Stack experiment

[tom]

Hi! Im new to Python and doing exercise found from internet. It is
supposed to evaluate expression given with postfix operator using
Stack() class.

class Stack:
def __init__(self):
self.items = []

def push(self, item):
self.items.append(item)

def pop(self):
return self.items.pop()

def isEmpty(self):
return (self.items == [])

def evaluatePostfix(expr):
import re
tokenList = re.split(" ([^0-9])", expr)
stack = Stack()
for token in tokenList:
if token == '' or token == ' ':
continue
if token == '+':
sum = stack.pop() + stack.pop()
stack.push(sum)
elif token == '*':
product = stack.pop() * stack.pop()
stack.push(product)
else:
stack.push(int(token))
return stack.pop()

print evaluatePostfix("56 47 + 2 *")

Errormsg:
Traceback (most recent call last):
File "C:\*\postfix1.py", line 31, in <module>
print evaluatePostfix("56 47 + 2 *")
File "C:\*\postfix1.py", line 28, in evaluatePostfix
stack.push(int(token))
ValueError: invalid literal for int() with base 10: '56 47'

How can I avoid the error and get desired result?

 

[irstas]

Hi! Im new to Python and doing exercise found from internet. It is
supposed to evaluate expression given with postfix operator using
Stack() class.

class Stack:
def __init__(self):
self.items = []

def push(self, item):
self.items.append(item)

def pop(self):
return self.items.pop()

def isEmpty(self):
return (self.items == [])

def evaluatePostfix(expr):
import re
tokenList = re.split(" ([^0-9])", expr)
stack = Stack()
for token in tokenList:
if token == '' or token == ' ':
continue
if token == '+':
sum = stack.pop() + stack.pop()
stack.push(sum)
elif token == '*':
product = stack.pop() * stack.pop()
stack.push(product)
else:
stack.push(int(token))
return stack.pop()

print evaluatePostfix("56 47 + 2 *")

Errormsg:
Traceback (most recent call last):
File "C:\*\postfix1.py", line 31, in <module>
print evaluatePostfix("56 47 + 2 *")
File "C:\*\postfix1.py", line 28, in evaluatePostfix
stack.push(int(token))
ValueError: invalid literal for int() with base 10: '56 47'

How can I avoid the error and get desired result?

Obviously your tokens are wrong since one of the tokens is '56 47' as
the error message indicates.

import re
print re.split(" ([^0-9])", "56 47 + 2 *")

It looks like you'd just want expr.split().

 

[kyosohma]

Hi! Im new to Python and doing exercise found from internet. It is
supposed to evaluate expression given with postfix operator using
Stack() class.

class Stack:
def __init__(self):
self.items = []

def push(self, item):
self.items.append(item)

def pop(self):
return self.items.pop()

def isEmpty(self):
return (self.items == [])

def evaluatePostfix(expr):
import re
tokenList = re.split(" ([^0-9])", expr)
stack = Stack()
for token in tokenList:
if token == '' or token == ' ':
continue
if token == '+':
sum = stack.pop() + stack.pop()
stack.push(sum)
elif token == '*':
product = stack.pop() * stack.pop()
stack.push(product)
else:
stack.push(int(token))
return stack.pop()

print evaluatePostfix("56 47 + 2 *")

Errormsg:
Traceback (most recent call last):
File "C:\*\postfix1.py", line 31, in <module>
print evaluatePostfix("56 47 + 2 *")
File "C:\*\postfix1.py", line 28, in evaluatePostfix
stack.push(int(token))
ValueError: invalid literal for int() with base 10: '56 47'

How can I avoid the error and get desired result?

I don't know why you're using the "re" module. For this, I would skip
it. Change your code so that it looks like this:

def evaluatePostfix(expr):
tokenList = expr.split(" ")
stack = Stack()
for token in tokenList:
if token == '' or token == ' ':
continue
if token == '+':
sum = stack.pop() + stack.pop()
stack.push(sum)
elif token == '*':
product = stack.pop() * stack.pop()
stack.push(product)
else:
stack.push(int(token))
return stack.pop()

# this worked for me. There may be something wrong with the "re" code
in your example, but I don't know enough about that to help in that
area.

