Question

I need code which creates boolean list depended on inserted list and count. For example, when the user gives List[0,1,2,3,4,5,6,7,8,9,10] and count=2 then then code makes bool List [true,false,true,false,true,false,true,false,true,false,true]

When count = 3 then it will make bool List [true, false, false, true, false, false, true, false, false, true, false]

If count = 4 then [true, false, false, false, true, false, false, false, true, false] and so on….

I´ve written following code but i think, that this code is wrong, i am new to f# so i need you help. Thanks.

   let Group (s1 : List) (c : int) =
        let lenght = List.length(s1)
        (lenght)
           let rec MakeBool (count : int) (boolist : List) =
                while lenght > 0 do
                    if lenght % count = 0 then boolist = true::boolist 
                    if lenght % count <> 0 then boolist = false::boolist    
                    lenght = lenght - 1
                    MakeBool count boolist

Answer 1

Using high-order function (recommended):

let group ls c = 
    ls |> List.mapi (fun i _ -> i%c = 0)

Rolling your own function:

let group ls c =
 let length = List.length ls    
 let rec makeBool count acc =
  if count = length then acc // Come to the end of ls, return the accummulator
  elif count%c=0 then // Satisfy the condition, prepend true to the accummulator
    makeBool (count+1) (true::acc)
  else  // Otherwise prepend false to the accummulator
    makeBool (count+1) (false::acc)
 List.rev (makeBool 0 []) // The accummulator is in backward order, reverse it