Compute the n-th Fibonacci number.

fibonacci.cpp

#include <iostream>

using namespace std;


/// Output the n-th Fibonacci number.
/// http://en.wikipedia.org/wiki/Fibonacci_number
///
int main(int argc, char* argv[])
{
  if (argc != 2)
  {
    cerr << "expected fibonacci n" << endl;
    return -1;
  }

  int n = atoi(argv[1]);

  if (n < 0)
  {
    cerr << "n must be greater than 0." << endl;
    return -2;
  }

  if (n == 0)
  {
    cout << 0;
  }
  else if (n == 1)
  {
    cout << 1;
  }
  else
  {
    int s0 = 0;
    int s1 = 1;

    n -= 1;
    do
    {
      const int sum = s1 + s0;
      s0 = s1;
      s1 = sum;

    } while (--n);

    cout << s1;
  }

  return 0;
}