Perl6入门指南：核心概念与操作符解析 - CSDN文库

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Chapter 2 THE BASICS

+= $r1 means increase the value in the variable on the le by $r1. In the ﬁrst iteration

%sets{$p1} is not yet set, so it defaults to a special value called Any. e addition and

incrementing operators treat Any as a number with the value of zero.

Before these two lines execute, %sets is empty. Adding to an entry not in the hash will

cause that entry to spring into existence just-in-time, with a value starting at zero. (is

is autoviviﬁcation). Aer these two lines have run for the ﬁrst 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 autoviviﬁed 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. If, as you can do in many other programming languages, you can use ++ as a

preﬁx, 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-ﬁrst 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 signatures 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.

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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

this property of Perl 6’s sort to achieve the goal by sorting twice: ﬁrst by the number of

sets won (the secondary criterion), then by the number of matches won.

Aer the ﬁrst sorting step, the names are in the order Beth Charlie Dave Ana. Aer

the second sorting step, it’s still the same, because no one has won fewer matches but

more sets than someone else. Such a situation is entirely possible, especially at larger

tournaments.

sort sorts in ascending order, from smallest to largest. is is the opposite of the desired

order. erefore, the code calls the .reverse method on the result of the second sort, and

stores the ﬁnal list in @sorted.

1 for @sorted -> $n {

2 say "$n has won %matches{$n} matches and %sets{$n} sets";

3 }

To print out the players and their scores, the code loops over @sorted, setting $n to the

name of each player in turn. Read this code as “For each element of sorted, set $n to

the element, then execute the contents of the following block.” say prints its arguments

to the standard output (the screen, normally), followed by a newline. (Use print if you

don’t want the newline at the end.)

When you run the program, you’ll see that say doesn’t print the contents of that string

verbatim. In place of $n it prints the contents of $n– the names of players stored in $n.

is automatic substitution of code with its contents is interpolation. is interpolation

happens only in strings delimited by double quotes "...". Single quoted strings '...'

do not interpolate:

1 my $names = 'things';

2 say 'Do not call me $names'; # Do not call me $names

3 say "Do not call me $names"; # Do not call me things

Double quoted strings in Perl 6 can interpolate variables with the $ sigil as well as blocks

of code in curly braces. Since any arbitrary Perl code can appear within curly braces,

Arrays and Hashes may be interpolated by placing them within curly braces.

Arrays within curly braces are interpolated with a single space character between each

item. Hashes within curly braces are interpolated as a series of lines. Each line will con-

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