[Tutor] Paper Rock Scissors game - User's choice not returned properly

[Joel M.]

On Mon, Oct 31, 2011 at 1:09 PM, Joel Montes de Oca
<joelmontes01 at 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
> <joelmontes01 at gmail.com> <joelmontes01 at 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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