问题描述
众所周知,Enumerable.SelectMany
将序列序列扁平化为单个序列.如果我们想要一种方法可以将序列序列的序列展平,等等,该怎么办?
As we all know, Enumerable.SelectMany
flattens a sequence of sequences into a single sequence. What if we wanted a method that could flatten sequences of sequences of sequences, and so on recursively?
我很快想出了一个使用 ICollection
的实现,即热切地评估,但我仍然在摸索如何制作一个懒惰的评估,比如,使用yield
关键字.
I came up quickly with an implementation using an ICollection<T>
, i.e. eagerly evaluated, but I'm still scratching my head as to how to make a lazily-evaluated one, say, using the yield
keyword.
static List<T> Flatten<T>(IEnumerable list) {
var rv = new List<T>();
InnerFlatten(list, rv);
return rv;
}
static void InnerFlatten<T>(IEnumerable list, ICollection<T> acc) {
foreach (var elem in list) {
var collection = elem as IEnumerable;
if (collection != null) {
InnerFlatten(collection, acc);
}
else {
acc.Add((T)elem);
}
}
}
有什么想法吗?欢迎使用任何 .NET 语言的示例.
Any ideas? Examples in any .NET language welcome.
推荐答案
这在具有递归序列表达式的 F# 中是微不足道的.
This is trivial in F# with recursive sequence expressions.
let rec flatten (items: IEnumerable) =
seq {
for x in items do
match x with
| :? 'T as v -> yield v
| :? IEnumerable as e -> yield! flatten e
| _ -> failwithf "Expected IEnumerable or %A" typeof<'T>
}
测试:
// forces 'T list to obj list
let (!) (l: obj list) = l
let y = ![["1";"2"];"3";[!["4";["5"];["6"]];["7"]];"8"]
let z : string list = flatten y |> Seq.toList
// val z : string list = ["1"; "2"; "3"; "4"; "5"; "6"; "7"; "8"]
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