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Re: generating permutations

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On 24/05/2016 05:36, luser droog wrote:
> On Monday, May 23, 2016 at 8:09:33 AM UTC-5, Bart wrote:

>> For example, take this is 2D array A of 3 rows of 5 columns:
>>
>> ((3 1 4 1 5)
>>   (9 2 6 5 3)
>>   (5 8 9 7 9)

>> So adding ;2 4 might yield:
>>
>> ((1 1)
>>   (8 7)
>>   (2 8))
>>
>> Nice and compact, perhaps the shape of your RHS on an assignment. But
>> take a look at the original with just these highlighted:
>>
>> ((- 1 - 1 -)
>>   (- - - - -)
>>   (- 8 - 7 -)
>>   (- 2 - 8 -))
>>
>> You selected 4 sub-arrays of the original: 2 individual elements, and 2
>> column vectors of two elements each, arranged as a 2x2 array of
>> sub-arrays; this is what I meant before. You're defining a rectangular
>> matrix of sub-arrays using this slicing scheme.

> Yes, all correct. And other APLs already have this power AFAIUI.

I tried adding this feature on one of my languages, using functions to 
do the slicing rather than changing the language.

It was surprisingly difficult!

As I chose to implement it (bear in mind I only spent an hour or so):

* It only handles rvalues (lvalues require language changes)

* The selection set for each dimension doesn't allow duplicates (they 
are ignored) and the indices are always sorted

* For a 2D array, the row-selector highlights a set of rows, and the 
column-selector highlights a set of columns. Where those intersect, 
those are the elements that are selected.

* Selected elements are copied (using a shallow copy in this case) to 
form a new 2D array consisting of the chosen elements in a compact 
non-sparse format.

* It needs a dedicated routine for each number of dimensions.

(The best I can do with lvalues, without changing anything deep, is to 
instead return a compact selection array consisting of /pointers/ to 
each element, but then code must explicitly dereference those pointers.)

-- 
Bartc

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generating permutations luser droog <mijoryx@yahoo.com> - 2016-05-22 02:46 -0700
  Re: generating permutations bart4858@gmail.com - 2016-05-22 05:36 -0700
    Re: generating permutations luser droog <mijoryx@yahoo.com> - 2016-05-22 14:40 -0700
      Re: generating permutations Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-05-22 22:58 +0100
      Re: generating permutations BartC <bc@freeuk.com> - 2016-05-23 14:09 +0100
        Re: generating permutations luser droog <mijoryx@yahoo.com> - 2016-05-23 21:36 -0700
          Re: generating permutations BartC <bc@freeuk.com> - 2016-05-24 12:56 +0100
            Re: generating permutations luser droog <mijoryx@yahoo.com> - 2016-05-25 19:03 -0700
              Re: generating permutations BartC <bc@freeuk.com> - 2016-05-26 13:08 +0100
                Re: generating permutations luser droog <mijoryx@yahoo.com> - 2016-05-26 08:35 -0700

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