python - Stack, Pop and Add -

this calculate function should work

calculate((1, 2, '+', 3, '*')) return 9 1. take 1, push 1   # stack [1] 2. take 2, push 2   #stack [1, 2] 3. take '+', pop 2, pop 1, calculate 1+2 = 3, push 3 #stack [3] 4. take 3, push 3 #stack [3, 3] 5. take '*', pop 3, pop 3, calculate 3 * 3 = 9, push 9 #stack [9] 6. input empty, pop 9, return 9 

i don't know how use string '+' here add 1 , 2 together. seems impossible.

here code. suppose convert tuple , put list pop first 2 number out combine binary operators '+', '-', '*', '/'

def calculate(inputs):  #(1, 2, '+', 3, '/')     if len(inputs) <= 1:         return inputs[0]     in range(len(inputs)+1):  ## (0,...5)         s = make_empty_stack() # []          first = push_stack(s, inputs[i])  #[1] # push_stack pushes item onto stack s, returns stack         second = push_stack(s, inputs[i + 1]) ##[2,1]         third = push_stack(s, inputs[i +2]) ##[2,1,+]         f = pop_stack(third)  #removes top item of stack #2 

here test cases

calculate((1, 2, '+', 3, '*')) ## 1+2 * 3 calculate((1, 2, '+', 3, '/'))  ## (1 +2) / 3 = 1 calculate((28,)) # 28 calculate((1, 2, '*', 3, '-', 2, '*', 5, '+'))  ## ((1*2) - 3) * 2)+5 = 3 

the operator module provides convenient functions python operators, including + , *:

import operator  ops = {'+': operator.add, '-': operator.sub,        '*': operator.mul, '/': operator.floordiv} 

now can use ops dictionary implement operations on stack:

def calculate(inputs):     if not inputs:         return     s = make_empty_stack()     item in inputs:         if item in ops:             op2, op1 = pop_stack(s), pop_stack(s)             res = ops[item](op1, op2)             push_stack(s, res)         else:             push_stack(s, item)     return pop_stack(s) 

the code pops 2 items stack , passes them operator function, provided item valid operator. need invert operands, however; want subtract top-most stack item second item on stack, not other way around. else pushed stack instead.

note need create empty stack once, outside loop.