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}) { $card_map{$num}; # return value } else { $num; # return value } } # 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
; consequently, the mapping is easy.
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 $num
as the return value.
The driver routine takes successive lines, chomp
ing off their newlines, and hands them one at a time to the &card
routine, printing the result.
sub card { ...; } # from previous problem print "Enter first number: "; chomp($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 $message
is then built up by calling &card
three times, once for each value, and once for the sum.
After the message is constructed, its first character is uppercased by the case-shifting backslash operator \u
. The message is then printed.
sub card { my %card_map; @card_map{0..9} = qw( zero one two three four five six seven eight nine ); my($num) = @_; my($negative); if ($num < 0) { $negative = "negative "; $num = - $num; } if ($card_map{$num}) { $negative . $card_map{$num}; # return value } else { $negative . $num; # return value } }
Here, we've given the %card_map
array a name for 0.
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 0. After this if
statement, the value of $num
is always non-negative, 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);