binary gcd algorithm - not working

Question

I read the binary gcd algorithm and tried to implement it .It worked. This is my code

int gcd2(int a, int b) {
   int sh;

   if (a == 0)
        return b;
   if (b == 0)
        return a;

   for (sh = 0; !((a | b) & 1); sh++) {
      a >>= 1;
      b >>= 1;
   }

   while (!(a & 1)) {
      a >>= 1;
   }

    while(b) {
      while (!(b & 1)) {
          b >>= 1;
      }

      if (a > b) {
          int t = a;
          a = b;
          b = t;
      }

      b = b - a;
   }

   return a << sh;
}

But doesn't work if I replace the last if with

    if (b > a)
    {
        int t = a;
        a = b;
        b = t;
    }

    b = a -b;

I just thought that both should work since they are doing the same.But it doesn't work. Can anyone explain it please?

Thanks in advance!

Answer 1

it is not the same: if you choose your second way, there is the chance that a always stays bigger then b. then you never get to svap variables, and b is always less then a after b=a-b, if b is positive

i think using

a=a-b

instead of

b=a-b

could do it