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Fastest way to list combinations
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More options Jul 13 2012, 4:51 pm
Newsgroups: comp.lang.idl-pvwave
From: antar3...@gmail.com
Date: 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
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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More options Jul 13 2012, 5:01 pm
Newsgroups: comp.lang.idl-pvwave
From: antar3...@gmail.com
Date: Fri, 13 Jul 2012 14:01:40 -0700 (PDT)
Local: Fri, Jul 13 2012 5:01 pm
Subject: Re: Fastest way to list combinations

Sorry, instead of small it should say slow...stupid autocorrect... ;)

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More options Jul 13 2012, 5:28 pm
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:

Dunno if it's the fastest and/or best way, but have a look at:
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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More options Jul 15 2012, 11:23 am
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:

You could try COMBIGEN in JBIU.

-Jeremy.

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More options Jul 16 2012, 6:12 am
Newsgroups: comp.lang.idl-pvwave
From: antar3...@gmail.com
Date: Mon, 16 Jul 2012 03:12:31 -0700 (PDT)
Local: Mon, Jul 16 2012 6:12 am
Subject: Re: Fastest way to list combinations

> You could try COMBIGEN in JBIU.

> -Jeremy.

That's the way to go I think! :)

Thanks