The Perl equivalent to a GROUP BY clause in SQL is a hash. You use the parameter of the GROUP BY ... clause as the keys of the hash, and aggregate the value(s) of the rows in the values of the hash.

A rough Perl implementation for a single aggregate (sum) could look like this:

use Scalar::MoreUtils qw(zip); # I'm lazy

sub sum_group_by {
    my ($key_columns, $aggregate_columns, $rows) = @_;

    my %result;

    for my $input_row (@$rows) {
        # Create the synthetic key, assuming that no value will contai
+n a \0:
        my @key_values = @{ $input_row }{ @$key_columns };
        my $key = join "\0", @key_values;

        # If this is a new key, create an new result row for it that c
+ontains the key values:
        $result{ $key } ||= { zip @$key_columns, @key_values };

        for my $aggregate (@$aggregate_columns) {
            $result{ $key }->{ $aggregate } += $input_row->{ $aggregat
+e };
        };
    };

    # Return the result as new rows, ordered asciibetically (because I
+'m lazy) by their key:
    map { $result{ $_ } } sort keys %result;
};

[download]

If you can manage that all your operations return lists sorted by their keys, or can allow an intermediate step that (pre)sorts all inputs according to their keys, you can make the aggregation much simpler by aggregating into a single value and outputting a new row whenever the key (after which you sorted) changes. I have seen no paper or anaylis that measures which is better, pre-sorting the input and then pipelined aggregation or aggregation into a hash and then (potentially sorted) output.

Update: Also consider just using SQL through either DBD::SQLite or DBD::AnyData as a way to manipulate data in a way that is very efficient to write for you, and likely better tested than a hand-rolled aggregation.

In reply to Re: Sum Fields in Array by Corion
in thread Sum Fields in Array by drodinthe559

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.