Path: csiph.com!usenet.pasdenom.info!gegeweb.org!de-l.enfer-du-nord.net!feeder1.enfer-du-nord.net!feeds.phibee-telecom.net!newsfeed.xs4all.nl!newsfeed4.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
MIME-Version: 1.0
In-Reply-To: <51ebb88e$0$29971$c3e8da3$5496439d@news.astraweb.com>
References: <6bf4d298-b425-4357-9c1a-192e6e6cd9f0@googlegroups.com> <9a207133-1f52-414a-bbbd-581bcee8dc93@googlegroups.com> <51ebb88e$0$29971$c3e8da3$5496439d@news.astraweb.com>
Date: Sun, 21 Jul 2013 20:48:46 +1000
Subject: Re: How can I make this piece of code even faster?
From: Chris Angelico <rosuav@gmail.com>
To: python-list@python.org
Content-Type: text/plain; charset=ISO-8859-1
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.4947.1374403735.3114.python-list@python.org>
Lines: 34
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:51004

On Sun, Jul 21, 2013 at 8:31 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> On Sun, 21 Jul 2013 03:19:24 -0700, pablobarhamalzas wrote:
>
>> Thank's for all the replies! I've tried some of the imporovements you
>> suggested (using math.exp() and sum() or math.fsum()). None of that made
>> the code faster, because they are functions you are calling lots of
>> times, and function calling is quite time expensive (same as x**(1/2) is
>> faster than math.sqrt(x)).
>
> You are *badly* mistaken. Not only is sqrt more accurate, but it is also
> much faster.
>
>
> [steve@ando ~]$ python3.3 -m timeit -s "x = 2.357e7" "x**0.5"
> 1000000 loops, best of 3: 0.319 usec per loop
> [steve@ando ~]$ python3.3 -m timeit -s "x = 2.357e7" -s "from math import
> sqrt" "sqrt(x)"
> 10000000 loops, best of 3: 0.172 usec per loop

Don't forget the cost of attribute lookup, which adds 50% to the
sqrt() figure. Still faster than exponentiation. (Figures from Python
3.4 alpha, but unlikely to be materially different.)

rosuav@sikorsky:~$ python3 -m timeit -s "x = 2.357e7" "x**0.5"
1000000 loops, best of 3: 0.239 usec per loop
rosuav@sikorsky:~$ python3 -m timeit -s "x = 2.357e7" -s "from math
import sqrt" "sqrt(x)"
10000000 loops, best of 3: 0.102 usec per loop
rosuav@sikorsky:~$ python3 -m timeit -s "x = 2.357e7" -s "import math"
"math.sqrt(x)"
10000000 loops, best of 3: 0.155 usec per loop

ChrisA