如何在OCaml中取两个清单的乘积? [英] How to take product of two list in OCaml?

问题描述

我有两个列表:

let a = ["a";"b"];
let b = ["c";"d"];

我想要一个输出列表c，例如:

I want an output list c such as :

c = ["a";"c";"a";"d";"b";"c";"b";"d"];

如何在ocaml中做到这一点，因为列表是不可变的?我是新来的.

How to do it in ocaml as lists are immutable? I am new to it.

推荐答案

您将返回一个新列表.如果您真的对列表的<笛卡尔直乘积感兴趣，那么就足够了:

You would return a new list. If you really are interested in the cartesian product of the lists, then this should be enough:

let cartesian l l' = 
  List.concat (List.map (fun e -> List.map (fun e' -> (e,e')) l') l)

# cartesian ["a";"b"] ["c";"d"];;
- : (string * string) list = [("a", "c"); ("a", "d"); ("b", "c"); ("b", "d")]

如果您需要这种奇怪的扁平结构，则可以使用其他列表串联.

If you need that strange flat structure instead, you can use an additional list concatenation.

let flat_cartesian l l' = 
  List.concat (List.concat (
    List.map (fun e -> List.map (fun e' -> [e;e']) l') l))

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