Another method to multiply two numbers without using the “*” operator [duplicate]

Answer 1

There is a method called Russian Peasant Multiplication. Demonstrate this with the help of a shift operator,

public static int multiply(int n, int m)
{ 
    int ans = 0, count = 0;
    while (m > 0)
    {
        if (m % 2 == 1)             
            ans += n << count;
        count++;
        m /= 2;
    }

    return ans;
}

The idea is to double the first number and halve the second number repeatedly till the second number doesn’t become 1. In the process, whenever the second number become odd, we add the first number to result (result is initialized as 0) One other implementation is,

static int russianPeasant(int n, int m) {
    int ans = 0;

    while (m > 0) {
        if ((m & 1) != 0)
            ans = ans + n;

        n = n << 1;
        m = m >> 1;
    }
    return ans;
}

refer :

• https://www.geeksforgeeks.org/russian-peasant-multiply-two-numbers-using-bitwise-operators/
• https://www.geeksforgeeks.org/multiplication-two-numbers-shift-operator/

Answer 2

The questions interviewers ask reflect their values. Many programmers prize their own puzzle-solving skills and mathematical acumen, and they think those skills make the best programmers.

They are wrong. The best programmers work on the most important thing rather than the most interesting bit; make simple, boring technical choices; write clearly; think about users; and steer away from stupid detours. I wish I had these skills and tendencies!

If you can do several of those things and also crank out working code, many programming teams need you. You might be a superstar.

---

But what should you do in an interview when you're stumped?

Ask clarifying questions. ("What kind of numbers?" "What kind of programming language is this that doesn't have multiplication?" And without being rude: "Why am I doing this?") If, as you suspect, the question is just a dumb puzzle with no bearing on reality, these questions will not produce useful answers. But common sense and a desire to get at "the problem behind the problem" are important engineering virtues.

The best you can do in a bad interview is demonstrate your strengths. Recognizing them is up to your interviewer; if they don't, that's their loss. Don't be discouraged. There are other companies.

Answer 3

If you don't have integer values, you can take advantage of other mathematical properties to get the product of 2 numbers. Someone has already mentioned log10, so here's a bit more obscure one:

public double multiply(double x, double y) {
    Vector3d vx = new Vector3d(x, 0, 0);
    Vector3d vy = new Vector3d(0, y, 0);
    Vector3d result = new Vector3d().cross(vx, vy);
    return result.length();
}