The documentation seems to indicate that partialShuffle: Partially shuffles the elements of r such that upon returning r[0..n] is a random subset of r, (which is what I want), but it seems that partialShuffle actually only shuffles the first subset of the range (which you could do probably also do by [0..n].randomShuffle).

This different behaviour was problem created since: https://issues.dlang.org/show_bug.cgi?id=11738. Does anyone know what the intended behaviour is/was?

On Tuesday, 19 May 2015 at 10:00:33 UTC, BlackEdder wrote:
> The documentation seems to indicate that partialShuffle: Partially shuffles the elements of r such that upon returning r[0..n] is a random subset of r, (which is what I want), but it seems that partialShuffle actually only shuffles the first subset of the range (which you could do probably also do by [0..n].randomShuffle).
>
> This different behaviour was problem created since: https://issues.dlang.org/show_bug.cgi?id=11738. Does anyone know what the intended behaviour is/was?

Reading the current documentation and unittests, I now also believe the fix was a mistake.  Reopened the issue for now with a comment: https://issues.dlang.org/show_bug.cgi?id=11738#c2

I hope Joseph Rushton Wakeling looks into it soon.

On Tuesday, 19 May 2015 at 14:31:21 UTC, Ivan Kazmenko wrote:
> On Tuesday, 19 May 2015 at 10:00:33 UTC, BlackEdder wrote:
>> The documentation seems to indicate that partialShuffle: Partially shuffles the elements of r such that upon returning r[0..n] is a random subset of r, (which is what I want), but it seems that partialShuffle actually only shuffles the first subset of the range (which you could do probably also do by [0..n].randomShuffle).
>>
>> This different behaviour was problem created since: https://issues.dlang.org/show_bug.cgi?id=11738. Does anyone know what the intended behaviour is/was?
>
> Reading the current documentation and unittests, I now also believe the fix was a mistake.  Reopened the issue for now with a comment: https://issues.dlang.org/show_bug.cgi?id=11738#c2
>
> I hope Joseph Rushton Wakeling looks into it soon.

Reading the documentation it does appear that the function behaviour is at odds with what is described.  I don't know how I came to that misunderstanding.

In the short term, if you want a randomly-shuffled random subset of a range, you could get it via something like,

    original_range.randomSample(n).array.randomShuffle;

or maybe better

    original_range.randomShuffle.randomSample(n);