Fastest way to list combinations
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Newsgroups: comp.lang.idl-pvwave
From: antar3...@gmail.comDate: Fri, 13 Jul 2012 13:51:41 -0700 (PDT)
Local: Fri, Jul 13 2012 4:51 pm
Subject: Fastest way to list combinations
Hi
I have to solve a problem which includes listing all possible combinations to form a triplet for a given number of objects.
Say you have the objects:
a,b,c,d,e, so in total 5 which gives you 5!/(2!*3!) = 10 possibilities to list them without any doubles:
abc
abd
abe
acd
ace
ade
bcd
bce
bde
cde
I have solved this problem for any number (well up to some computer limit anyway) in three FOR loops but for large numbers this is rather small...
can you think of a better way than using loops to list all these possibilites?
thanks!!
:)
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Newsgroups: comp.lang.idl-pvwave
From: antar3...@gmail.comDate: Fri, 13 Jul 2012 14:01:40 -0700 (PDT)
Local: Fri, Jul 13 2012 5:01 pm
Subject: Re: Fastest way to list combinations
On Friday, July 13, 2012 10:51:41 PM UTC+2, (unknown) wrote:
> Hi > I have to solve a problem which includes listing all possible combinations to form a triplet for a given number of objects.
> Say you have the objects:
> a,b,c,d,e, so in total 5 which gives you 5!/(2!*3!) = 10 possibilities to list them without any doubles:
> abc
> abd
> abe
> acd
> ace
> ade
> bcd
> bce
> bde
> cde
> I have solved this problem for any number (well up to some computer limit anyway) in three FOR loops but for large numbers this is rather small...
> can you think of a better way than using loops to list all these possibilites?
> thanks!!
> :)
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Newsgroups: comp.lang.idl-pvwave
From: Paul van Delst <paul.vande...@noaa.gov>Date: Fri, 13 Jul 2012 17:28:29 -0400
Local: Fri, Jul 13 2012 5:28 pm
Subject: Re: Fastest way to list combinations
Hello,
On 07/13/12 16:51, antar3...@gmail.com wrote:
> Hi > I have to solve a problem which includes listing all possible combinations to form a triplet for a given number of objects.
> Say you have the objects:
> a,b,c,d,e, so in total 5 which gives you 5!/(2!*3!) = 10 possibilities to list them without any doubles:
> abc
> abd
> abe
> acd
> ace
> ade
> bcd
> bce
> bde
> cde
> I have solved this problem for any number (well up to some computer limit anyway) in three FOR loops but for large numbers this is rather small...
> can you think of a better way than using loops to list all these possibilites?
> thanks!!
> :)
http://ftp.emc.ncep.noaa.gov/jcsda/CRTM/idl/
There should be three files.
If you look at the test_combination.pro you will see how to use it.
For example, getting a list of all the combos of three letters from 26:
IDL> t=systime(1) & Test_Combination, 26, 3 & print, systime(1)-t
...lots and lots of output listing the combinations....
22 23 25
22 24 25
23 24 25
Total combinations: 2600
0.032832861
IDL>
You could use this sort of output to generate index arrays to extract your combination of letters from string arrays.
Anyhoo, hope they're useful.
cheers,
paulv
p.s. There may be a native solution for this as well. I wrote those routines a while back... perhaps even as an
exercise. Can't recall.
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Newsgroups: comp.lang.idl-pvwave
From: Jeremy Bailin <astroco...@gmail.com>Date: Sun, 15 Jul 2012 12:53:26 -0230
Local: Sun, Jul 15 2012 11:23 am
Subject: Re: Fastest way to list combinations
On 7/13/12 6:21 PM, antar3...@gmail.com wrote:
> Hi
> I have to solve a problem which includes listing all possible combinations to form a triplet for a given number of objects.
> Say you have the objects:
> a,b,c,d,e, so in total 5 which gives you 5!/(2!*3!) = 10 possibilities to list them without any doubles:
> abc
> abd
> abe
> acd
> ace
> ade
> bcd
> bce
> bde
> cde
> I have solved this problem for any number (well up to some computer limit anyway) in three FOR loops but for large numbers this is rather small...
> can you think of a better way than using loops to list all these possibilites?
> thanks!!
> :)
http://astroconst.org/jbiu/jbiu-doc/math/combigen.html
-Jeremy.
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Newsgroups: comp.lang.idl-pvwave
From: antar3...@gmail.comDate: Mon, 16 Jul 2012 03:12:31 -0700 (PDT)
Local: Mon, Jul 16 2012 6:12 am
Subject: Re: Fastest way to list combinations
That's the way to go I think! :)
Thanks
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