如何在Perl 6中缩短数组？

How could I cut off an array or an array-reference in Perl 6?

我如何在Perl 6中切断数组或数组引用？

In Perl 5, I can do this:

在Perl 5中，我可以这样做：

my $d = [0 .. 9];
$#$d = 4;

In Perl 6, I get an error if I try this:

在Perl 6中，如果我尝试这个，我会收到错误：

my $d = [0 .. 9];
$d.end = 4; # Cannot modify an immutable Int

This works, but it looks less beautiful than the Perl 5 way and may be expensive:

这可行，但它看起来不如Perl 5方式漂亮，可能很昂贵：

 $d.=splice(0, 5);

3 个解决方案

#1

There is a simple way:

有一个简单的方法：

my $d = [0..9];

$d[5..*] :delete;

That is problematic if the array is an infinite one.

如果阵列是无限的，则存在问题。

$d.splice(5) also has the same problem.

$ d.splice（5）也有同样的问题。

Your best bet is likely to be $d = [ $d[^5] ] in the average case where you may not know anything about the array, and need a mutable Array.

在平均情况下你最好的选择可能是$ d = [$ d [^ 5]]，你可能对阵列一无所知，需要一个可变数组。

If you don't need it to be mutable $d = $d[^5] which returns a List may be better.

如果你不需要它是可变的$ d = $ d [^ 5]，它返回一个List可能会更好。

#2

splice is probably the best choice here, but you can also shorten to five elements using the ^N range constructor shortcut (I call this the "up until" "operator" but I am sure there is a more correct name since it is a constructor of a Range):

splice可能是这里最好的选择，但你也可以使用^ N范围构造函数快捷方式缩短到五个元素（我称之为“直到”“运算符”但我确信有一个更正确的名称，因为它是一个构造函数范围）：

> my $d = [ 0 .. 9 ];
> $d.elems
> 10
> $d = [ $d[^5] ]
[0 1 2 3 4]
> $d.elems
5
> $d
[0 1 2 3 4]

"The caret is ... a prefix operator for constructing numeric ranges starting from zero".
(From the Range documentation)

“插入符号是......用于构造从零开始的数值范围的前缀运算符”。（来自Range文档）

One can argue that perl6 is "perl-ish" in the sense it usually has an explicit version of some operation (using a kind of "predictable" syntax - a method, a routine, and :adverb, etc.) that is understandable if you are not familiar with the language, and then a shortcut-ish variant.

有人可以说perl6是“perl-ish”，因为它通常具有某种操作的显式版本（使用一种“可预测”的语法 - 方法，例程和：副词等），这是可以理解的你不熟悉这种语言，然后是一种快捷方式。

I'm not sure which approach (splice vs. the shortcut vs. using :delete as Brad Gilbert mentions) would have an advantage in speed or memory use. If you run:

我不确定哪种方法（拼接与快捷方式与使用方式：删除方式为Brad Gilbert提及）在速度或内存使用方面具有优势。如果您运行：

perl6 --profile -e 'my $d = [ 0 .. 9 ]; $d=[ $d[^5] ]'
perl6 --profile -e 'my $d = [ 0 .. 9 ]; $d.=splice(0, 5);'

you can see a slight difference. The difference might be more significant if you compared with a real program and workload.

你可以看到一点点差异。如果与实际程序和工作负载进行比较，差异可能会更大。

#3

Another option is using the xx operator:

另一种选择是使用xx运算符：

my $d = [0..9];

$d.pop xx 4;  #-> (9 8 7 6)
say $d;       #-> [0 1 2 3 4 5]

$d = [0..9];

$d.shift xx 5 #-> (0 1 2 3 4)
say $d;       #-> [5 6 7 8 9)

#1