• BlackEdder (9/9) May 19 2015 The documentation seems to indicate that partialShuffle:
• Ivan Kazmenko (5/14) May 19 2015 Reading the current documentation and unittests, I now also
• Joseph Rushton Wakeling (9/24) May 20 2015 Reading the documentation it does appear that the function

"BlackEdder" <edder tkwsping.nl> writes:

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?

"Ivan Kazmenko" <gassa mail.ru> writes:

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.

"Joseph Rushton Wakeling" <joseph.wakeling webdrake.net> writes:

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);