Question

Using java, I am trying to recreate an arithmetic logic unit which performs an integer multiplication. I should able to get an output like this:

387637653 x 366499587 = 142069039730149311

But I am not sure because something is wrong apparently.

The algorithm of the program goes like:

1. Test multiplier0
   1. If multiplier0 = 0
      1. Shift the multiplicand register left 1 bit
      2. Shift the multiplier register right 1 bit
   2. If multiplier0 = 1
      1. Add multiplicand to product and place the result in product register
      2. Shift the multiplicand register left 1 bit
      3. Shift the multiplier register right 1 bit
2. Do the above 32 times
3. End

import java.util.*;

public class HWMult {

       public static void main(String[] args)

       {

              Scanner x = new Scanner(System.in);

              System.out.println("Please enter your multiplier: ");

              int multiplier0 = x.nextInt();

              System.out.println("Please enter your multiplicant: ");

              long multiplicant = x.nextLong();

              long product = 0;

              int rep = 0;

             

              while(rep<=32)

              {

                     if (rep ==0 || multiplier0 == 1)

                     {

                           product = multiplicant + product;

                           multiplicant = multiplicant << 1;

                           multiplier0 = multiplier0 >> 1;

                     }

                    

                     else if(multiplier0 == 0)

                     {

                           multiplicant = multiplicant << 1;

                           multiplier0 = multiplier0 >> 1;

                     }

                          

                     rep++;

              }

             

              System.out.println("your product:"+ product);

       }

}

Answer 1

Hi,

You were checking always for multiplier only but since this is in binary, we need to check for the last digit whether it is divisible by 2 or not using multiplier0%2.

Caution: Please ensure that integer and long overflow didn’t happen. Please take care of the range.

The input given by you:

387637653 x 366499587 = 142069039730149311

works fine now.

Code:

import java.util.*;

public class Main

{

              public static void main(String[] args)

       {

              // scanner to take input from the user

              Scanner x = new Scanner(System.in);

              System.out.println("Please enter your multiplier: ");

             

              // take multiplier input from user

              int multiplier0 = x.nextInt();

             

              System.out.println("Please enter your multiplicant: ");

             

              // take multiplicand input from user

              long multiplicant = x.nextLong();

             

              // initialize the variables

              long product = 0;

              int rep = 0;

             

              // loop for 32 times and

              // repeat the process mentioned in the algorithm

              while(rep<=32)

              {

                     // if the last bit is 1

                     if (multiplier0%2 == 1)

                     {

                           // add product to multiplicant

                           // store it back in product

                           product = multiplicant + product;

                          

                           // left shift multiplicant by 1 bit

                           multiplicant = multiplicant << 1;

                          

                           // right shift multiplier0 by 1 bit

                           multiplier0 = multiplier0 >> 1;

                     }

                     // if the last bit is 0

                     else if(multiplier0%2 == 0)

                     {

                        // left shift multiplicant by 1 bit

                        multiplicant = multiplicant << 1;

                       

                        // right shift multiplier0 by 1 bit

                        multiplier0 = multiplier0 >> 1;

                     }

                    

                     // increment the counter

                     rep++;

              }

              System.out.println("your product:"+ product);

       }

}

Screenshot of the output: