Perl小知识点之排序sort

脚本这种东西，就是要常用，否则一段时间不用就生疏了，因此决定时时记一些小知识点，一来回顾一下，二来需要的时候可以迅速获得提示。

Sort by number

You could now write a numeric sort subroutine like this:

sub by_number {
    # a sort subroutine, expect $a and $b
    if ($a < $b) { –1 } elsif ($a > $b) { 1 } else { 0 }
}

To use the sort subroutine, just put its name (without an ampersand) between the keyword sort and the list you’re sorting. This example puts a numerically sorted list of numbers into @result:

my @result = sort by_number @some_numbers;

Notice that you don’t have to do anything in the sort subroutine to declare $a and $b and set their values—and if you did, the subroutine wouldn’t work right. We just let Perl set up $a and $b for us, so all you need to write is the comparison.

In fact, you can make it even simpler (and more efficient).  Perl has a convenient shortcut to use to write it. In this case, you use the spaceship operator (<=>). This operator compares two numbers and returns –1, 0, or 1 as needed to sort them numerically.

sub by_number { $a <=> $b }

Sort by strings

three-way string-comparison operator: cmp. These two are easy to remember and keep straight.

• >=有两种可能的返回值，而<=>有三种可能的返回值，因为>=是两个字符，<=>是三个字符。同样道理，
• ge有两种可能的返回值，而cmp有三种可能的返回值，因为ge有两个字符，而cmp有三个字符。

sub by_code_point { $a cmp $b }
my @strings = sort by_code_point @any_strings;

But you can use cmp to build a more complex sort order, like a case-insensitive sort: 

sub case_insensitive { "L$a" cmp "L$b" }

In this case, you’re comparing the string from $a (forced to lowercase) against the string from $b (forced to lowercase), giving a case-insensitive sort order. 

Hash sort: Sorting a Hash by Value

my %score = ("barney" => 195, "fred" => 205, "dino" => 30);
my @winners = sort by_score keys %score;
sub by_score { $score{$b} <=> $score{$a} }

注意这里$b在$a前面，因为我们希望是按照分数降序排列。

如果有分数相同的怎么办？按照名字字母排序：

my %score = (
    "barney" => 195, "fred" => 205,
    "dino" => 30, "bamm-bamm" => 195,
);

my @winners = sort by_score_and_name keys %score;
sub by_score_and_name {
    $score{$b} <=> $score{$a} # by descending numeric score
    or
    $a cmp $b                 # code point order by name
}

当分数相同是，$score{$b} <=> $score{$a}返回0，因此执行短路操作符or之后的语句:$a cmp $b，按照名字字母排序。

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以上摘自：《Learning Perl 6th Edition》