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data_structures/stacks/infix_to_postfix_conversion.py

@@ -0,0 +1,64 @@
+from __future__ import print_function
+from __future__ import absolute_import
+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))