list - 操作 Fsharp 列表

Question

好的，最近几天我一直在玩 F#，发现一些教程在网上流传（没有解决方案！）如果我有以下列表 -

let new = [
 (1808,"RS");
 (1974,"UE");
 (1066,"UE");
 (3005,"RS");
 (2007,"UE");
 (2012,"UE");
 ]

我将如何过滤此列表以首先显示 RS 组中的项目数，然后显示 UE？我在想 List.filter，List.length，不太确定从这两个中去哪里以获得每个组的特定数字。谢谢你的帮助

Answer 1

一般来说，分组操作很容易由Seq.groupBy处理。

如果您只想计算每个组中的项目数，Seq.countBy是要走的路。

[ (1808,"RS");
  (1974,"UE");
  (1066,"UE");
  (3005,"RS");
  (2007,"UE");
  (2012,"UE"); ]
// 'countBy' returns a sequence of pairs denoting unique keys and their numbers of occurrences
// 'snd' means that we try to pick a key as the *second* element in each tuple
|> Seq.countBy snd
// convert results back to an F# list
|> Seq.toList

// val it : (string * int) list = [("RS", 2); ("UE", 4)]