两表跨产品 [英] Cross product of two lists

问题描述

与扩展功能的列表模块瞎搞。 （我花了相当长的一段发展mapfold - 这螺纹状褶皱蓄能器，而是使用它作为一个参数创建像地图新的价值 - 然后发现，这就是 List.scan_left 一样）的

有关生成测试数据，我需要做的两个列表交叉的产品，这就是我想出了：

  ///执行两个列表交叉的产品，回报元组
让双重交叉L1 L2 =
    让产品LST V2 = List.map（乐趣V1  - ＆GT;（V1，V2））LST
    List.map_concat（产品L1）12
 

这是什么好，或者是有已经有一些更好的方法来做到这一点？

这一个同样的问题：

  ///执行三个列表交叉的产品，回报元组
让crossproduct3 L1 L2 L3 =
    让tuplelist =双重交叉​​L1 L2 //不知道这是最好的方式...
    让产品3 LST2 V3 = List.map（FUN（V1，V2） - ＆GT;（V1，V2，V3））LST2
    List.map_concat（产品3 tuplelist）13
 

解决方案

另一种选择是使用F＃序列的前pressions和这样写：

 让双重交叉L1 L2 =
  序列{为EL1的L1做
          为EL2在12做
            产量EL1，EL2} ;;
 

（实际上，它几乎是同样的事情，你写的是什么，因为为..在..做'序列中前pression可以被看作是map_concat）。这适用于（懒惰）序列，但如果你想用列表来上班，你只是包装内的code [...]，而不是内部的序列{...}。

Messing around with 'extension functions' for the List module. (I spent quite a while developing 'mapfold' - which threads an accumulator like fold, but uses it as a parameter to create new values like map - then discovered that that is what List.scan_left does)

For generation of test data, I needed to do a cross product of two lists, This is what I came up with:

///Perform cross product of two lists, return tuple
let crossproduct l1 l2 =
    let product lst v2 = List.map (fun v1 -> (v1, v2)) lst
    List.map_concat (product l1) l2

Is this any good, or is there already some better way to do this?

Same question for this one:

///Perform cross product of three lists, return tuple
let crossproduct3 l1 l2 l3 =
    let tuplelist = crossproduct l1 l2 //not sure this is the best way...
    let product3 lst2 v3 = List.map (fun (v1, v2) -> (v1, v2, v3)) lst2
    List.map_concat (product3 tuplelist) l3

解决方案

another option is to use F# "sequence expressions" and write something like this:

let crossproduct l1 l2 =
  seq { for el1 in l1 do
          for el2 in l2 do
            yield el1, el2 };;

(actually, it is almost the same thing as what you wrote, because 'for .. in .. do' in sequence expression can be viewed as map_concat). This works with (lazy) sequences, but if you want to work with lists, you'd just wrap the code inside [ ... ] rather than inside seq { ... }.

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