Splitting a list into list of lists based on predicate

Question

(I am aware of this question, but it relates to sequences, which is not my problem here)

Given this input (for example):

let testlist = 
    [  
       \         


        

Answer 1

there is a List.partition function in the F# core library (in case you wanted to implement this just to have it working and not to learn how to write recursive functions yourself). Using this function, you can write this:

> testlist |> List.partition (fun s -> s.StartsWith("*"))
val it : string list * string list =
  (["*text1"; "*text2"; "*text5"; "*text6"; "*text7"], ["text3"; "text4"])

Note that this function returns a tuple instead of returning a list of lists. This is a bit different to what you wanted, but if the predicate returns just true or false, then this makes more sense.

The implementation of partition function that returns tuples is also a bit simpler, so it may be useful for learning purposes:

let partition pred list = 
  // Helper function, which keeps results collected so
  // far in 'accumulator' arguments outTrue and outFalse
  let rec partitionAux list outTrue outFalse =
    match list with 
    | [] -> 
        // We need to reverse the results (as we collected
        // them in the opposite order!)
        List.rev outTrue, List.rev outFalse
    // Append element to one of the lists, depending on 'pred'
    | x::xs when pred x -> partitionAux xs (x::outTrue) outFalse
    | x::xs -> partitionAux xs outTrue (x::outFalse)

  // Run the helper function
  partitionAux list [] []