Your code does something like 
if (X) {
    $Znx = ...;
} elsif (Y) {
    $Znx = ...;
}

[download]
If neither X nor Y is true, $Znx does not get assigned a value, so it remains undefined. Perl treats that as zero, which may or may not be what you want.

In reply to Re: calculate distance between atoms and a target atom by educated_foo
in thread calculate distance between atoms and a target atom by mishimakaz

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