partialShuffle only shuffles subset.

[BlackEdder via Digitalmars-d-learn]

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?

Re: partialShuffle only shuffles subset.

[Ivan Kazmenko via Digitalmars-d-learn]

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.

Re: partialShuffle only shuffles subset.

[Joseph Rushton Wakeling via Digitalmars-d-learn]

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);