Path: csiph.com!x330-a1.tempe.blueboxinc.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!weretis.net!feeder1.news.weretis.net!news.mb-net.net!open-news-network.org!.POSTED!not-for-mail
From: Bart Kastermans <bkasterm@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: List comprehension timing difference.
Date: Fri, 02 Sep 2011 07:54:50 -0600
Organization: MB-NET.NET for Open-News-Network e.V.
Lines: 52
Message-ID: <87sjofjbh1.fsf@gmail.com>
References: <87zkinkcht.fsf@gmail.com> <mailman.676.1314925972.27778.python-list@python.org>
NNTP-Posting-Host: kasterma.colorado.edu
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Trace: gwaiyur.mb-net.net 1314971694 1602 128.138.150.65 (2 Sep 2011 13:54:54 GMT)
X-Complaints-To: abuse@open-news-network.org
NNTP-Posting-Date: Fri, 2 Sep 2011 13:54:54 +0000 (UTC)
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)
X-User-ID: U2FsdGVkX18Ijpl7SIJi1OtL+pWPfjHSqNCv28gSLxEjBP9+kotzmzI66+3edWAz
Cancel-Lock: sha1:drLmV4wfl0qJu6xNIR8AVQA2Ceo= sha1:hyY9Q3MxpcNQH4ktSHyD+IOKzUU=
Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:12642

MRAB <python@mrabarnett.plus.com> writes:

> On 02/09/2011 01:35, Bart Kastermans wrote:

>> graph = [[a,b] for a in data for b in data if d(a,b) ==1 and a<  b]

>> graph2 = []
>> for i in range (0, len(data)):
>>      for j in range(0,len(data)):
>>          if d(data[i],data[j]) == 1 and i<  j:
>>              graph2.append ([i,j])

>
> Are they actually equivalent? Does graph == graph2?
>
> The first version (list comprehension) creates a list of pairs of
> values:
>
>     [a, b]
>
> whereas the second version (for loops) creates a list of pairs of
> indexes:
>
>     [i, j]
>
> The second version has subscripting ("data[i]" and "data[j]"), which
> will slow it down.

You are absolutely right.  I had changed the code from the
equivalent:

    graph2 = []
    for i in range (0, len(data)):
        for j in range(0,len(data)):
            if d(data[i],data[j]) == 1 and i < j:
                graph2.append ([data[i],data[j]])

But then also tried the equivalent

    for a in data:
        for b in data:
            if d(a,b) == 1 and a < b:
                graph2.append([a,b])

Which does away with the indexing, and is just about exactly as
fast as the list comprehension.


That'll teach me; I almost didn't ask the question thinking it might 
be silly.  And it was, but I thought it for the wrong reason.  I tell my
students there are no stupid questions, I should listen to myself
more when I do.  Thanks!