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digitalmars.D.learn - Generating all combinations of length X in an array

reply "wobbles" <grogan.colin gmail.com> writes:
Hi folks,

While trying to generate all combinations of length X in an array,
I came across the question on stackoverflow. [1]

Theres a couple good answers there, but one that caught my eye 


public static IEnumerable<IEnumerable<T>> Combinations<T>(this 
IEnumerable<T> elements, int k)
{
   return k == 0 ? new[] { new T[0] } :
     elements.SelectMany((e, i) =>
       elements.Skip(i + 1).Combinations(k - 1).Select(c => (new[] 
{e}).Concat(c)));
}


I spent a couple hours trying to translate this to D, but couldnt 
get my head around the SelectMany statement.
I think it's analogous to std.algorithm.map, but it seems quite 
difficult to mimic the behaviour using map.

Anyone with more knowledge and/or skills like to have a crack?

Thanks!

[1] 
http://stackoverflow.com/questions/127704/algorithm-to-return-all-combinations-of-k-elements-from-n
Apr 08 2015
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
wobbles:


 While trying to generate all combinations of length X in an 
 array,
 I came across the question on stackoverflow. [1]

 Theres a couple good answers there, but one that caught my eye 



Often short code is not the best code.

Take a look at the versions here, the usable one is the third:
http://rosettacode.org/wiki/Combinations#D

Bye,
bearophile
Apr 08 2015
parent reply "wobbles" <grogan.colin gmail.com> writes:
On Wednesday, 8 April 2015 at 10:54:45 UTC, bearophile wrote:

 wobbles:


 While trying to generate all combinations of length X in an 
 array,
 I came across the question on stackoverflow. [1]

 Theres a couple good answers there, but one that caught my eye 



 Often short code is not the best code.

 Take a look at the versions here, the usable one is the third:
 http://rosettacode.org/wiki/Combinations#D

 Bye,
 bearophile


Ah, excellent! Dunno why I didnt think of rosettacode before.

Is the 3rd version usable at compile time?

Thanks, all the D stuff on rosettacode is an excellent resource!
Apr 08 2015
next sibling parent reply "wobbles" <grogan.colin gmail.com> writes:
On Wednesday, 8 April 2015 at 11:08:00 UTC, wobbles wrote:

 On Wednesday, 8 April 2015 at 10:54:45 UTC, bearophile wrote:

 wobbles:


 While trying to generate all combinations of length X in an 
 array,
 I came across the question on stackoverflow. [1]

 Theres a couple good answers there, but one that caught my 



 Often short code is not the best code.

 Take a look at the versions here, the usable one is the third:
 http://rosettacode.org/wiki/Combinations#D

 Bye,
 bearophile


 Ah, excellent! Dunno why I didnt think of rosettacode before.

 Is the 3rd version usable at compile time?

 Thanks, all the D stuff on rosettacode is an excellent resource!


Have just tested, it is!
Apr 08 2015
parent "bearophile" <bearophileHUGS lycos.com> writes:
wobbles:


 Have just tested, it is!


But with the current D front-end it's not a good idea to generate 
too many combinations at compile-time. Efficient code doesn't 
save you from bad usages.

Bye,
bearophile
Apr 09 2015
prev sibling parent reply "Messenger" <dont shoot.me> writes:
On Wednesday, 8 April 2015 at 11:08:00 UTC, wobbles wrote:

 On Wednesday, 8 April 2015 at 10:54:45 UTC, bearophile wrote:

 wobbles:


 Thanks, all the D stuff on rosettacode is an excellent resource!


He's likely not the *only* contributor but I think bearophile is 
the big D champion there. <3
Apr 09 2015
parent reply "weaselcat" <weaselcat gmail.com> writes:
On Thursday, 9 April 2015 at 23:21:17 UTC, Messenger wrote:

 On Wednesday, 8 April 2015 at 11:08:00 UTC, wobbles wrote:

 On Wednesday, 8 April 2015 at 10:54:45 UTC, bearophile wrote:

 wobbles:


 Thanks, all the D stuff on rosettacode is an excellent 
 resource!


 He's likely not the *only* contributor but I think bearophile 
 is the big D champion there. <3


Rosetta Code should really be linked somewhere on the site, 
bearophile put a lot of effort into it.
Apr 09 2015
parent "Marc =?UTF-8?B?U2Now7x0eiI=?= <schuetzm gmx.net> writes:
On Thursday, 9 April 2015 at 23:49:22 UTC, weaselcat wrote:

 On Thursday, 9 April 2015 at 23:21:17 UTC, Messenger wrote:

 On Wednesday, 8 April 2015 at 11:08:00 UTC, wobbles wrote:

 On Wednesday, 8 April 2015 at 10:54:45 UTC, bearophile wrote:

 wobbles:


 Thanks, all the D stuff on rosettacode is an excellent 
 resource!


 He's likely not the *only* contributor but I think bearophile 
 is the big D champion there. <3


 Rosetta Code should really be linked somewhere on the site, 
 bearophile put a lot of effort into it.


There are already lots of useful links on the start page of the 
wiki:
http://wiki.dlang.org/The_D_Programming_Language
Apr 10 2015