C++ Nested Loop

A loop within another loop is called a nested 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 nested loop to iterate three times (3 weeks). And inside the loop, we can create another loop to iterate 7 times (7 days). For example,

Example: Nested for Loop

// C++ program to display 7 days of 3 weeks

#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) {
            cout << "    Day:" << j << endl;
        }
    }

    return 0;
}

Output

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

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

Example: Displaying a Pattern

// C++ program to display a pattern
// with 5 rows and 3 columns

#include <iostream>
using namespace std;

int main() {

   int rows = 5;
   int columns = 3;

   for (int i = 1; i <= rows; ++i) {
      for (int j = 1; j <= columns; ++j) {
         cout << "*  ";
      }
      cout << endl;
   }

   return 0;
}

Output

*  *  *  
*  *  *  
*  *  *  
*  *  *  
*  *  *  

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

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

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,

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) {
        // break during the 2nd week    
        if (i == 2) {
            break;
        }
        cout << "    Day:" << j << endl;
    }
}

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,

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 odd number     
        if (j%2 != 0) {
            continue;
        }
        cout << "    Day:" << j << endl;
    }
}

Output

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

This program prints only those days that are even.

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