Testing for membership speed

Larry Bates lbates at swamisoft.com
Wed Jun 23 16:38:42 CEST 2004


The dictionaries are faster because they are indexed.
Lists/tuples must be searched serially from the
beginning.

Dictionary indexes are hashed and the
hashing algorithm has more to do on a string
than on an integer (see first article link
below for explanation).

You could check into pre-hashing your strings and using
these as integer keys.  It will take longer to build
the dictionary, but searching should be faster.

Here are some articles that might interest you:

http://effbot.org/zone/python-hash.htm

http://mail.python.org/pipermail/python-dev/2003-June/036556.html

http://www.egenix.com/files/python/mxTextTools.html

Searching list or tuple serially from the beginning
should take approximately the same time.  Nothing
about the mutability can help the search speed.

HTH,
Larry Bates
Syscon, Inc.


<brianc at temple.edu> wrote in message
news:mailman.42.1087998564.27577.python-list at python.org...
> I just ran this stuff for my own knowledge. Though it might be
> useful to some other people to know and maybe spark a discussion.
>
> I needed a fast way to test for membership, so naturally the
> choices were the builtin containers: lists, dictionaries, and
> tuples. The following is the test code and results:
>
> import timeit
>
> lst_i=timeit.Timer('random.randrange(10000) in l','import
> random; l=range(10000)')
> dct_i=timeit.Timer('l.has_key(random.randrange(10000))','import
> random; l=dict([(i,None) for i in xrange(10000)])')
> tup_i=timeit.Timer('random.randrange(10000) in l','import
> random; l=tuple(range(10000))')
>
> lst_str=timeit.Timer('md5.md5(str(random.randrange(10000))).hexdigest()
> in l','import random,md5; l=[md5.md5(str(i)).hexdigest() for i
> in xrange(10000)]')
>
dct_str=timeit.Timer('l.has_key(md5.md5(str(random.randrange(10000))).hexdig
est())','import
> random,md5; l=dict([(md5.md5(str(i)).hexdigest(),None) for i
> in xrange(10000)])')
> tup_str=timeit.Timer('md5.md5(str(random.randrange(10000))).hexdigest()
> in l','import random,md5; l=tuple([md5.md5(str(i)).hexdigest()
> for i in xrange(10000)])')
>
> print 'Integer lookup'
> r=lst_i.repeat(100,100); print 'list:',min(r),max(r);
> r=dct_i.repeat(100,100); print 'dict:',min(r),max(r);
> r=tup_i.repeat(100,100); print 'tupl:',min(r),max(r);
> print 'String lookup'
> r=lst_str.repeat(100,100); print 'list:',min(r),max(r);
> r=dct_str.repeat(100,100); print 'dict:',min(r),max(r);
> r=tup_str.repeat(100,100); print 'tupl:',min(r),max(r);
>
> [[[Ran on IRIX64 6.5, 24 processors, 12G Memory, 4G Swap, this
> code only uses one processor at %100 the full length of the run]]]
> Python 2.2.3 (#1, Nov 25 2003, 18:58:21) [C] on irix646-n32
> Type "help", "copyright", "credits" or "license" for more
> information.
> >>> ## working on region in file
> /usr/tmp/python-119959673PMu.py...
> Integer lookup
> list: 0.126830816269 0.160212993622
> dict: 0.00362300872803 0.00385618209839
> tupl: 0.119297981262 0.161748170853
> String lookup
> list: 0.142526865005 0.188524961472
> dict: 0.00711393356323 0.00760197639465
> tupl: 0.143892049789 0.186873912811
> >>>
>
> The results are conclusive, go for dictionaries. But this
> surprised me a little, anyone have insight as to why?
>
> I was also surprised that tuples and lists scored exactly the
> same. I was hoping that immutable tuples might earn it some
> speed over lists.
>
> I will eventually need this for testing strings. So the
> doubling of speed for strings over integers for dictionaries
> is a little alarming. Lists and tuples only saw a modest increase.
>
> Thank you in advance for any clever tricks you suggest.
>
> -Brian
>





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