Nested Loops in C++

February 3, 2022

Nested Loops

Here, in this page we will discuss about nested loops in C++.

C++ allows 256 levels of nesting
It is a procedure to construct a loop inside another loop body

Premise

C++ allows nesting of looping structures like – For, while, Do-while, for-each etc. To allow for complex code logic and control structure.

We will first look at an example syntax for nesting and then a few programs to understand how it can be done.

This is an Alert Even though nesting is allowed for all types of loops. For loop nesting is most common.

Both homogeneous and heterogeneous nesting is allowed. Example -

1. For loop within For loop / while loop within while loop
2. For loop within while loop / Do-while within for loop

Homogeneous nesting

For Loop

for (initialization; condition(s); increment) {
   // statement(s) of outer loop
    for (initialization; condition(s); increment) {
        // statement(s) of inner loop
   }
   // statement(s) of outer loop
}

While Loop

while(condition(s)) {
   // statement(s) of outer loop
   while(condition(s)) {
        // statement(s) of inner loop
   }
   // statement(s) of outer loop
}

Do-while Loop

do{
   // statement(s) of outer loop
   do{
        // statement(s) of inner loop
   }while(condition);
   // statement(s) of outer loop
}while(condition);

Heterogeneous nesting

While all kinds of combinations of nesting can be done, we have taken an example of for loop inside a while loop and that while loop inside do-while

Example

do{
    // statement(s) of do-while loop
    while(condition) {
        // statement(s) of while loop
        for ( initialization; condition; increment ) {
            // statement(s) of for loop
        }
        // statement(s) of while loop
    }
    // statement(s) of do-while loop
}while(condition);

Execution flow

When we are working with nested loops always execution starts from the outer loop test condition, if it is true control will pass to the outer loop body
Now the inner loop test condition is checked, if the inner loop test condition is true, control will pass within the inner loop until the inner loop test condition becomes false
In the inner loop if the condition becomes false control goes to the Outer loop
Until the outer loop test condition becomes false outer loop executes n number of times.

Let us look at a few programs to understand these nesting

Example 1

Print table of numbers from 1 to 5

#include<iostream>
using namespace std;

int main(){
    
    for(int i = 1; i <= 5; i++)
    {
        cout << "Table of " << i << " : ";
        
        for(int j = 1; j <= 10; j++){
            cout << i*j << " ";
        }
        cout << "\n";
    }
    
    return 0;
}

Output

Table of 1 : 1 2 3 4 5 6 7 8 9 10 
Table of 2 : 2 4 6 8 10 12 14 16 18 20 
Table of 3 : 3 6 9 12 15 18 21 24 27 30 
Table of 4 : 4 8 12 16 20 24 28 32 36 40 
Table of 5 : 5 10 15 20 25 30 35 40 45 50

Example 2

Program to print a 2D matrix

#include<iostream>
using namespace std;
  
// Driver program
int main()
{
    // Declare the matrix
    int mat[3][3] = { { 1, 2, 3 },
                        { 4, 5, 6 },
                        { 7, 8, 9 } };
    cout << "Matrix \n";
  
    // Here we are using two for loops to print two matrices
    for (int i = 0; i < 3; i++) {
  
        for (int j = 0; j < 3; j++)
            cout << mat[i][j] << " ";
  
        cout << "\n";
    }
  
    return 0;
}

Output

Matrix 
1 2 3 
4 5 6 
7 8 9

Example 3

Program to print tables of 1 to 5. But using a for and a while loop

#include<iostream>
using namespace std;

int main(){
    
    for(int i = 1; i <= 5; i++)
    {
        cout << "Table of " << i << " : ";
        
        int j = 1;
        while(j <= 10){
            cout << i * j << " ";
            j++;
        }
        cout << "\n";
    }
    
    return 0;
}

Output

Table of 1 : 1 2 3 4 5 6 7 8 9 10 
Table of 2 : 2 4 6 8 10 12 14 16 18 20 
Table of 3 : 3 6 9 12 15 18 21 24 27 30 
Table of 4 : 4 8 12 16 20 24 28 32 36 40 
Table of 5 : 5 10 15 20 25 30 35 40 45 50