randomSample

[Meta via Digitalmars-d-learn]

On Sunday, 18 May 2014 at 04:19:05 UTC, David Held wrote:
> How do I get an array from randomSample()?
>
> int[] source = [ ... ];
> int[] sample = randomSample(source, 3);
>
> src\main.d(30): Error: cannot implicitly convert expression 
> (randomSample(source, 3u)) of type RandomSample!(int[], void) 
> to int[]
>
> I get that RandomSample is a struct which implements the 
> necessary interface to work with foreach.  But the fact that 
> this struct is considered a proprietary implementation detail 
> of std.random is a constant source of frustration for me.  
> Quite a few D libraries use this trick, but the opacity of 
> these data structures make it impossible for users to know 
> exactly how to use them outside of the one or two examples 
> given in the documentation.  I don't know how to improve the 
> situation, other than documenting them explicitly rather than 
> treating them as a magical black box.
>
> In the mean time, can anyone suggest a solution to the above, 
> short of actually iterating over it?  I suppose someone will 
> say to use map!().
>
> Dave

You need to use the function array from std.array.

import std.array;

int[] source = [ ... ];
int[] sample = randomSample(source, 3).array();

Array has the ability to turn any range into an array, and it's 
generally how you convert from these structs (which generally 
implement a range interface) returned by most range-based 
functions in D. It can be a bit confusing at first, but returning 
a range struct is much more flexible than returning a simple 
array of values.