how to multiply two negative values without using the multiplication (*) function in java

Question

For example, I have this code to multiply 2 numbers without using the * symbol, but how can I achieve the same with negative numbers?

public static void main(String[] args) {
    // TODO code application logic here
    Scanner in = new Scanner(System.in);

    int result = 0;

    System.out.println("Enter the first number: ");
    int num1 = in.nextInt();
    System.out.println("Enter the second number: ");
    int num2 = in.nextInt();
   if(num1==0 || num2==0) {
       System.out.println("the profuct of these numbers is: 0");
     }

    for(int i =0; i <num1; i++)

    {
        result += num2;            
    }
    System.out.println("the profuct od these numbers is: "+result);

}

}

Hint: if X is less than 0, then treat it as `X = -1 * abs(X);`. — Luiggi Mendoza, Feb 13 '14 at 21:56
Hint: your code works with adding additional minus signs in front of the num1 in for loop and result at the end. — B.J. Smegma, Feb 13 '14 at 21:59

Franz Ebner · Answer 1 · 2014-02-13T22:48:49.153

2

java.lang.Maths.abs(num1/2) for iteration and

something like:

if(num1<0^num2<0){
    result = 0 - result;
}

between interation and output.

edited Feb 13 '14 at 22:48

answered Feb 13 '14 at 22:00

Franz Ebner

4,951
3
39
57

score 1 · Answer 2 · edited May 23 '17 at 10:24

1

Maybe BigInteger

BigInteger num1 = new BigInteger(-12);
BigInteger num2 = new BigInteger(-13);
BigInteger result  = num1.multiply(num2);

Also you may use bitwise shifting and adding and try this on negative numbers.

Of course you can mutiply values your way and just make it positive or negative - you know the rules.

edited May 23 '17 at 10:24

Community

1
1

answered Feb 13 '14 at 22:00

Piotr Gwiazda

12,080
13
60
91

Well, this clearly multiplies without using the `*` operator, but I don't think that's what OP is going for. – Reinstate Monica -- notmaynard Feb 13 '14 at 22:00
-1: The question seems obviously to be a homework/exercise to emulate multiplication using loops. While this *solves* the problem directly, won't help OP to learn about programming. – Luiggi Mendoza Feb 13 '14 at 22:03

score 1 · Accepted Answer · answered Feb 13 '14 at 22:01

If only one number is negative, you can use the same approach, but you need to make sure that you use the positive one in the cycle (as num1 in your code). If both numbers are negative, you can change both of them to positive using abs(num) and nothing changes.

score 1 · Answer 4 · answered Feb 13 '14 at 22:01

Keep what you have but keep track of how many negatives you have and use the absolute value.

bool resultNeg = false;
if( num1 < 0 && num2 > 0 || num1 > 0 && num2 < 0 )
    resultNeg = true;

num1 = Math.abs( num1 ); num2 = Math.abs( num2 );

for( int i = 0; i < num1; ++i )
{
    // ...
}

if( resultNeg ) result = 0 - result;

// ...

score 0 · Answer 5 · answered Feb 13 '14 at 22:11

public static void main(String[] args)
{
    // TODO code application logic here
    Scanner in = new Scanner(System.in);

    int result = 0;

    System.out.println("Enter the first number: ");
    int num1 = in.nextInt();
    System.out.println("Enter the second number: ");
    int num2 = in.nextInt();
    if(num1==0 || num2==0) {
        System.out.println("the profuct of these numbers is: 0");
        return;
    }

    String sign = "";
    if (num1 < 0 && num2 > 0)
    {
        sign = "-";
    }
    if (num2 < 0 && num1 > 0)
    {
        sign = "-";
    }

    num1 = abs(num1);
    num2 = abs(num2);
    for(int i =0; i <num1; i++)
    {
        result += num2;            
    }
    System.out.println("the profuct of these numbers is: "+sign+result);
}

how to multiply two negative values without using the multiplication (*) function in java

5 Answers5