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Competitive Programming - Crack Your Coding Interview, C++

Multiplication Without Using * Operator

Use Recursion to multiply two numbers without using the * operator.

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Problem statement

In this problem, we need to find the result of the multiplication of two numbers, a and b but without using * operator. This looks like a pretty simple problem. We can easily iterate a loop from 1 to b and add a to our result. But there is a catch: you need to write a solution with a Recursive approach.

Implementation

Let’s write the solution first, and then visualize how recursion is working.

C++
#include <iostream>
using namespace std;
int prod(int a , int b){
    if(b==0){
        return 0;
    }
    else if(b==1){
        return a;
    }
    else if(b>0){
        return a + prod(a,b-1);
    }
    else {
        return -prod(a,-b);
    }
}
int main() {
    int a = 2, b = -3;
    cout << prod(a,b);
    return 0;
}

Explanation:

  • On line 4, we define our recursive function prod().
  • On line 5, we check for the base case as
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