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63 lines
1.8 KiB
Python
63 lines
1.8 KiB
Python
import string
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from Stack import Stack
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__author__ = 'Omkar Pathak'
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def is_operand(char):
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return char in string.ascii_letters or char in string.digits
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def precedence(char):
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""" Return integer value representing an operator's precedence, or
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order of operation.
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https://en.wikipedia.org/wiki/Order_of_operations
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"""
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dictionary = {'+': 1, '-': 1,
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'*': 2, '/': 2,
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'^': 3}
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return dictionary.get(char, -1)
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def infix_to_postfix(expression):
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""" Convert infix notation to postfix notation using the Shunting-yard
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algorithm.
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https://en.wikipedia.org/wiki/Shunting-yard_algorithm
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https://en.wikipedia.org/wiki/Infix_notation
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https://en.wikipedia.org/wiki/Reverse_Polish_notation
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"""
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stack = Stack(len(expression))
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postfix = []
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for char in expression:
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if is_operand(char):
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postfix.append(char)
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elif char not in {'(', ')'}:
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while (not stack.is_empty()
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and precedence(char) <= precedence(stack.peek())):
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postfix.append(stack.pop())
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stack.push(char)
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elif char == '(':
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stack.push(char)
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elif char == ')':
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while not stack.is_empty() and stack.peek() != '(':
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postfix.append(stack.pop())
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# Pop '(' from stack. If there is no '(', there is a mismatched
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# parentheses.
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if stack.peek() != '(':
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raise ValueError('Mismatched parentheses')
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stack.pop()
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while not stack.is_empty():
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postfix.append(stack.pop())
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return ' '.join(postfix)
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if __name__ == '__main__':
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expression = 'a+b*(c^d-e)^(f+g*h)-i'
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print('Infix to Postfix Notation demonstration:\n')
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print('Infix notation: ' + expression)
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print('Postfix notation: ' + infix_to_postfix(expression))
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