table =: 5 4 $ i. 20
positions =: 0 2 0 1 2
(0 0;1 2;2 0;3 1;4 2){table
0 6 8 13 18
(i.5),&.> positions
┌───┬───┬───┬───┬───┐
│0 0│1 2│2 0│3 1│4 2│
└───┴───┴───┴───┴───┘
((i.5),&.> positions){table
0 6 8 13 18
2010/5/22 Roman Odaisky <[email protected]>:
> Hi,
>
> I’ve only recently gotten into J. It’s very interesting but I’m often unable
> to find simple solutions to simple problems. I’m now struggling with finding a
> way of taking a table and a vector and picking elements from the table with
> positions in rows according to the vector, that is, the i-th element of the
> vector specifies the element to be taken from the i-th row of the table. How
> can this be done?
>
> For example, given table =: 5 4 $ i. 20 and positions =: 0 2 0 1 2, the result
> should be 0 6 8 13 8:
>
> <0> 1 2 3
> 4 5 <6> 7
> <8> 9 10 11
> 12<13>14 15
> 16 17<18>19
>
> The best I was able to come up with is
>
> pick =: {~ (<"1 @: ,.~ i.@:#)
> table pick positions
> 0 6 8 13 18
>
> which is rather ugly. I expected this to look more like “positions {"something
> table” but nothing of that kind worked.
>
> In Python that would look like this:
>
> def pick(table, positions):
> return [row[p] for row, p in zip(table, positions)]
>
> or in a J-ish point-free way:
>
> pick = compose(partial(starmap, getitem), zip)
>
> It looks like no more than five verbs should be necessary. How can J do this?
>
> --
> TIA
> Roman.
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
--
Björn Helgason, Verkfræðingur
Fornustekkum II
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