How to avoid index out of range error?

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October 22nd, 2003, 12:29 AM

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How to avoid index out of range error?

How to switch places two names in a long file name?
I have a directory with lots of files (don't know how many), where I want
to switch first name with second but leave the remaing names as they are. So if the file name is like that

First Second Description it should become
Second First Description

I know this must be really simple so this is what I tried to do.

Code:

import os, string 
count = 0 
for title in os.listdir('c:/files'): 
   a = string.split(title) 
   b = len(a[0]) + len(a[1]) + 1 
   newstring = a[1] + ', ' + a[0] + title[-(len(title)-b):] 
   os.rename('c:/files/' + title,'c:/files/' + newstring) 
   count = count + 1 
print count

Well this code worked if I had a couple of files but failed with quantity
with "IndexError: list index out of range" error. Please tell me how to fix this or rewrite it better.

Thanks
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October 22nd, 2003, 07:15 AM

FLNHST

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Code:


test_list = ['This','is','a','Test']

if test_list.__contains('is') == 1:
      print 'True'
else:
      print 'False'

October 22nd, 2003, 07:16 AM

FLNHST

404 Error

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oops, error in the reply from me, __contains must be __contains__

October 22nd, 2003, 08:19 AM

netytan

Hello World :)

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I've reworked your code, this new script has been tested it with up to 30 files without a glitch so it should work fine!? I'm guessing the reason your program fails at a sertain point is that not all files in the directory fit a standard partern (so can't be parsed in the same way)

The following is a line by line explination of this script:

1. shebang pointing at python
3. import the os module
5. set the default directory (./ is the dir which our program is in)
6. get a list of all the files in dir which fit a basic pattern (you could make this more accurate using regex)
8. loop though each filename in our list ('filenames')
9. split the filename on only the first two spaces (using the built-in string method split())
10. create the new format for our file using string formating
11. rename 'name'
13. print the number of files in filenames using len
15. wait for the user to press enter before exiting..

Code:

#!/usr/bin/env python

import os

directory = './'
filenames = filter(lambda x: x.count(' ') >= 2, os.listdir(directory))

for name in filenames: 
	name1 = name.split(' ', 2)  
	name2 = '%s %s %s' % (name1[1], name1[0], name1[2])
	os.rename(directory + name, directory + name2)

print len(filenames)

raw_input('Press ENTER to exit..')

FLN, in Python 2.x+ You can do the same thing with the 'in' keyword i.e 'is' in list.. just thought i'd mention it

Hope this helps,
Mark.

__________________
programming language development: www.netytan.com – Hula

Last edited by netytan : October 22nd, 2003 at 08:28 AM.

October 22nd, 2003, 08:35 AM

cvchen

Hi, I'm Calvin

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here's what i did

i tried to keep your code format where i could

Code:

import os, string 
count = 0 
for title in os.listdir('c:/files'): 
   a = string.split(title)
   newstring = '%s %s %s'%(a[1],a[0],string.join(a[2:]))
   os.rename('c:/files/' + title,'c:/files/' + newstring) 
   count = count + 1 
print count

i used string.join() because i did not want to assume that all your files would have three tokens when split by spaces, perhaps there are some files that have more than two spaces? if not, if all your files only have three tokens when split by spaces, i'd use netytan's code instead... but if there are files that have more than three tokens when split by spaces, string.join() as i used it is the way to go... it works either way, but if there are only three space delimited tokens then string.join() is a bit overkill.

October 22nd, 2003, 08:38 AM

cvchen

Hi, I'm Calvin

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oh yeah

count = count + 1

can be replaced by

count += 1

i don't know... somehow that's just more visually pleasing to me lol...

you know, (this has nothing to do with your program or question) i really am curious as to why python doesn't have an icrementer and decrementer operator like ++ or --

count++ to me looks better still than count+=1

ok i'm done rambling

October 22nd, 2003, 08:54 AM

netytan

Hello World :)

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Me too

i've asked that question to myself a few times, the answer i was given by strike (if i remember this correctly) was that +=/-= is more flexable than ++/-- but personally, i like the whole icrementer and decrementer operator idea.. ah well

In CV's program he used string.join(), but you can also use 'string object'.join() which will do the exactly same thing

. I havn't touched the string module in ages!

For more info on this you can type help(str) at Pythons interactive prompt, this will give you a list of the internals and methods for the string type, this also works for other types/modules/functions i.e. lists, tuples and dicts etc.

Mark.

October 22nd, 2003, 09:46 AM

cvchen

Hi, I'm Calvin

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yeah...

and i suppose flexibility is pythonic, or something

October 22nd, 2003, 09:55 AM

netytan

Hello World :)

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uh huh, but theres no halm in having both surly!? Ah well, maybe one day they'll put one in the mix..

Mark.

#10

October 22nd, 2003, 07:42 PM

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Thanks Mark and cvchen for your help. This is one of the most helpful forums I have visited. Now it's time to learn from your code and see how it works.
Again Thanks.
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#11

October 28th, 2003, 09:36 AM

lazy_yogi

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The main things I dislike about perl (and some other languages, but epecially prevalient in perl) is that there are so many ways to do the same task.

You can come back after a few months/years and look at perl code and have to look up every second line and figure out what it means.

The thing I love about python is that in many cases there is only one way to do something.
It makes the code almost immediately readable since there is only one way to do it.

Cheers,
Eli

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