CODEWITHSUNDEEP
listfunctional-programmingsmlml
user5910724
user5910724

Reputation:

How to check if x value exists in SML List

I need to write an SML function that takes in as input a list of tuples (x and y coordinates) and an integer value. The function needs to return true if the integer is an x value in the list and false otherwise. For example if the list was:

val list = [(1,1),(2,4),(3,9),(4,16)];

The function would behave as follows:

memberOf(2, list) = true
memberOf(4, list) = true
memberOf(9, list) = false

Can somebody help me write a function that accomplishes this? I only need to test if the x value is in the list, the y value of the tuple is unimportant.

Thank you!

Upvotes: 1

Views: 7724

Answers (2)

Michael Norrish
Michael Norrish

Reputation: 412

fun member_of (item, list) = List.exists (fn (x,y) => x = item) list

Upvotes: 2

Geo Angelopoulos
Geo Angelopoulos

Reputation: 1187

You can access the first argument of your tuple using #1 tuple and the second one with #2 tuple. So you check if the item exists either as the first argument or the second of the tuple list until you reach the end of your list.

(* fn : int * (int * int) list -> bool *)
fun exists_in (item: int, mylist: (int*int) list) =
    if null mylist
    then false
    else if #1 (hd mylist) = item orelse #2 (hd mylist) = item
    then true
    else exists_in (item, tl mylist)

Upvotes: 1

Related Questions