Manipulating an Fsharp list

Question

Ok so i've been playing around with F# the last few days and found some tutorials kicking around online (no solutions!) If i had the following list-

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

How would I filter this list to show first the number of items in group RS, and secondly UE? I'm thinking List.filter, List.length, not too sure where to go from these two to get specific numbers for each group. Thanks for any help

pad · Accepted Answer · 2013-02-23T20:05:02.303

2

In general, grouping operations are easily handled by Seq.groupBy.

If you only want to count number of items in each group, Seq.countBy is the way to go.

[ (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)]

edited Feb 23 '13 at 20:05

answered Feb 23 '13 at 19:54

pad

41,040
7
92
166

Can you explain to me what the functions Seq.count By snd and Seq.toList all essentially mean, in layman's terms? – user2093122 Feb 23 '13 at 20:00
I added comments to the code fragment. You can follow the links in my answer to understand these operations. – pad Feb 23 '13 at 20:05
are there any ways to do this same operation via List.filter and list.length? – user2093122 Feb 25 '13 at 19:22

Manipulating an Fsharp list

1 Answers1