I would be interested to see the C routine. J Forum msgs
do not seem to permit attachments, so you may have to
post the code in a J wiki page.
Since the J interpreter is written in C, in theory it should
be the case that a C function would be no slower than
a J function. In this particular case, I think the speed of J
relative to C comes from
- Working with the entire file at once rather than byte-by-byte
- Well-crafted routines, in particular for E., that get to "see"
large chunks of data at a time. It's not necessarily that
the J implementors are better C programmers than you,
but that (probably) more effort had been spent in the
C functions in the J interpreter.
Some comments on your J function:
- better to use 6!:2 than 6!:1 for benchmarking (higher resolution)
- #x is the same as 0{$x
Finally, I am interested to know how much time it took you
to write the fully debugged J function vs. the fully debugged
C function.
----- Original Message -----
From: Robert Cyr <[EMAIL PROTECTED]>
Date: Thursday, October 2, 2008 9:59
Subject: [Jprogramming] performance : J or C ?
To: Programming forum <[email protected]>
> Good afternoon all,
>
> Having this seemingly well written routine in C, I wrote
> it's equivalent in
> J, wishing to estimate the penalty for using J.
>
> I am amazed and a bit mystified to find J nearly twice as
> fast: on this
> Acer One with Gentoo Linux.
>
>
> - C
> test: 2.0 secs
> - J first
> test 1.13 secs
> - J second test: 0.71 secs
>
> >From a 38 Meg file, find all diff rent IP addresses following a given
> label. In this test the number of addresses is about
> 16000, the required
> different addresses is 7.
>
> Attached is the C routine, and a 20K sample of the 38 meg data file.
>
> Is the J use of mapped files the trick in favor of J? In C
> the entire file
> is scanned, byte by byte.*
>
>
> J routine:*
> require'jmf'
> require'files dir'
>
> find=: 4 : 0
> NB. x is the label preceding the IP adress,
> NB. y is the file name
> NB. produces a list of different IP adresses.
> deb=. 6!:1 ''
> JCHAR map_jmf_ 'file';y
> nos=. I. x E. file
> n=:(nos+/(0{$x)+i.16){file
> unmapall_jmf_ ''
> a=.'time in secs to read 38 megs, find IP
> adrs. etc. ',6j2":secs=. (6!:1
> '')-deb
> a=.a,:'diffreent IP adresses'
> a,~.((n i."1 (' ')){."0 1 (n))
> )
>
> findTxt=: 3 : 0
> NB. for testing find.
> fn=:'/media/KINGSTON/logParse/messages.2'
> NB.fn=:'G:\logParse\messages.2'
> txt =.' rhost='
> txt find fn
> )
>
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