        for(int j=1;j<=10;j++)  // inner loop  
10.        {  
11.            printf(“%d\t”,(i*j)); // printing the value.  
12.        }  
13.        printf(“\n”);  
14.    }  

Explanation of the above code

• First, the ‘i’ variable is initialized to 1 and then program control passes to the i<=n.
• The program control checks whether the condition ‘i<=n’ is true or not.
• If the condition is true, then the program control passes to the inner loop.
• The inner loop will get executed until the condition is true.
• After the execution of the inner loop, the control moves back to the update of the outer loop, i.e., i++.
• After incrementing the value of the loop counter, the condition is checked again, i.e., i<=n.
• If the condition is true, then the inner loop will be executed again.
• This process will continue until the condition of the outer loop is true.

Output:

Nested while loop

The nested while loop means any type of loop which is defined inside the ‘while’ loop.

1. while(condition)  
2. {  
3.     while(condition)  
4.     {  
5.          // inner loop statements.  
6.     }  
7. // outer loop statements.  
8. }  

Example of nested while loop

1. #include <stdio.h>  
2. int main()  
3. {  
4.    int rows;  // variable declaration  
5.    int columns; // variable declaration  
6.    int k=1; // variable initialization  
7.    printf(“Enter the number of rows :”);  // input the number of rows.  
8.    scanf(“%d”,&rows);  
9.    printf(“\nEnter the number of columns :”); // input the number of columns.  
10.    scanf(“%d”,&columns);  
11.       int i=1;  
12.    while(i<=rows) // outer loop  
13.    {  
14.        int j=1;  
15.       while(j<=columns)  // inner loop  
16.        {  
17.            printf(“%d\t”,k);  // printing the value of k.  
18.            k++;   // increment counter  
19.            j++;  
20.        }  
21.        i++;  
22.        printf(“\n”);  
23.    }  
24. }  

Explanation of the above code.

• We have created the 2d array, i.e., int a[rows][columns].
• The program initializes the ‘i’ variable by 1.
• Now, control moves to the while loop, and this loop checks whether the condition is true, then the program control moves to the inner loop.
• After the execution of the inner loop, the control moves to the update of the outer loop, i.e., i++.
• After incrementing the value of ‘i’, the condition (i<=rows) is checked.
• If the condition is true, the control then again moves to the inner loop.
• This process continues until the condition of the outer loop is true.

Output:

Nested do..while loop

The nested do..while loop means any type of loop which is defined inside the ‘do..while’ loop.

1. do  
2. {  
3.    do  
4.   {   
5.       // inner loop statements.  
6.    }while(condition);  
7. // outer loop statements.  
8. }while(condition);  

Example of nested do..while loop.

1. #include <stdio.h>  
2. int main()  
3. {  
4.   /*printing the pattern 
5.      ******** 
6.      ******** 
7.      ******** 
8.      ******** */  
9. int i=1;  
10. do           // outer loop  
11. {  
12.     int j=1;  
13.     do       // inner loop  
14.    {  
15.       printf(“*”);  
16.       j++;  
17.    }while(j<=8);  
18.     printf(“\n”);  
19.     i++;  
20.      }while(i<=4);  
21. }  

Output:

Explanation of the above code.

• First, we initialize the outer loop counter variable, i.e., ‘i’ by 1.
• As we know that the do..while loop executes once without checking the condition, so the inner loop is executed without checking the condition in the outer loop.
• After the execution of the inner loop, the control moves to the update of the i++.
• When the loop counter value is incremented, the condition is checked. If the condition in the outer loop is true, then the inner loop is executed.
• This process will continue until the condition in the outer loop is true.