UPDATE: Please disregard this suggestion. I did not notice the explicit reverse.

Your book ran the loop backwards to reverse the string. You can run it forward, but you must reverse the concatenation: 

$complement = $base . $complement;

[download]
Bill

In reply to Re: Trying to use substring to find a reverse complement in DNA, by BillKSmith
in thread Trying to use substring to find a reverse complement in DNA, by tryingnottofailmytes

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