SixShot:

When your input data is sorted, I normally approach it like this:

# Compute average value for each group
my ($prev_key, $cnt, $sum);
foreach my $line (@data) {
    my ($key, $val) = parse_line($line);
    if (defined($prev_key) and $prev_key eq $key) {
        # Same group as last time, accumulate results
        ++$cnt, $sum += $val;
    }
    else {
        # New group starting.  Print results from
        # previous group, then start new one.
        print "Group $prev_key avg=", $sum/$cnt, ", cnt=", $cnt, "\n";
        $prev_key = $key, $cnt=1, $sum=$val;
    }
}
# Print results from last group.
print "Group $prev_key avg=", $sum/$cnt, ", cnt=", $cnt, "\n";
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And when you're data isn't sorted, you can always sort it first!

...roboticus

When your only tool is a hammer, all problems look like your thumb.