On Mon, Oct 31, 2011 at 1:09 PM, Joel Montes de Oca <joelmonte...@gmail.com>wrote:
> On Mon 31 Oct 2011 12:14:40 PM EDT, Hugo Arts wrote: > > > On Mon, Oct 31, 2011 at 4:41 PM, Joel Montes de Oca > <joelmonte...@gmail.com> <joelmonte...@gmail.com> wrote: > > > Hello everyone, > > I am having a little trouble with a block of code that isn't behaving the > way I would expect. Maybe you can give me a hand and point where it is > going > wrong. > > The function that is not working correctly belongs to a Paper Rock Scissor > game I am making. > > This particular function is responsible to: > a) Get the user's choice (Paper, Rock, or Scissors) > b) Return the user's choice within the variable choice to the function > that called it. > > The function works correctly as long as the user does not try to enter a > string other than 'P', 'R', or 'S'. > > Logic: > Take the user's string and put it in the variable choice. > If choice is not 'P', 'R', or 'S' then pass a message to the user and > call the function again. > If the choice is 'P', 'R', or 'S' then return choice to where it was > called from. > > The problem is this. > > When the user enters a string other than the valid ones, the if statements > catches it and calls the same function again so the user can enter a valid > string. But the variable choice does not get assigned the new string > entered > by the user, instead it is empty. > > I would expect if the function runs again, the variable choice would be > updated to the last value given by the user. > > The function: ( http://dpaste.com/644857/) > > def UserChoice (): # The function that returns the choice from the user > print 'Please select (P) for paper, (R) for Rock, or (S) for Scissors.' > choice = raw_input('What is your selection?: ') > > if choice.lower() not in ('p', 'r','s'): # Converts the user's choice to > lowercase and confirms the choice is valid > print 'I am sorry, you entered \'' + choice.upper() + '\' which is an > invalid response. Please try again.' > raw_input('Press Enter to try again.') > UserChoice () # If the choice is not valid, run the function over > else: > return choice > > > > Your problem is that you don't quite understand recursion. > > When a function calls itself, it's not actually any different from > when a function A calls another function B. Once function B is done > running and returns, control goes back to function A. It doesn't > matter if function B is actually the same function as A. In fact, even > if A and B are the same function, they don't "share" any variables and > are totally separate as if they were two different functions that just > happened to do the same thing. > > So, how does this apply to your function? Let's go through a run of > it. We call UserChoice (this is function A) and we input a wrong > letter. So, A calls UserChoice again (this is function B), and this > time we input something valid. So, function B runs the line "return > choice." > > Control now returns to function A, right at the point where we called > function B. So what do we do here with the choice that was just > returned from function B? Well, looking at the line where it's called, > it's just "UserChoice()". So we do nothing. We just throw it away. > Then, we continue on with function A, move out of the if statement, > and "fall off the end" of the function. And when that happens, Python > returns None from function A to show you that nothing was returned. > > If you understood all that, you should be able to fix your problem. > > Hugo > > > Hey Hugo, > > I think I understand your explanation. Let me see if I get it. > > The code: > > def UserChoice (): *# I WILL CALL THIS FUNCTION A* > > > print 'Please select (P) for paper, (R) for Rock, or (S) for Scissors.' > choice = raw_input('What is your selection?: ') > > if choice.lower() not in ('p', 'r','s'): > > print 'I am sorry, you entered \'' + choice.upper() + '\' which is an > invalid response. Please try again.' > raw_input('Press Enter to try again.') > > UserChoice () *# I WILL CALL THIS FUNCTION B* > else: > return choice *# I WILL CALL THE ORIGINAL CALLING FUNCTION, FUNCTION > MAIN* > > > OK so when the user uses a valid letter, the variable choice gets > returned to FUNCTION MAIN. Everything works fine. > > When the user enters an invalid letter, FUNCTION B calls FUNCTION A. > FUNCTION A returns choice to FUNCTION B. FUNCTION B does nothing with the > return, FUNCTION MAIN gets nothing to returned to it, thus choice is NONE. > > FUN MAIN > | > | > |__ FUN A > | > | > |_ FUN B > > This is how I understand it. So if I want this to work, I need FUN B to > give something back to FUN A so that FUN A will have something to give back > to FUN MAIN but that doesn't feel right. > > Is there a way to say GO TO FUN A instead of calling the function? > > > > I also found this Youtube video on the topic of recursion in python. > http://www.youtube.com/watch?v=72hal4Cp_2I > > -- > -Joel M. > > After a little digging around, I found that there's no goto or label functions. http://entrian.com/goto/ had me excited for a few minutes until I read the top red text where it reads that the goto module was an April Fools joke. haha
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