Python/data_structures/stacks/infix_to_postfix_conversion.py

import string

from .stack import Stack

__author__ = "Omkar Pathak"


def is_operand(char):
    return char in string.ascii_letters or char in string.digits


def precedence(char):
    """Return integer value representing an operator's precedence, or
    order of operation.

    https://en.wikipedia.org/wiki/Order_of_operations
    """
    dictionary = {"+": 1, "-": 1, "*": 2, "/": 2, "^": 3}
    return dictionary.get(char, -1)


def infix_to_postfix(expression):
    """Convert infix notation to postfix notation using the Shunting-yard
    algorithm.

    https://en.wikipedia.org/wiki/Shunting-yard_algorithm
    https://en.wikipedia.org/wiki/Infix_notation
    https://en.wikipedia.org/wiki/Reverse_Polish_notation
    """
    stack = Stack(len(expression))
    postfix = []
    for char in expression:
        if is_operand(char):
            postfix.append(char)
        elif char not in {"(", ")"}:
            while not stack.is_empty() and precedence(char) <= precedence(stack.peek()):
                postfix.append(stack.pop())
            stack.push(char)
        elif char == "(":
            stack.push(char)
        elif char == ")":
            while not stack.is_empty() and stack.peek() != "(":
                postfix.append(stack.pop())
            # Pop '(' from stack. If there is no '(', there is a mismatched
            # parentheses.
            if stack.peek() != "(":
                raise ValueError("Mismatched parentheses")
            stack.pop()
    while not stack.is_empty():
        postfix.append(stack.pop())
    return " ".join(postfix)


if __name__ == "__main__":
    expression = "a+b*(c^d-e)^(f+g*h)-i"

    print("Infix to Postfix Notation demonstration:\n")
    print("Infix notation: " + expression)
    print("Postfix notation: " + infix_to_postfix(expression))