[Chicago-talk] Array Slices - of columns

[Steven Lembark]

> If I have a 10x10 array of arrays like:
>
> [40 8 79 8 73 71 77 35 12 67]
> [4 79 10 90 45 17 30 29 65 86]
> [56 86 5 34 23 79 17 83 24 85]
> [58 85 46 53 15 31 34 4 14 15]
> [68 15 97 21 81 95 71 82 26 90]
> [59 70 11 82 39 25 36 84 88 13]
> [66 6 85 1 84 17 27 58 60 94]
> [34 10 38 90 32 0 56 14 23 70]
> [71 23 5 97 4 71 27 20 11 87]
> [5 8 23 30 97 16 10 93 16 67]
>
> Is there any notation I can use to get "column 1", i.e. $a[0][0], $a[1][0],
> $a[2][0]... without looping over the whole structure
>
> I need to find the max value in each column
>
> I know how to do it in a loop, but I thought maybe there was some neato way
> in a slice.

Not really: "slices" are sequential within the
array. Since you have a collection of separate
arrays a slice does not apply.

If you have to do this regularly, an alternative
is storing the data in a single list and taking
every Nth item:

    @big_array[ 0, 10, 20, 30, ... ]

will take the first item out of a 10x10 array,

    @big_array [ 0 .. 9 ]

will take the first row.

This is more-or-less the way that languages with
multi-dimensinal arrays handle the issue, but is
a pain to manage yourself by hand if the language
doesn't deal with stride issues for you.

It also requires that you pre-size the array, which
isn't always easy or effecient.

If you regularly have to slice the data, it may 
be effective to store it in a hash:

    $data{ $x, $y } = $value;

This also helps in cases where you regularly have
sparse data. You can "slice" the data by factoring
out the interesting keys:

    grep { /$x ; .*/x } keys %data;

will grab a given x value, 

    grep { /.* $; $y/x } keys %data;

will grab a given y value.

That or you can buffer the keys onto arrays when
they are first used and then just use:


    my @col_valz = max @data{ @{ $col_keyz[$col] } };

or

    my @row_valz = max @data{ @{ $row_keyz[$row] } };

For a moderately small matrix or somewhat sparse
matrix that is frequently sliced this actually
works pretty well -- at the expense of setting up
the column and row key arrays once.

Otherwise, as usual, map is your friend :-)

-- 
Steven Lembark                                            85-09 90th St.
Workhorse Computing                                 Woodhaven, NY, 11421
lembark at wrkhors.com                                      +1 888 359 3508