infix-to-postfix.py

# Copyright (C) Deepali Srivastava - All Rights Reserved
# This code is part of DSA course available on CourseGalaxy.com  

from StackArray import Stack

def infix_to_postfix(infix):
    postfix = ""

    st = Stack()

    for symbol in infix:
        if symbol == ' ' or symbol == '\t': #ignore blanks and tabs
            continue

        if symbol == '(':
            st.push(symbol)
        elif symbol == ')':
            next = st.pop()
            while next != '(':
                postfix = postfix + next
                next = st.pop()
        elif symbol in "+-*/%^":
            while not st.is_empty() and precedence(st.peek()) >= precedence(symbol):
                postfix = postfix + st.pop()
            st.push(symbol)
        else: #operand
            postfix = postfix + symbol
            
    while not st.is_empty():
        postfix = postfix + st.pop()
    return postfix

def precedence(symbol):
    if symbol == '(':
        return 0
    elif symbol in '+-':
        return 1
    elif symbol in '*/%':
        return 2
    elif symbol == '^':
        return 3
    else:
        return 0
            
def evaluate_postfix(postfix):
    st = Stack()
    
    for symbol in postfix:
        if symbol.isdigit():
            st.push( int(symbol) )
        else:
            x = st.pop()
            y = st.pop()

            if symbol == '+':
                st.push(y + x)
            elif symbol == '-':
                st.push(y - x)
            elif symbol == '*':
                st.push(y * x)
            elif symbol == '/':
                st.push (y / x)
            elif symbol == '%':
                st.push(y % x)
            elif symbol == '^':
                st.push(y ** x)
            
    return st.pop()

####################################################
			
while True:
    print("Enter infix expression (q to quit) : ", end = '')

    expression = input()
    if expression == 'q':
        break
    
    postfix = infix_to_postfix(expression)
    print("Postfix expression is : " ,postfix)
    print("Value of expression : " , evaluate_postfix(postfix) )