Mike

 

[Richard Brodie]

There may be something wrong with the "re" code in your example,
but I don't know enough about that to help in that area.

There is a stray leading space in it.

 

[Steve Holden]

Hi! Im new to Python and doing exercise found from internet. It is
supposed to evaluate expression given with postfix operator using
Stack() class.

class Stack:
def __init__(self):
self.items = []

def push(self, item):
self.items.append(item)

def pop(self):
return self.items.pop()

def isEmpty(self):
return (self.items == [])

def evaluatePostfix(expr):
import re
tokenList = re.split(" ([^0-9])", expr)

If you add a print statement here I think you will find that the
tokenisation here isn't really what you want:

>>> expr = "56 47 + 2 *"
>>> re.split(" ([^0-9])", expr)

['56 47', '+', ' 2', '*', '']

stack = Stack()
for token in tokenList:
if token == '' or token == ' ':
continue
if token == '+':
sum = stack.pop() + stack.pop()
stack.push(sum)
elif token == '*':
product = stack.pop() * stack.pop()
stack.push(product)
else:
stack.push(int(token))
return stack.pop()

print evaluatePostfix("56 47 + 2 *")

Errormsg:
Traceback (most recent call last):
File "C:\*\postfix1.py", line 31, in <module>
print evaluatePostfix("56 47 + 2 *")
File "C:\*\postfix1.py", line 28, in evaluatePostfix
stack.push(int(token))
ValueError: invalid literal for int() with base 10: '56 47'

How can I avoid the error and get desired result?

I'd try using

tokenList = split(expr)

instead - this has the added benefit of removing the spaces, so you can
simplify your code by removing the case that handles empty tokens and
sapaces, I suspect.

regards
Steve

 

[irstas]

There is a stray leading space in it.

Nah, I'd say there's a stray ([^0-9]) after the space. With just space
in it (same as basic split) one would get the tokens 56, 47, +, 2 and
*. Without the space one would get: ['56', ' ', '47', ' + ', '2', '
*', '']

 

[Matimus]

Hi! Im new to Python and doing exercise found from internet. It is
supposed to evaluate expression given with postfix operator using
Stack() class.

class Stack:
def __init__(self):
self.items = []

def push(self, item):
self.items.append(item)

def pop(self):
return self.items.pop()

def isEmpty(self):
return (self.items == [])

def evaluatePostfix(expr):
import re
tokenList = re.split(" ([^0-9])", expr)
stack = Stack()
for token in tokenList:
if token == '' or token == ' ':
continue
if token == '+':
sum = stack.pop() + stack.pop()
stack.push(sum)
elif token == '*':
product = stack.pop() * stack.pop()
stack.push(product)
else:
stack.push(int(token))
return stack.pop()

print evaluatePostfix("56 47 + 2 *")

Errormsg:
Traceback (most recent call last):
File "C:\*\postfix1.py", line 31, in <module>
print evaluatePostfix("56 47 + 2 *")
File "C:\*\postfix1.py", line 28, in evaluatePostfix
stack.push(int(token))
ValueError: invalid literal for int() with base 10: '56 47'

How can I avoid the error and get desired result?

Two things:
1. The regular expression contains a space, so it is trying to split
on a space character followed by a non number, when you really want to
split on any nonnumber. Remove the space.

tokenList = re.split(" ([^0-9])", expr) -> tokenList =
re.split("([^0-9])", expr)

2. The return statement in evaluatePostfix is part of the for loop, it
will exit on the first loop.

This:

for token in tokenList:
if token == '' or token == ' ':
continue
if token == '+':
sum = stack.pop() + stack.pop()
stack.push(sum)
elif token == '*':
product = stack.pop() * stack.pop()
stack.push(product)
else:
stack.push(int(token))
return stack.pop()

