Infix To Postfix conversion in python

About Infix and Postfix

In an expression if the operators are placed between the operands, it is known as infix notation ( eg A+B) . On the other hand if the operators are placed after the operands then the expression is in postfix notation .( eg AB+)

Infix Notation                            Postfix Notation

(A-C)*B                                              AC-B*

A+(B*C)                                            ABC*+

(A+B)/(C-D)                                      AB+CD-/

Code

#!/usr/bin/python
#http://ragsagar.wordpress.com

postfix = []
temp = []
operator = -10
operand = -20
leftparentheses = -30
rightparentheses = -40
empty = -50

def precedence(s):
	if s is '(':
		return 0
	elif s is '+' or '-':
		return 1
	elif s is '*' or '/' or '%':
		return 2
	else:
		return 99 

def typeof(s):
	if s is '(':
		return leftparentheses
	elif s is ')':
		return rightparentheses
	elif s is '+' or s is '-' or s is '*' or s is '%' or s is '/':
		return operator
	elif s is ' ':
		return empty
	else :
		return operand							

infix = raw_input("Enter the infix notation : ")
for i in infix :
	type = typeof(i)
	if type is leftparentheses :
		temp.append(i)
	elif type is rightparentheses :
		next = temp.pop()
		while next is not '(':
			postfix.append(next)
			next = temp.pop()
	elif type is operand:
		postfix.append(i)
	elif type is operator:
		p = precedence(i)
		while len(temp) is not 0 and p <= precedence(temp[-1]) :
			postfix.append(temp.pop())
		temp.append(i)
	elif type is empty:
		continue 

while len(temp) > 0 :
	postfix.append(temp.pop())

print "It's postfix notation is ",''.join(postfix)		

Code Explanation

Above code converts infix notation in variable infix into postfix notation and stores in postfix list. This algorithm makes use of list temp to hold operators and left parantheses in the infix notation. The postfix list will be constructed from left to right using operands from infix and operators which are removed from temp.

output