Learning Perl, Second Edition

A.7. Chapter 8

1. Here's one way to do it:

   						sub card {
       my %card_map;
       @card_map{1..9} = qw(
           one two three four five six seven eight nine
       );
   
       my($num) = @_;
       if ($card_map{$num}) {
          return $card_map{$num};
       } else {
         return $num;
       }
   }
   # driver routine:
   while (<>) {
       chomp;
       print "card of $_ is ", &card($_), "\n";
   }

   The &card subroutine (so named because it returns a cardinal name for a given value) begins by initializing a constant hash called %card_map. This array has values such that $card_map{6} is six, making it fairly easy to do the mapping.

   The if statement determines if the value is in range by looking the number up in the hash: if there's a corresponding hash element, the test is true, so that array element is returned. If there's no corresponding element (such as when $num is 11 or -4), the value returned from the hash lookup is undef, so the else-branch of the if statement is executed, returning the original number. You can also replace that entire if statement with the single expression:

   $card_map{$num} || $num;

   If the value on the left of the || is true, it's the value for the entire expression, which then gets returned. If it's false (such as when $num is out of range), the right side of the || operator is evaluated, returning $num as the return value.

   The driver routine takes successive lines, chomping off their newlines, and hands them one at a time to the &card routine, printing the result.

2. Here's one way to do it:

   sub card { ...; } # from previous problem print "Enter first number: ...