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Passing Values

Remco Gerlich scarblac at pino.selwerd.nl
Tue Feb 13 18:06:50 EST 2001 
mike.mellor at tbe.com <mike.mellor at tbe.com> wrote in comp.lang.python:
> I am working on an RPN calculator program, using a four level stack.  
> I initialize it as:
> 
> stack = [0.0,0.0,0.0,0.0]
> 
> I am noticing problems with the following two functions (xStack and 
> yStack are Entry widgets).  clearer is supposed to clear the values 
> of the stack, and RollDown is supposed to "roll the stack" down.
> 
> def clearer(event):
>     yStack.delete(0,END)
>     xStack.delete(0,END)
>     stack = [0.0, 0.0, 0.0, 0.0]
>     print 'Clear ' + str(stack)
>     for i in range(4):
>        print 'Stack ' + `i` + ' ' + str(stack[i])

The stack you use here is a local variable. It has no effect on the global
variable also named stack.

Solution: put 'global stack' as the first line of the function.

> def RollDown(event):
>     stack[0] = stack[1]
>     stack[1] = stack[2]
>     stack[2] = stack[3]
>     try:
>         stack[3] = float(xStack.get())
>     except ValueError:
>         stack[3] = 0.0
>     xStack.delete(0,END)
>     xStack.insert(0,stack[0])
>     yStack.delete(0,END)
>     yStack.insert(0,str(stack[1]))
>     print 'Roll down ' + str(stack)
>     for i in range(4):
>        print 'Stack ' + `i` + ' ' + str(stack[i])

Here, you never directly assign to stack, so Python assumes you meant the
global. This function uses the global stack.

> The console reports that the stack is cleared, but when I roll the 
> stack down, old values pop in.  Do I need to use a deepcopy to pass 
> the values from one stack location to the next (stack[0] = deepcopy
> (stack[1])) or is there a better way to do this?

Instead of stack[0]=stack[1]; stack[1]=stack[2]; stack[2]=etc, you
simply want to do 'del stack[0]'. Then add the new element with
stack.append(0.0).

-- 
Remco Gerlich
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