Python: I'm misunderstanding stack logic, but I'm not sure where -


i have function evaluating infix expression via use of stacks. (bet you've never seen 1 before.) takes expression in list format, wherein each item in list single character (operand, operator, or parenthesis.)

s = stack() tempvar = none ops = ['+','-','/','*']  in expression:     if == '('         s.push(i)      elif i.isdigit():         if tempvar none:             tempvar =         else:             tempvar = tempvar +      elif in ops:         if tempvar not none:             s.push(tempvar)             tempvar = none          popped = str(s.pop() + str(s.pop())         last = s.pop()         if last not ')':             popped += str(last)          x in popped:             print 'popping',x,'from stack.'          print 'popping',last,'from stack.'          math = eval(popped)         s.push(math)         print 'pushing',math,'back onto stack.

my intention using "tempvar" string that, since i'm iterating on expression character character, encounter issue multi-digit numbers. i'm making string out of them , pushing string onto stack instead.

i having 2 problems this, , i'm not sure why i'm getting 1 of them.

for input

((21*2)-(4/2))

i output

pushing ( onto stack pushing ( onto stack pushing 21 onto stack pushing * onto stack pushing 2 onto stack popping 2 stack **popping * stack popping 2 stack popping 1 stack popping 21 stack**

i've set off problem portion asterisks. understanding of stacks function last-in-first-out, , don't care sitting on top, pop takes off. 21 first thing pushed onto stack, looks it's getting read individually (as 21, last entry pop off) , split in half '2' , '1'- expect 1 pop off before 2! misunderstanding either stacks, or i've written?

for x in popped:     print 'popping',x,'from stack.'

popped string, loop iterate on 1 character @ time.


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