extracting a substring

[b83503104@yahoo.com]

Hi,
I have a bunch of strings like
a53bc_531.txt
a53bc_2285.txt
....
a53bc_359.txt

and I want to extract the numbers 531, 2285, ...,359.

One thing for sure is that these numbers are the ONLY part that is
changing; all the other characters are always fixed.

I know I should use regular expressions, but I'm not familar with
python, so any quick help would help, such as which commands or idioms
to use. Thanks a lot!

[Felipe Almeida Lessa]

Re: extracting a substring

Em Ter, 2006-04-18 às 17:25 -0700, b83503104@yahoo .com escreveu:[color=blue]
> Hi,
> I have a bunch of strings like
> a53bc_531.txt
> a53bc_2285.txt
> ...
> a53bc_359.txt
>
> and I want to extract the numbers 531, 2285, ...,359.[/color]

Some ways:

1) Regular expressions, as you said:[color=blue][color=green][color=darkred]
>>> from re import compile
>>> find = compile("a53bc_ ([1-9]*)\\.txt").find all
>>> find('a53bc_531 .txt\na53bc_228 5.txt\na53bc_35 9.txt')[/color][/color][/color]
['531', '2285', '359']

2) Using ''.split:[color=blue][color=green][color=darkred]
>>> [x.split('.')[0].split('_')[1] for x in 'a53bc_531.txt[/color][/color][/color]
\na53bc_2285.tx t\na53bc_359.tx t'.splitlines()]
['531', '2285', '359']

3) Using indexes (be careful!):[color=blue][color=green][color=darkred]
>>> [x[6:-4] for x in 'a53bc_531.txt\ na53bc_2285.txt[/color][/color][/color]
\na53bc_359.txt '.splitlines()]
['531', '2285', '359']

Measuring speeds:

$ python2.4 -m timeit -s 'from re import compile; find =
compile("a53bc_ ([1-9]*)\\.txt").find all; s = "a53bc_531. txt
\na53bc_2285.tx t\na53bc_359.tx t"' 'find(s)'
100000 loops, best of 3: 3.03 usec per loop

$ python2.4 -m timeit -s 's = "a53bc_531.txt\ na53bc_2285.txt
\na53bc_359.txt \n"[:-1]' "[x.split('.')[0].split('_')[1] for x in
s.splitlines()]"
100000 loops, best of 3: 7.64 usec per loop

$ python2.4 -m timeit -s 's = "a53bc_531.txt\ na53bc_2285.txt
\na53bc_359.txt \n"[:-1]' "[x[6:-4] for x in s.splitlines()]"
100000 loops, best of 3: 2.47 usec per loop


$ python2.4 -m timeit -s 'from re import compile; find =
compile("a53bc_ ([1-9]*)\\.txt").find all; s = ("a53bc_531. txt
\na53bc_2285.tx t\na53bc_359.tx t\n"*1000)[:-1]' 'find(s)'
1000 loops, best of 3: 1.95 msec per loop

$ python2.4 -m timeit -s 's = ("a53bc_531.txt \na53bc_2285.tx t
\na53bc_359.txt \n" * 1000)[:-1]' "[x.split('.')[0].split('_')[1] for x
in s.splitlines()]"
100 loops, best of 3: 6.51 msec per loop

$ python2.4 -m timeit -s 's = ("a53bc_531.txt \na53bc_2285.tx t
\na53bc_359.txt \n" * 1000)[:-1]' "[x[6:-4] for x in s.splitlines()]"
1000 loops, best of 3: 1.53 msec per loop


Summary: using indexes is less powerful than regexps, but faster.

HTH,

--
Felipe.

[Gary Herron]

Re: extracting a substring

b83503104@yahoo .com wrote:
[color=blue]
>Hi,
>I have a bunch of strings like
>a53bc_531.tx t
>a53bc_2285.t xt
>...
>a53bc_359.tx t
>
>and I want to extract the numbers 531, 2285, ...,359.
>
>One thing for sure is that these numbers are the ONLY part that is
>changing; all the other characters are always fixed.
>
>I know I should use regular expressions, but I'm not familar with
>python, so any quick help would help, such as which commands or idioms
>to use. Thanks a lot!
>
>
>[/color]
Try this:
[color=blue][color=green][color=darkred]
>>> import re
>>> pattern = re.compile("a53 bc_([0-9]*).txt")
>>>
>>> s = "a53bc_531. txt"
>>> match = pattern.match(s )
>>> if match:[/color][/color][/color]
.... print int(match.group (1))
.... else:
.... print "No match"
....
531[color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Hope that helps,
Gary Herron

[Dale Strickland-Clark]

Re: extracting a substring

You don't need a regex for this, as long as the prefix and suffix are fixed
lengths, the following will do:
[color=blue][color=green]
>> "a53bc_531. txt"[6:-4][/color][/color]
'531'[color=blue][color=green][color=darkred]
>>> "a53bc_2285.txt "[6:-4][/color][/color][/color]
'2285'



b83503104@yahoo .com wrote:
[color=blue]
> Hi,
> I have a bunch of strings like
> a53bc_531.txt
> a53bc_2285.txt
> ...
> a53bc_359.txt
>
> and I want to extract the numbers 531, 2285, ...,359.
>
> One thing for sure is that these numbers are the ONLY part that is
> changing; all the other characters are always fixed.
>
> I know I should use regular expressions, but I'm not familar with
> python, so any quick help would help, such as which commands or idioms
> to use. Thanks a lot![/color]

--
Dale Strickland-Clark
Riverhall Systems - www.riverhall.co.uk

[Kent Johnson]

Re: extracting a substring

b83503104@yahoo .com wrote:[color=blue]
> Hi,
> I have a bunch of strings like
> a53bc_531.txt
> a53bc_2285.txt
> ...
> a53bc_359.txt
>
> and I want to extract the numbers 531, 2285, ...,359.
>
> One thing for sure is that these numbers are the ONLY part that is
> changing; all the other characters are always fixed.[/color]

In that case a fixed slice will do what you want:

In [1]: s='a53bc_531.tx t'

In [2]: s[6:-4]
Out[2]: '531'

Kent

[rx]

Re: extracting a substring

[color=blue]
> and I want to extract the numbers 531, 2285, ...,359.
>
> One thing for sure is that these numbers are the ONLY part that is
> changing; all the other characters are always fixed.
>[/color]

I'm not sure about what you mean by "always fixed" but I guess it means that
you have n files with a fixed start and a changing ending, and m files with
a fixed start and a changing ending, ....

import re
filenames=['ac99_124.txt', 'ac99_344.txt', 'ac99_445.txt']
numbers=[]
for i in filenames:
numbers.append( int(re.compile( '[^_]*_(?P<number>[^.]*).txt').match( i).group('numbe r')))



this sets numbers to: [124, 344, 445]