What is wrong with this program. if a number is perfect for example num is 6 it should print 1*2*3. used BigInteger

Question

package perfect;

import java.math.BigInteger;
import java.util.Scanner;

public class Perfect {

 public static void main(String[] args) 
   {

 Scanner input = new Scanner(System.in);
 System.out.println("Enter the number");

 BigInteger n = input.nextBigInteger();
 BigInteger sum  = BigInteger.valueOf(0);
 BigInteger i = BigInteger.valueOf(1);
 BigInteger mul = BigInteger.valueOf(1);

 for(; i.compareTo(n)< 0; i.add(BigInteger.ONE))
      {

       if(n.mod(i).equals(BigInteger.ZERO))
         {

             sum = sum.add(i);
             mul = mul.multiply(i) ;
         }
     }
     if(sum == n)
     {

       System.out.println(n+ "=" +mul) ;
     }
     else
     {
         System.out.println("the given number " +n+ " is not a perfect 
      number");
      }
        }

        }

as it has to print 6 = 1*2*3 i used BigInteger. but it is not showing any error but the program after taking a number from user in the console i am not getting any output.

Answer 1

Three problems:

• BigInteger is immutable, so you should do i = i.add(BigInteger.ONE) instead
• When comparing sum with n, you should do sum.equals(n) instead
• Store factors into a list instead of cumulatively multiplying them back to the input
• Code will look nicer if you format it

import java.math.BigInteger;
import java.util.Scanner;
import java.util.ArrayList;

public class Perfect {

  public static void main(String[] args) {

    Scanner input = new Scanner(System.in);
    System.out.println("Enter the number");

    BigInteger n = input.nextBigInteger();
    BigInteger sum = BigInteger.valueOf(0);
    BigInteger i = BigInteger.valueOf(1);
    ArrayList<BigInteger> factors = new ArrayList<BigInteger>();

    for (; i.compareTo(n) < 0; i = i.add(BigInteger.ONE)) {

      if (n.mod(i).equals(BigInteger.ZERO)) {

        sum = sum.add(i);
        factors.add(i);
      }
    }
    if (sum.equals(n)) {

      System.out.print(n + "=" + factors.get(0));
      for (int idx = 1; idx < factors.size(); idx++) {
        System.out.print("*" + factors.get(idx));
      }
      System.out.println();
    } else {
      System.out.println("the given number " + n + " is not a perfect number");
    }
  }

}