The condition on your for loop is the wrong way round, you need to use less than not greater than, here:-

$i>$length-1;

If you want to know how to fix these things for yourself try the basic debugging checklist

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

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