CODEWITHSUNDEEP
pythonstackpostfix-operatorinfix-operator
Bella
Bella

Reputation: 1

Evaluate postfix (Stack python)

what I have so far is:

Stack = ArrayStack()

def evaluate(postfix_str, Stack):
    for c in postfix_str:
        if c not in '+-*/()':
            Stack.push(int(c))
        else:   
            c.split(" ") 
            if c == '+':
                sum = Stack.pop() + Stack.pop()
                Stack.push(sum)
            elif c == '-':
                difference = Stack.pop() - Stack.pop()
                Stack.push(difference)
            elif c == '*':
                product = Stack.pop() * Stack.pop()
                Stack.push(product)
            elif c == '/':
                quotient = Stack.pop() / Stack.pop()
                Stack.push(quotient)
            else:
                Stack.push(int(c))
                
    return Stack.pop()

print(evaluate('234*-', Stack))  # 10

This code is supposed to find an operator, pop its operands off the stack, run the operation and then push the result back on the stack. but when I run my code let's say one of my test cases is ('234*-'), so instead of getting -10, what I am getting is 10.

Upvotes: 0

Views: 393

Answers (1)

ErnestBidouille
ErnestBidouille

Reputation: 1146

Actually, 10 is the correct answer. Stack is a LIFO (Last In, First Out) structure which is, with a Python implementation, simply an inherited list class with push==append (add an element to the end) and pop which remove the last element of the list.

Step 1: c=(2) ; Stack = [] ; action = Push(2) ; Stack = [2]
Step 2: c=(3) ; Stack = [2] ; action = Push(3) ; Stack = [2, 3]
Step 3: c=(4) ; Stack = [2, 3] ; action = Push(4) ; Stack = [2, 3, 4]
Step 4: c=(*) ; Stack = [2, 3, 4] ; action = Push(Pop(4) * Pop(3)) ; Stack = [2, 12]
Step 5: c=(-) ; Stack = [2, 12] ; action = Push(Pop(12) - Pop(2)) ; Stack = [10]

Upvotes: 0

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