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

How to make IRC bot in python? – my first post in Tuxopia

My first post in tuxopia is a how-to for writing basic IRC bot in python.Thought of sharing the link here

Weather Info Utility in Python

Here is a script to provide weather information of a city with the help of google api.

#!/usr/bin/python
#Rag Sagar <ragsagar @gmail.com>
#Free to copy, modify and redistribute

import sys
from urllib2 import urlopen, URLError
from xml.sax import make_parser
from xml.sax.handler import ContentHandler

class WeatherFinder(ContentHandler):
	"""the handler class"""
	def __init__(self):
		self.weather_list = []
		self.go_on = False

	def startElement(self, tag, attrs):
		"handles the opening tags"
		if tag == 'current_conditions':
			self.go_on = True
		if self.go_on : # so that only values inside current_conditions tag is appended
			if tag == 'condition' :
				self.weather_list.append('Condition: '+attrs.get('data',""))
			elif tag == 'humidity' :
				self.weather_list.append(attrs.get('data',""))
			elif tag == 'temp_c' :
				self.weather_list.append('Temperature:'+attrs.get('data',"")+'C')
			elif tag == 'wind_condition' :
				self.weather_list.append(attrs.get('data',""))
		return

	def endElement(self, tag):
		"handles closing tags"
		if tag == 'current_conditions':
			self.go_on = False

if len(sys.argv) is not 2 :
	print "Syntax : ",sys.argv[0]," <city> \n"
	sys.exit(1)
else :
	city = sys.argv[1]
	url = 'http://www.google.com/ig/api?weather='+city
	parser = make_parser()
	obj = WeatherFinder()
	parser.setContentHandler(obj)
	try :
		parser.parse(urlopen(url))
	except URLError:
		print "\n\33[33m Unable to fetch data..\33[0m\n"
		sys.exit(1)
	if obj.weather_list == [] :
		print "\n\33[33m Invalid city name, Try another.. \33[0m\n "
	else :
		print "\33[33mWeather Conditions of",city,"\33[36m"
		for i in obj.weather_list:
			print i
		print "\33[0m"

Checkout its screenshot

Weather utility

Thanks to rohit