A.7 Chapter 8, Functions

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: "; chomp($first = <STDIN>); print "Enter second number: "; chomp($second = <STDIN>); $message = card($first) 
   
   . " plus " .     card($second) . " equals " .     card($first+$second) . ".\n"; print "\u$message";

   The first two print statements prompt for two numbers, with the immediately following statements reading the values into $first and $second .

   A string called $message is then built up by calling &card three times, once for each value and once for the sum.

   Once the message is constructed, its first character is uppercased by the case-shifting backslash operator \u . The message is then printed.

3. Here's one way to do it:

   sub card {     my %card_map;     @card_map{0..9} = qw(         zero one two three four five six seven eight nine     );      my($num) = @_;     my($negative);     if ($num < 0) {         $negative = "negative ";         $num = - $num;     }     if ($card_map{$num}) {         return $negative . $card_map{$num};     } else {         return $negative . $num;     } }

   Here, we've given the %card_map array a name for zero.

   The first if statement inverts the sign of $num and sets $negative to the word negative, if the number is found to be less than zero. After this if statement, the value of $num is always nonnegative, but we will have an appropriate prefix string in $negative .

   The second if statement determines if the (now positive) $num is within the hash. If so, the resulting hash value is appended to the prefix within $negative and returned. If not, the value within $negative is attached to the original number.

   That last if statement can be replaced with the expression:

     $negative . ($card_map{$num} || $num);