As a mental exercise, I wanted to see if I could use PDL
's matrix functions to do this. My idea was to transpose the matrix, add it to itself, then subtract the original diagonal. This would extend it's usefullness to huge arrays. I got as far as
adding the transpose to itself, and getting the original diagonal, which would be then be zero padded, and subtracted from the result of the matrix addition.
This might be a way to go, if you want to use a module.
my $a = pdl([
[1, , , ],
[4, -5, , ],
[7, -8, -9, ],
[17, -18, -90, 114],
my $b = transpose($a);
my $c = $a+ $b;
my $d = $a->diagonal(0,1);
print "$d\nNeeds to be padded with zeros and subtracted from \$c\n";
print "I'll leave that to you. :-)\n";
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