Hi. Obviously I'm new here. I'm a biology student playing in a CS class this semester and I have a final on Tuesday. Theres a lot of stuff I don't understand, so I'm hoping you guys can help me out. I'm trying to get the reverse complement of a string of DNA w/o using s// or tr/// and HAVE to use substring. Its a practice problem in our book that I'm working on. What I have so far 
my $dna = 'atgAAAAAAAAAAGGGGGGGGGGTTTTTTTTCCCCCCCCCCCCagtcatc';
$dna = uc$dna;
print "$dna\n";
my $revDNA= reverse $dna;
print "$revDNA\n";
my $length = length$revDNA;
my $complement = '';
for (my $i=0;$i>$length-1; $i++)
{
    my $base= substr($revDNA,$i,1);
    if ($base eq 'A') { 
    $base = 'T'; 
    }
    elsif ($base eq 'T') { 
    $base = 'U'; 
    }
    elsif ($base eq 'G') { 
    $base = 'C'; 
    }
    elsif ($base eq 'C') { 
    $base = 'G'; 
    }
     $complement.=$base;
     
}
print $complement, "\n";
exit;

[download]
I don't really understand why this isn't working I'm not sure what the differences is between my and the posted solution, which has the for loop go backwards instead of forwards.... Can anyone offer any help?

In reply to Trying to use substring to find a reverse complement in DNA, by tryingnottofailmytes

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