# Recursive Multiply

**8.5 Recursive Multiply:**Write a recursive function to multiply two positive integers without using the * operator (or / operator). You can use addition, subtraction, and bit shifting, but you should minimize the number of those operations.

- With this problem, I give myself a big fuck ya. 5 min solution.
- The multiplication of two numbers is negative only if either of the numbers are negative. XOR is perfect for this.
- Multiplication is the same thing as adding the other number n amount of times. Either one can work, but I chose the max number to count the min number n amount of times, that way I minimize the number of times, or operations that I need to count.

int rec_mult(int a, int b, int sol, int count) {

if (count >= abs(min(a, b))) {

bool neg = (a < 0 ^ b < 0);

if (neg) return (-sol);

return sol;

}

return rec_mult(a, b, sol + abs(max(a, b)), ++count);

}

int mult(int a, int b) {

return rec_mult(a, b, 0, 0);

}

int main() {

cout << mult(10, 3) << endl;

cout << mult(2, -3) << endl;

cout << mult(23, -10) << endl;

cout << mult(-500, 0) << endl;

cout << mult(10, -1) << endl;

}

Last modified 3yr ago