CODEWITHSUNDEEP
recursionf#tuples
Shinji-san
Shinji-san

Reputation: 1001

Creating a tuple from two lists of two different types

I'm trying to create a list of tuples from two different lists where each head from the two lists will become a tuple(ex: headList1 and headList2 become a tuple), and this continues for each element) and I'm getting type errors in F# and I'm not sure what I'm doing wrong. I tried taking away the parentheses putting in braces etc and still to no avail. 

let rec pairToTuple letter count = // assume that each list is the same, will return a list with (letter, count) in itself
  match letter with
  | [] -> [()]
  | e1::rest1, e2::rest2 -> let tup = (e1, e2) 
                            tup::(pairToTuple rest1 rest2 )

ex: (a,b,c) and (10,20,30) would become [ (a,10);(b,20);(c,30) ]

/home/codio/workspace/program/Program.fs(180,5): error FS0001: This expression was expected to have type    ''a list'    but here has type    ''b * 'c'
[/home/codio/workspace/program/program.fsproj]

Upvotes: 0

Views: 383

Answers (2)

gileCAD
gileCAD

Reputation: 2493

What about List.zip function?

> List.zip ["a"; "b"; "c"; "d"] [1; 2; 3; 4];;
val it : (string * int) list = [("a", 1); ("b", 2); ("c", 3); ("d", 4)]

Upvotes: 2

Nghia Bui
Nghia Bui

Reputation: 3784

You can simple use the List.map2 function:

let pairToTuple letterList countList =
    List.map2 (fun letter count -> (letter, count)) letterList countList

Or you can write it shorter in F# idioms:

let pairToTuple = List.map2 (fun letter count -> (letter, count))

If your question is an exercise and you don't want to use List.map2, then:

let rec pairToTuple letterList countList =
    match letterList, countList with
    | [], _ -> []
    | _, [] -> []
    | letter :: res1, count :: res2 ->
        (letter, count) :: pairToTuple res1 res2

Upvotes: 1

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