int factorial( int N )
{
  if ( N == 0 )
    return 1;
  else
    return N * factorial( N-1 ) ;
}

Defensive Programming

Defensive programming is when the programmer anticipates problems and writes code to deal with them. To avoid the disaster a negative parameter would cause, sometimes factorial() is written like this:

int factorial( int N )
{
  if ( N <= 0 )
    return 1;
  else
    return N * factorial( N-1 ) ;
}

But, according to the math-like definition, this is not correct. The value of factorial( -1 ) is undefined, not equal to one. Potentially, returning an incorrect value and continuing on as if nothing is wrong might cause a greater disaster than stopping the computer system.

Sometimes the method is written to throw an exception when an illegal argument is detected. But this adds complication since now the caller must deal with a possible exception.

Perhaps the best solution is to write factorial() so that it follows the math-like definition exactly. It is the responsibility of the caller to make sure that the argument is within range.