F # I want to filter my output
I have a problem which I simplified for this question.
Let's say I have 2 lists. The first is actually a list of classes, but for that, let's say it just represents a list of ints (2,4,6,8,10). I have another ints list that represents flags, indicating if I want to include / exclude the corresponding values from the first set.
(This is not the best example, but should be sufficient to help me solve my real problem.)
let set1 = [2;4;6;8;10]
let set2 = [1;0;0;1;1]
My desired output :
[2;8;10]
This is my code:
let solution =
List.map2 (fun a b ->
match b with
| 1 -> a
| _ -> 0
) set1 set2
this outputs the following output:
val solution : int list = [2; 0; 0; 8; 10]
How can I filter out these unwanted zeros?
Instead, | _ -> 0
I ideally want to return zero and then filter out all zeros.
Your help will be much appreciated!
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This seems reasonable:
let filterWith set2 set1 =
List.zip set1 set2
|> List.filter (fun (_,x) -> x=1)
|> List.map fst
using:
let set1 = [2;4;6;8;10]
let set2 = [1;0;0;1;1]
set1 |> filterWith set1
if you decide to use the list bool
for your own set2
, it might get a little nicer:
let filterWith set2 set1 =
List.zip set1 set2
|> List.filter snd
|> List.map fst
using:
let set1 = [2;4;6;8;10]
let set2 = [true;false;false;true;true]
set1 |> filterWith set1
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Here's another version using the fold function and a predicate to maintain a common flag. I had fun with flags :)
let filterByFlag pred l flags =
List.zip l flags
|> List.fold (fun s (x,flag) -> if pred(flag) then x::s else s) []
|> List.rev
let l = [2;4;6;8;10]
let flags = ["👍";"👎";"👎";"👍";"👍"]
filterByFlag (fun t -> t = "👍") l flags
>val it : int list = [2; 8; 10]
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I'll add 3 options: :)
let set1 = [2;4;6;8;10]
let set2 = [1;0;0;1;1]
let filterWith2 (set1:int list) (set2:int list) =
[0..set1.Length-1]
|> List.choose (fun i ->
match set2.[i] with
| 1 -> Some set1.[i]
| _ -> None)
let filterWith3 (set1:int list) (set2:int list) =
List.foldBack2(fun x y acc -> if y=1 then x::acc else acc) set1 set2 []
open System.Linq
let filterWith4 (set1:int list) (set2:int list) =
set1.Where(fun _ i -> set2.[i]=1) |> List.ofSeq
filterWith2 set1 set2 |> printfn "%A"
filterWith3 set1 set2 |> printfn "%A"
filterWith4 set1 set2 |> printfn "%A"
Printing
[2; 8; 10]
[2; 8; 10]
[2; 8; 10]
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