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 shows a C# code snippet that is quite nice and short:

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

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 shows a C# code snippet that is quite nice and short:

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

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 shows a C# code snippet that is quite nice and short:
>
> 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!

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 eye shows a C# code snippet that is quite nice and short:
>>
>> 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!

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

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

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.

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