是否可以在地图块中使用数组的索引？

Question

我想在映射例程中使用数组的索引。例如，这个 Raku 代码：

raku -e 'my @a = "First", "Second", "Third", "First"; say @a.map({ "Index of $_ is: ;" })'

打印：

(Index of First is: ; Index of Second is: ; Index of Third is: ; Index of First is: ;)

是否可以获取数组的索引，例如：

(Index of First is: 0; Index of Second is: 1; Index of Third is: 2; Index of First is: 3;)

谢谢！

Answer 1

有

.kv

来生成“键”和值。对于数组，键是索引 0、1、2...

 >>> my @a = "First", "Second", "Third", "First";
[First Second Third First]

>>> @a.kv
(0 First 1 Second 2 Third 3 First)

这是一个长度为2N的平坦序列；我们可以通过显式签名告诉

map

一次获取 2 件事：

>>> @a.kv.map(-> $index, $value { "Index of $value is $index;" })
(Index of First is 0; Index of Second is 1; Index of Third is 2; Index of First is 3;)

或通过占位符变量：

>>> @a.kv.map({ "Index of $^value is $^index;" })
(Index of First is 0; Index of Second is 1; Index of Third is 2; Index of First is 3;)

由于 index 按字典顺序出现在 value 之前，因此其工作方式与之前的显式签名情况相同，即在幕后为我们生成相同的签名。我们可以看到这个：

>>> &{ $^value, $^index }
-> $index, $value { #`(Block|140736965530488) ... }

请注意，重要的是变量名称的 Unicode 顺序，而不是出现的顺序。

---

本着 TIMTOWDI 的精神，一些替代方案（IMO 不是更好）：

• 依靠匿名标量状态变量$产生0、1、2...的流

# Limitation: generated anew per closure and per mention,
# so cannot use, e.g., in string interpolations with closure and more than once
>>> @a.map({ "Index of $_ is " ~ $++ ~ ";" })
(Index of First is 0; Index of Second is 1; Index of Third is 2; Index of First is 3;)

• 使用

  .pairs

  （

  .kv

  的表弟）来生成“键=>值”对而不是平面列表

>>> @a.pairs
(0 => First 1 => Second 2 => Third 3 => First)

# Reach over .key & .value
>>> @a.pairs.map({ "Index of $_.value() is $_.key();" })
(Index of First is 0; Index of Second is 1; Index of Third is 2; Index of First is 3;)

# Unpack with signature
>>> @a.pairs.map(-> (:key($index), :$value) { "Index of $value is $index;" })
(Index of First is 0; Index of Second is 1; Index of Third is 2; Index of First is 3;)

• 我们可以映射数组的键，而不是映射数组的值本身：

# Array.keys produces 0, 1, 2...
>>> @a.keys.map({ "Index of @a[$_] is $_;" })
(Index of First is 0; Index of Second is 1; Index of Third is 2; Index of First is 3;)

# ^@a is equivalent to ^@a.elems === 0 ..^ @a.elems, i.e., 0, 1, 2... again
>>> ^@a .map({ "Index of @a[$_] is $_;" })
(Index of First is 0; Index of Second is 1; Index of Third is 2; Index of First is 3;)

我认为最后一个是最不惯用的。