Accessing a specific member in a F# tuple

Question

In F# code I have a tuple:

let myWife=("Tijana",32)

I want to access each member of the tuple separately. For instance this what I want to achieve by I can't

Console.WriteLine("My wife is {0} and her age is {1}",myWife[0],myWife[1])

This code doesn't obviously work, by I think you can gather what I want to achieve.

It doesn't work because the PC is scared of spitting out your wife age. — Kafo, Mar 24 '17 at 18:48

score 91 · Accepted Answer · edited Feb 08 '15 at 16:24

91

You want to prevent your wife from aging by making her age immutable? :)

For a tuple that contains only two members, you can fst and snd to extract the members of the pair.

let wifeName = fst myWife;
let wifeAge = snd myWife;

For longer tuples, you'll have to unpack the tuple into other variables. For instance,

let _, age = myWife;;
let name, age = myWife;;

edited Feb 08 '15 at 16:24

david.barkhuizen

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answered Mar 01 '09 at 18:33

Curt Hagenlocher

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score 24 · Answer 2 · answered Mar 01 '09 at 22:05

Another quite useful thing is that pattern matching (just like when extracting elements using "let" binding) can be used in other situations, for example when writing a function:

let writePerson1 person =
  let name, age = person
  printfn "name = %s, age = %d" name age

// instead of deconstructing the tuple using 'let', 
// we can do it in the declaration of parameters
let writePerson2 (name, age) = 
  printfn "name = %s, age = %d" name age

// in both cases, the call is the same
writePerson1 ("Joe", 20)
writePerson2 ("Joe", 20)

score 17 · Answer 3 · answered Jul 16 '10 at 14:32

17

You can use the function fst to get the first element, and snd to get the second ekement. You can also write your own 'third' function:

let third (_, _, c) = c

Read more here: F# Language reference, Tuples

answered Jul 16 '10 at 14:32

Johan Franzen

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+1 but surely `trd` or `tri` would represent more idiomatic naming :) – Ruben Bartelink Dec 01 '13 at 02:17

CodeMonkey · Answer 4 · 2017-02-02T09:53:43.160

You can also write an unpack function for a certain length:

let unpack4 tup4 ind =
    match ind, tup4 with
    | 0, (a,_,_,_) -> a
    | 1, (_,b,_,_) -> b
    | 2, (_,_,c,_) -> c
    | 3, (_,_,_,d) -> d
    | _, _ -> failwith (sprintf "Trying to access item %i of tuple with 4 entries." ind)

or

let unpack4 tup4 ind =
    let (a, b, c, d) = tup4
    match ind with
    | 0 -> a
    | 1 -> b
    | 2 -> c
    | 3 -> d
    | _ -> failwith (sprintf "Trying to access item %i of tuple with 4 entries." ind)

Accessing a specific member in a F# tuple

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