lenti214 [~/perltest]>more hash2.pl
#!/usr/bin/perl -w
#

# assign keys and values to a hash
# keys = 1 letter codes, values = aa names
%aa_codes = ("A" => "alanine", "R" => "arginine", "D" => "aspartate",
"N" => "asparagine", "C" => "cysteine", "E" => "glutamate",
"Q" => "glutamine", "G" => "glycine", "H" => "histidine",
"I" => "isoleucine", "L" => "leucine", "K" => "lysine",
"M" => "methionine", "F" => "phenylalanine", "P" => "proline",
"S" => "serine", "T" => "threonine", "Y" => "tyrosine",
"W" => "tryptophan", "V" => "valine");

# assign keys and values to a hash
# keys = 1 letter codes, values = codes for aa properties
%aa_props = ("A" => "tsn", "R" => "pc+", "D" => "scp-",
"N" => "sp", "C" => "stap", "E" => "cp-",
"Q" => "p", "G" => "st", "H" => "n+cpr",
"I" => "an", "L" => "an", "K" => "+cpn",
"M" => "n", "F" => "rn", "P" => "s",
"S" => "tsp", "T" => "tsp", "Y" => "rnp",
"W" => "rnp", "V" => "ans");

# keys = aa property codes, values = full name of properties
%prop_names = ("a" => "aliphatic", "s" => "small", "t" => "tiny",
"r" => "aromatic", "n" => "non-polar", "p" => "polar",
"+" => "positive", "-" => "negative", "c" => "charged");

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

# get code from user
print "\nEnter a single letter amino acid code: ";
chomp ($code = <STDIN>);
$code = uc ($code);

# if legal print code, aa name and properties, else print message
if (defined ($aa_codes{$code})) {
print $aa_codes{$code}, "  ", $code, "  ", $aa_props{$code}, "\n\n";
                              }
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);
}

exit;

lenti215 [~/perltest]>perl hash2.pl
Name "main::prop_names" used only once: possible typo at hash2.pl line 25.

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

Again (y/n)? y

Enter a single letter amino acid code: h
histidine  H  n+cpr

Again (y/n)? y

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

Again (y/n)? n
lenti216 [~/perltest]>