Should Be:

for token in tokenList:
if token == '' or token == ' ':
continue
if token == '+':
sum = stack.pop() + stack.pop()
stack.push(sum)
elif token == '*':
product = stack.pop() * stack.pop()
stack.push(product)
else:
stack.push(int(token))
return stack.pop()

 

[kyosohma]

Hi! Im new to Python and doing exercise found from internet. It is
supposed to evaluate expression given with postfix operator using
Stack() class.

class Stack:
def __init__(self):
self.items = []

def push(self, item):
self.items.append(item)

def pop(self):
return self.items.pop()

def isEmpty(self):
return (self.items == [])

def evaluatePostfix(expr):
import re
tokenList = re.split(" ([^0-9])", expr)

If you add a print statement here I think you will find that the
tokenisation here isn't really what you want:

expr = "56 47 + 2 *"
re.split(" ([^0-9])", expr)

['56 47', '+', ' 2', '*', '']

stack = Stack()
for token in tokenList:
if token == '' or token == ' ':
continue
if token == '+':
sum = stack.pop() + stack.pop()
stack.push(sum)
elif token == '*':
product = stack.pop() * stack.pop()
stack.push(product)
else:
stack.push(int(token))
return stack.pop()

print evaluatePostfix("56 47 + 2 *")

Errormsg:
Traceback (most recent call last):
File "C:\*\postfix1.py", line 31, in <module>
print evaluatePostfix("56 47 + 2 *")
File "C:\*\postfix1.py", line 28, in evaluatePostfix
stack.push(int(token))
ValueError: invalid literal for int() with base 10: '56 47'

How can I avoid the error and get desired result?

I'd try using

tokenList = split(expr)

instead - this has the added benefit of removing the spaces, so you can
simplify your code by removing the case that handles empty tokens and
sapaces, I suspect.

regards
Steve
--
Steve Holden +44 150 684 7255 +1 800 494 3119
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://del.icio.us/steve.holden
Recent Ramblings http://holdenweb.blogspot.com

Steve,

How do you do "tokenList = split(expr)"? There is no builtin called
"split".

Mike

 

[Richard Brodie]

There is a stray leading space in it.

Nah, I'd say there's a stray ([^0-9]) after the space.

If you regard the spaces as being a required part of the postfix
grammar, it would be simpler. But who would design a language
where white space was significant

 

[Terry Reedy]

| Hi! Im new to Python and doing exercise found from internet. It is
| supposed to evaluate expression given with postfix operator using
| Stack() class.
|
| class Stack:
| def __init__(self):
| self.items = []
|
| def push(self, item):
| self.items.append(item)
|
| def pop(self):
| return self.items.pop()
|
| def isEmpty(self):
| return (self.items == [])
|
| def evaluatePostfix(expr):
| import re
| tokenList = re.split(" ([^0-9])", expr)

To see what is wrong, print tokenList.
tokenList = expr.split() is probably what you want

| stack = Stack()
| for token in tokenList:
| if token == '' or token == ' ':
| continue
| if token == '+':
| sum = stack.pop() + stack.pop()
| stack.push(sum)
| elif token == '*':
| product = stack.pop() * stack.pop()
| stack.push(product)
| else:
| stack.push(int(token))
| return stack.pop()
|
| print evaluatePostfix("56 47 + 2 *")
|
| Errormsg:
| Traceback (most recent call last):
| File "C:\*\postfix1.py", line 31, in <module>
| print evaluatePostfix("56 47 + 2 *")
| File "C:\*\postfix1.py", line 28, in evaluatePostfix
| stack.push(int(token))
| ValueError: invalid literal for int() with base 10: '56 47'
|
| How can I avoid the error and get desired result?
| --
| http://mail.python.org/mailman/listinfo/python-list
|

 

[tom]

Ok, I got it running. Thank you!

I removed the space and top of that I had foul indentation in return
statement.

I'll try the approaches you suggest.

 

[Steve Holden]

Steve,

How do you do "tokenList = split(expr)"? There is no builtin called
"split".

Mike

Sorry, that should have been a call to the .split() method of expr, i.e.:

tokenList = expr.split()

regards
Steve