C++ Nested Loop

Introduction of Nested Loop in C++

A loop within another loop is called a nested loop. Nested loop means a loop statement inside another loop statement. That's why nested loop are also called as loop inside loop.

Working of Nested Loop

• Execution of statement within the loop flows in a way that the inner loop of the nested loop gets declared, initialized and then incremented.
• Once all the condition within the inner loop gets satisfied and becomes true it moves for the search of the outer loop. It is often called a loop within a loop.

Let's take an example:-
Suppose we want to loop through each day of a week for 3 weeks. To achieve this, we can create a loop to iterate three times (3 weeks). And inside the loop, we can create another loop to iterate 7 times (7 days). This is how we can use nested loops.

Nested for Loop

A for loop within another for loop is called Nested For loop

The syntax of nested for loop is:

for (initialization; condition; update) {
    for (initialization; condition; update) {
        // body of inner for-loop 
    }
    // body of outer for-loop 
}

Nested while Loop

A while loop within another while loop is called Nested while loop.

The syntax of nested while loop is:

while (condition) {
    while (condition) {
        // body of inner while-loop 
    }
    // body of outer while-loop 
}

Nested do-while Loop

A do-while loop within another do-while loop is called Nested do-while loop.

The syntax of nested do-while loop is:

do {
    do{
    // body of inner do-while-loop 
    }while (condition);
    // body of outer do-while-loop 
}while (condition);

Example 1: C++ Nested for loop

// C++ program to display 7 days a weeks
      
#include <iostream>
using namespace std;

int main() {
    int weeks = 1, days_in_week = 7;

    for (int i = 1; i <= weeks; ++i) {
        cout << "Week: " << i << endl;

        for (int j = 1; j <= days_in_week; ++j) {
            cout << "    Day:" << j << endl;
        }
    }
    return 0;
}

Output

Week: 1
    Day:1
    Day:2
    Day:3
    Day:4
    Day:5
    Day:6
    Day:7

We can create nested loops with while and do...while in a similar way.

Example 2: Displaying a Pattern

// C++ program to display a triangular pattern 
// Number is entered by the user
                                                                                        
#include <iostream>
using namespace std;
                                                                                        
int main() {
                                                                                        
   int i, j, n;
   cout << "Enter Number :  ";
   cin >> n;
                                                                                        
   for (i = 1; i <= n; i++) {
      for (j = 1; j <= i; j++) {
         cout << "*  ";
      }
      cout << endl;
   }

    return 0;
}

Output

Enter Number :  4
*    
*  *   
*  *  *  
*  *  *  *

In this program, the outer loop iterates from 1 to rows.

The inner loop iterates from 1 to columns. Inside the inner loop, we print the character '*'.

Example 3: C++ Nested while Loop

// C++ program to display a triangular pattern of numbers using nested while loop 

#include <iostream>
using namespace std;
                                            
int main() {
                                            
   int rows, i = 1;
   cout << "Enter the number of rows: ";
   cin >> rows;

   while (i <= rows) {
        int j = 1;
      while(j <= i) {
         cout << i;
         j++;
      }
    cout << "\n";
    i++;
   }
    return 0;
}

Output

Enter the number of rows: 4
1
22
333
4444

In this program, the outer loop iterates from 1 to rows.

The inner loop iterates from 1 to i. Inside the inner loop, we print the numbers.

Example 4: C++ Nested do-while Loop

// C++ program to display a triangular pattern of numbers using nested do-while loop

#include <iostream>
using namespace std;
    
int main() {
    
int rows, i,j;
i = 1;
cout << "Enter the number of rows: ";
cin >> rows;
    do {
        j = 1;
        do{
            cout << j;
            j++;
        }while(j <= i);
        cout << "\n";
        i++;
    }while(i <= rows);
                                
    return 0;
}

Output

Enter the number of rows: 4
1
12
123
1234

break and continue Inside Nested Loops

When we use a break statement inside the inner loop, it terminates the inner loop but not the outer loop. For example,

Example 4: C++ break Inside Nested Loops

// C++ program to display a triangular pattern of numbers using nested do-while loop

#include <iostream>
using namespace std;
    
int main() {
    int weeks = 3, days_in_week = 2;

    for (int i = 1; i <= weeks; ++i) {
        cout << "Week: " << i << endl;

        for (int j = 1; j <= days_in_week; ++j) {
            // break during the 2nd week
            if (i == 2) {
                break;
            }
            cout << "    Day:" << j << endl;
        }
    }
    return 0;
}

Output

Week: 1
    Day:1
    Day:2
Week: 2
Week: 3
    Day:1
    Day:2

This program does not run the inner loop when the value of i is 2 i.e. it does not print the days of the 2nd week. The outer loop that prints the weeks is unaffected.

Similarly, when we use a continue statement inside the inner loop, it skips the current iteration of the inner loop only. The outer loop is unaffected. For example,

Example 4: C++ break Inside Nested Loops

// C++ program to display a triangular pattern of numbers using nested do-while loop

#include <iostream>
using namespace std;
    
int main() {
    int weeks = 3, days_in_week = 7;

    for (int i = 1; i <= weeks; ++i) {
        cout << "Week: " << i << endl;

        for (int j = 1; j <= days_in_week; ++j) {
            // continue if the day is an even number
            if (j % 2 == 0) {
                continue;
            }
            cout << "    Day:" << j << endl;
        }
    }
    return 0;
}

Output

Week: 1
    Day:1
    Day:3
    Day:5
    Day:7
Week: 2
    Day:1
    Day:3
    Day:5
    Day:7
Week: 3
    Day:1
    Day:3
    Day:5
    Day:7

This program prints only those days that are even.

Whenever the days_in_week is even, the continue statement skips that iteration of the inner loop.

We hope that this tutorial helped you develop better understanding of the concept of Nested Loops in C++.

Keep Learning : )