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Exercise Answers
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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);


Previous: A.6 Chapter 7, Regular ExpressionsLearning PerlNext: A.8 Chapter 9, Miscellaneous Control Structures
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