I've inherited a set of scripts and, not being an expert in Perl, there's a particular chunk of code which i'm struggling to understand:

while (<IN>) {
    our(@F) = split(/\s+/, $_, 0);
    push @{$r{join ' ' x 8, @F[0..3]};}, [@F[4, 6]];
    sub END {
        foreach $k (keys %r) {
            my($x, $y);
            map {$x += $$_[0]; $y += $$_[1];} @{$r{$k};};
            my @g = split(/\s+/,$k);
            print OUT "$g[0]\t@g[1]\t@g[2]\t@g[3]\t", $x / scalar(@{$r
+{$k};}), "\t$y\n";
        }
    }
}

[download]

This takes an input file in this format:

1    111    C    T    1    0    6
1    136    G    A    1    0    6
1    136    G    A    1    0    9
1    244    C    CT    1    0    2
1    262    A    G    1    0    2
1    268    A    C    1    0    2
1    268    A    C    1    0    4
1    270    C    T    1    0    2

[download]

Finds all unique entries (based on the first four columns), averages the 5th and sums the 6th.

There's a few functions and pieces of syntax i'm unfamiliar with. Specifically, the use "$$_" (the use of a double $), exactly why 'map' is used here and how this line:

map {$x += $$_[0]; $y += $$_[1];} @{$r{$k};};

[download]

Is producing the desired result in general. If anybody could explain or clarify how this chunk of code works, that would be great, thanks!

In reply to Code clarification - use of map and $$_ by Anonymous Monk

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