Gigs_ wrote: > Steve Holden wrote: [...] >> >> regards >> Steve > >>> def factorial(n): > print "n =", n > if n==0: > return 1 > else: > return n * factorial(n-1) > > >>> factorial(3) > n = 3 > n = 2 > n = 1 > n = 0 > 6 > > > now i understand. but one question at the end this function return 1. how > python > knows that it needs to multiply 1 with all recursive returns. (why python > didnt > sum recursive return with 1?) > > > that will be all, thanks in advance
Aah, that's the magic of recursion (not that it's really magic at all). When you call factorial(3), the function sees the value of n as 3. So the if condition is false, so it must execute the return statement. In order to do that it has to multiply n by the value of factorial n-1. So it makes a call to a *new copy* of factorial, and this one has the value 2 for n. The if statement again needs to execute the return statement, and before it can do that it needs the value of factorial n-1, so it makes a call to a *new copy* of factorial, and this one has the value 1 for n. The if statement again needs to execute the return statement, and before it can do that it needs the value of factorial n-1, so it makes a call to a *new copy* of factorial, and this one has the value 0 for n. [Are you detecting a pattern here?]. Finally *this* copy of factorial can immediately return the value of 1 to its caller, which then multiplies that by 1 and returns it ti *its caller, which multiplies it by 2 and returns that to *its* caller, when multiplies it by 3 and returns the result, 6. In other words, the computer builds a "stack" of partially-completed functions, and unwinds it when the innermost (topmost, whatever) finally sees that it can return a result without creating another stacked call to factorial. Hope this straightens it out for you, it's a bit of a head-twister when you first come across it. regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden Sorry, the dog ate my .sigline -- http://mail.python.org/mailman/listinfo/python-list
