How can I perform multiplication without the '*' operator?

Question

I was just going through some basic stuff as I am learning C. I came upon a question to multiply a number by 7 without using the * operator. Basically it\'s like this

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Answer 1

When it comes down to it, multiplication by a positive integer can be done like this:

int multiply(int a, int b) {
  int ret = 0;
  for (int i=0; i

Efficient? Hardly. But it's correct (factoring in limits on ints and so forth).

So using a left-shift is just a shortcut for multiplying by 2. But once you get to the highest power-of-2 under b you just add a the necessary number of times, so:

int multiply(int a, int b) {
  int ret = a;
  int mult = 1;
  while (mult <= b) {
    ret <<= 1;
    mult <<= 1;
  }
  while (mult < b) {
    ret += a;
  }
  return ret;
}

or something close to that.

To put it another way, to multiply by 7.

• Left shift by 2 (times 4). Left shift 3 is 8 which is >7;
• Add b 3 times.