Python/data_structures/stacks/infix_to_postfix_conversion.py
Du Yuanchao 786b32431c
Update infix to postfix (#3817)
* add test to infix_to_postfix_conversion

* fixed pre-commit error

* fixed build error

* updating DIRECTORY.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
2020-11-02 00:35:31 +08:00

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Python

"""
https://en.wikipedia.org/wiki/Infix_notation
https://en.wikipedia.org/wiki/Reverse_Polish_notation
https://en.wikipedia.org/wiki/Shunting-yard_algorithm
"""
from .balanced_parentheses import balanced_parentheses
from .stack import Stack
def precedence(char: str) -> int:
"""
Return integer value representing an operator's precedence, or
order of operation.
https://en.wikipedia.org/wiki/Order_of_operations
"""
return {"+": 1, "-": 1, "*": 2, "/": 2, "^": 3}.get(char, -1)
def infix_to_postfix(expression_str: str) -> str:
"""
>>> infix_to_postfix("(1*(2+3)+4))")
Traceback (most recent call last):
...
ValueError: Mismatched parentheses
>>> infix_to_postfix("")
''
>>> infix_to_postfix("3+2")
'3 2 +'
>>> infix_to_postfix("(3+4)*5-6")
'3 4 + 5 * 6 -'
>>> infix_to_postfix("(1+2)*3/4-5")
'1 2 + 3 * 4 / 5 -'
>>> infix_to_postfix("a+b*c+(d*e+f)*g")
'a b c * + d e * f + g * +'
>>> infix_to_postfix("x^y/(5*z)+2")
'x y ^ 5 z * / 2 +'
"""
if not balanced_parentheses(expression_str):
raise ValueError("Mismatched parentheses")
stack = Stack()
postfix = []
for char in expression_str:
if char.isalpha() or char.isdigit():
postfix.append(char)
elif char == "(":
stack.push(char)
elif char == ")":
while not stack.is_empty() and stack.peek() != "(":
postfix.append(stack.pop())
stack.pop()
else:
while not stack.is_empty() and precedence(char) <= precedence(stack.peek()):
postfix.append(stack.pop())
stack.push(char)
while not stack.is_empty():
postfix.append(stack.pop())
return " ".join(postfix)
if __name__ == "__main__":
from doctest import testmod
testmod()
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))