#!/bin/perl
##################################################
# This program accepts as input a sequence 
# position and outputs the codon starting at that 
# position. The first nt is position 1.
#
# This problem requires familiarity with basic I/O,
# and scalar operators.
#
##################################################

# Initalization
$sequence = "GTAACCGAACCTAGGAGTTAAA";
print "DNA sequence: $sequence\n";

# Get Sequence Position from user (use $sequencePosition)
print "Enter sequence position: ";
$input = <STDIN>;
chomp($input);
$sequencePosition = $input;

$seqLen = length ($sequence);
if ($sequencePosition < 1) { 
   print "Sequence Position must be between 1 and ", $seqLen-2,"\n";
   exit;
}

if ($sequencePosition > $seqLen -2) {
   print "Sequence Position must be between 1 and ", $seqLen-2,"\n";
   exit;
}

# Print the codon (from $sequence) starting at $position
$codon = substr($sequence,$sequencePosition-1,3);

print "The codon at position $sequencePosition is: $codon\n";

exit;