Nested Loops in C++ with Examples - GeeksforGeeks

03 Dec, 2019
Nested loop means a loop statement inside another loop statement. That is why nested loops are also called as “loop inside loop“.
Syntax for Nested For loop:
for ( initialization; condition; increment ) {

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

   // statement of outer loop
}

Syntax for Nested While loop:
while(condition) {

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

   // statement of outer loop
}

Syntax for Nested Do-While loop:
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);

Below are some examples to demonstrate the use of Nested Loops:
Example 1: Below program uses a nested for loop to print a 2D matrix of 3×3.
// C++ program that uses nested for loop// to print a 2D matrix  #include <bits/stdc++.h>using namespace std;  #define ROW 3#define COL 3  // Driver programint main(){      int i, j;      // Declare the matrix    int matrix[ROW][COL] = { { 1, 2, 3 },                             { 4, 5, 6 },                             { 7, 8, 9 } };    cout << "Given matrix is \n";      // Print the matrix using nested loops    for (i = 0; i < ROW; i++) {          for (j = 0; j < COL; j++)            cout << matrix[i][j];          cout << "\n";    }      return 0;}
Given matrix is 
123
456
789

Example 2: Below program uses a nested for loop to print all prime factors of a number.
// C++ Program to print all prime factors// of a number using nested loop  #include <bits/stdc++.h>using namespace std;  // A function to print all prime factors of a given number nvoid primeFactors(int n){    // Print the number of 2s that divide n    while (n % 2 == 0) {        cout << 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) {            cout << i;            n = n / i;        }    }      // This condition is to handle the case when n    // is a prime number greater than 2    if (n > 2)        cout << n;}  /* Driver program to test above function */int main(){    int n = 315;    primeFactors(n);    return 0;}
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