Python/data_structures/Stacks/infix_to_postfix_conversion.py

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from __future__ import print_function
from __future__ import absolute_import
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import string
from .Stack import Stack
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__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))