[Pdl-general] How do you dice based on a boolean matrix?

[Eric Wheeler]

Hello all!

I would like to get a diced/sliced/indexed piddle that is derived from a 
larger matrix based on rows indicated by a mask.

For example, lets say I have a mask and I want to dice a matrix's rows
where the mask has a value of 1 (in any location):

        pdl> p $mask = random(10)->transpose > 0.5

        [
         [0]
         [0]
         [1]
         [0]
         [1]
         [1]
         [1]
         [0]
         [1]
         [0]
        ]

and lets say that this is the origin matrix I want to dice:

        pdl> p $target = sequence(5, 10)

        [
         [ 0  1  2  3  4]
         [ 5  6  7  8  9]
         [10 11 12 13 14]
         [15 16 17 18 19]
         [20 21 22 23 24]
         [25 26 27 28 29]
         [30 31 32 33 34]
         [35 36 37 38 39]
         [40 41 42 43 44]
         [45 46 47 48 49]
        ]

I would like to have a result that has only the rows where the mask is
marked with a 1, like this:

        $result = filter_somehow_from($target by $mask);
        [
         [30 31 32 33 34]
         [35 36 37 38 39]
         [40 41 42 43 44]
         [45 46 47 48 49]
        ]

so I can modify the result in-place, like square it:

        $result **= 2;

         [ 900  961 1024 1089 1156]
         [1225 1296 1369 1444 1521]
         [1600 1681 1764 1849 1936]
         [2025 2116 2209 2304 2401]

so the `**= 2` will modify to the original matrix:

        [
         [ 0  1  2  3  4]
         [ 5  6  7  8  9]
         [10 11 12 13 14]
         [15 16 17 18 19]
         [20 21 22 23 24]
         [25 26 27 28 29]
         [ 900  961 1024 1089 1156]
         [1225 1296 1369 1444 1521]
         [1600 1681 1764 1849 1936]
         [2025 2116 2209 2304 2401]
        ]


I know I could do something with ->slice to modify in-place, but ':,6:9' is not 
$mask:

        pdl> p $target->slice(':,6:9') **= 2

        [
         [ 900  961 1024 1089 1156]
         [1225 1296 1369 1444 1521]
         [1600 1681 1764 1849 1936]
         [2025 2116 2209 2304 2401]
        ]

        pdl> p $target

        [
         [   0    1    2    3    4]
         [   5    6    7    8    9]
         [  10   11   12   13   14]
         [  15   16   17   18   19]
         [  20   21   22   23   24]
         [  25   26   27   28   29]
         [ 900  961 1024 1089 1156]
         [1225 1296 1369 1444 1521]
         [1600 1681 1764 1849 1936]
         [2025 2116 2209 2304 2401]
        ]

So $mask defines the rows I need to modify, and $mask could have 1's in
any location, and those are the locations I need to twiddle. 

Ideas?

Thanks for your help!

--
Eric Wheeler


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