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Java Nested Loops with Examples

  • Difficulty Level : Medium
  • Last Updated : 01 Oct, 2021
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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.
 

Java




// Java program to print the elements of
// a 2 D array or matrix
 
import java.io.*;
 
class GFG {
 
    public static void print2D(int mat[][])
    {
        // Loop through all rows
        for (int i = 0; i < mat.length; i++) {
 
            // Loop through all elements of current row
            for (int j = 0; j < mat[i].length; j++)
                System.out.print(mat[i][j] + " ");
 
            System.out.println();
        }
    }
 
    public static void main(String args[]) throws IOException
    {
        int mat[][] = { { 1, 2, 3, 4 },
                        { 5, 6, 7, 8 },
                        { 9, 10, 11, 12 } };
        print2D(mat);
    }
}
Output: 
1 2 3 4 
5 6 7 8 
9 10 11 12

 

Example 2: Below program uses a nested for loop to print all prime factors of a number.
 

Java




// Java program to print all prime factors
 
import java.io.*;
import java.lang.Math;
 
class GFG {
    // A function to print all prime factors
    // of a given number n
    public static void primeFactors(int n)
    {
        // Print the number of 2s that divide n
        while (n % 2 == 0) {
            System.out.print(2 + " ");
            n /= 2;
        }
 
        // n must be odd at this point. So we can
        // skip one element (Note i = i +2)
        for (int i = 3; i <= Math.sqrt(n); i += 2) {
            // While i divides n, print i and divide n
            while (n % i == 0) {
                System.out.print(i + " ");
                n /= i;
            }
        }
 
        // This condition is to handle the case when
        // n is a prime number greater than 2
        if (n > 2)
            System.out.print(n);
    }
 
    public static void main(String[] args)
    {
        int n = 315;
        primeFactors(n);
    }
}
Output: 
3 3 5 7

 

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