CODEWITHSUNDEEP
f#
kelperg1
kelperg1

Reputation: 81

F# Generating all possible combinations between two lists

I want to generate all possible combinations between two lists. For example, if I had list1: [1,2] and list2: [a,b]. It would produce a list that contains [(1,a),(1,b),(2,a),(2,b)]. I am trying to figure out how to do this with recursion and match expressions. I am currently stuck with this:

let rec combo (a1: 'a list) (a2: 'b list) =
    match a1,a2 with
    | [],[] -> []

Upvotes: 0

Views: 373

Answers (1)

Attila Karoly
Attila Karoly

Reputation: 1031

Divide your task. First, write a function that returns a list of pairs formed from a single value and the items in a list (it'll be your second list).

let rec makePairs x lst acc = 
    match lst with
    | [] -> acc
    | _  -> makePairs x (List.tail lst) (acc @ [(x, List.head lst)])

// makePairs 'a' [5;7] [] yields [('a',5), ('a',7)]

Then, by the same pattern as in makePairs, write a function makeAllPairs lst1 lst2 acc that enumerates the items of the first list and collects the pairs for the item returned from makePairs.

A simpler version of makePairs:

let makePairs2 x lst = List.map (fun b -> (x, b)) lst

Upvotes: 0

Related Questions