Frequently, when processing rows returned by a DB query, we need to group sets of rows. We often want to display a single header for the group and/or calculate some aggregate values. Here is an abstract example.

@a = (['a',1],['a',2],['b',3],['b',4]); # "db read"

my $old;
my $total = 0;                          # init
foreach my $row (@a) {
        if ($old && $old ne $row->[0]) {
                print "$old: $total\n"; # finish row block
                $total = 0;             # init 
        }    
        $total += $row->[1];            # processrow block
        $old = $row->[0];               # check variable
}
print "$old: $total\n";                 # finish row block
[download]

Bad things:

• the first finish appears in a non-intuitive place -- at the very beginning of the loop!
• init and finish sections are repeated! In the real code, these sections are both a dozen lines, and they constantly get out-of-sync: a major source of bugs.
• needing to keep track of the $old (last group) is annoying.
• This gets very messy if there are nested groups, e.g. if you want to total by group and by subgroup.

But, I'd like not to:

• have to iterate through the array with a counter variable

Anyone developed a clean solution for this? Any advice would be appreciated.

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