lenti234 [~/perltest]>more count.pl
#!/user/bin/perl -w
#

# assign values to three arrays
@aa_names = qw (alanine arginine aspartate asparagine cysteine glutamate
glutamine glycine histidine isoleucine leucine lysine methionine
phenylalanine proline serine threonine tyrosine tryptophan valine);

@aa_1codes = qw (A R D N C E Q G H I L K M F P S T Y W V);

@aa_3codes = qw (Ala Arg Asp Asn Cys Glu Gln Gly His Ile Leu Lys Met
Phe Pro Ser Thr Tyr Trp Val);

# initialize (set to initial value (zero)) counters
$total = 0; # counter for total number of codes
$good = 0; # counter for number of legal codes

# start main program loop
$again = "y";
until ($again ne "y") {

# get code from user, convert to upper case
print "\nEnter a single letter amino acid code: ";
chomp ($code = <STDIN>);
$code = uc ($code);
$total++; # increment total counter

# check if it is a legal code
$i = 0;
while (defined ($aa_1codes[$i]) and $code ne $aa_1codes[$i]) {
 $i ++;
  }

# if legal, print code, 3 letter code, and aa name, else print message
if (defined ($aa_1codes[$i])) {
print $aa_names[$i], "  ", $aa_3codes[$i], "  ", $aa_1codes[$i], "\n\n";
$good++; #increment good counter
                              }
else {
print "$code is not a legal code.\n\n";
     }

# end main program loop
print "Again (y/n)? ";
chomp ($again = <STDIN>);
$again = lc ($again);
$again = substr ($again,0,1);
}

# print summary statistics
print "$total codes entered \n";
print "$good (",int(($good*1000/$total)+0.5)/10,"%) legal codes \n\n";
exit;
lenti235 [~/perltest]>perl count.pl

Enter a single letter amino acid code: A
alanine  Ala  A

Again (y/n)? y

Enter a single letter amino acid code: b
B is not a legal code.

Again (y/n)? y

Enter a single letter amino acid code: c
cysteine  Cys  C

Again (y/n)? n
3 codes entered
2 (66.7%) legal codes

lenti236 [~/perltest]>