Chapter 2 THE BASICS
Before these two lines execute, %sets is empty. Adding to an entry that is not in the hash
yet will cause that entry to spring into existence just-in-time, with a value starting at zero.
(is is autovivication). Aer these two lines have run for the rst time, %sets contains
'Ana' => 3, 'Dave' => 0. (e fat arrow => separates key and value in a Pair.)
1 if $r1 > $r2 {
2 %matches{$p1}++;
3 } else {
4 %matches{$p2}++;
5 }
If $r1 has a larger value than $r2, %matches{$p1} increments by one. If $r1 is not larger
than $r2, %matches{$p2} increments. Just as in the case of +=, if either hash value did not
exist previously, it is autovivied by the increment operation.
$thing++ is short for $thing += 1 or $thing = $thing + 1, with the small exception that
the return value of the expression is $thing before the increment, not the incremented
value. As in many other programming languages, you can use ++ as a prex. en it
returns the incremented value; my $x = 1; say ++$x prints 2.
1 my @sorted = @names.sort({ %sets{$_} }).sort({ %matches{$_} }).reverse;
is line consists of three individually simple steps. An array’s sort method returns a
sorted version of the array’s contents. However, the default sort on an array sorts by its
contents. To print player names in winner-rst order, the code must sort the array by
the scores of the players, not their names. e sort method’s argument is a block used
to transform the array elements (the names of players) to the data by which to sort. e
array items are passed in through the topic variable $ .
You have seen blocks before: both the for loop -> $line { ... } and the if state-
ment worked on blocks. A block is a self-contained piece of Perl 6 code with an optional
signature (the -> $line part). See sec:signatures on page 30 for more information.
e simplest way to sort the players by score would be @names.sort({ %matches{$ } }),
which sorts by number of matches won. However Ana and Dave have both won two
matches. at simple sort doesn’t account for the number of sets won, which is the sec-
ondary criterion to decide who has won the tournament.
When two array items have the same value, sort leaves them in the same order as it
found them. Computer scientists call this a stable sort. e program takes advantage of
10
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