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## to Custom Set in the OCaml Track

Published at Feb 12 2021 · 0 comments
Instructions
Test suite
Solution

Create a custom set type.

Sometimes it is necessary to define a custom data structure of some type, like a set. In this exercise you will define your own set. How it works internally doesn't matter, as long as it behaves like a set of unique elements.

## Getting Started

1. For library documentation, follow Useful OCaml resources.

## Running Tests

A `Makefile` is provided with a default target to compile your solution and run the tests. At the command line, type:

``````make
``````

## Submitting Incomplete Solutions

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## Feedback, Issues, Pull Requests

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If you have feedback about an exercise, or want to help implementing a new one, head over there and create an issue or submit a PR. We welcome new contributors!

### test.ml

``````(* custom-set - 1.3.0 *)
open OUnit2

module type EXPECTED = sig
type t
val of_list : int list -> t
val is_empty : t -> bool
val is_member : t -> int -> bool
val is_subset : t -> t -> bool
val is_disjoint: t -> t -> bool
val equal : t -> t -> bool
val add : t -> int -> t
val intersect : t -> t -> t
val difference : t -> t -> t
val union : t -> t -> t
end

module CSet : EXPECTED = Custom_set.Make(struct
type t = int
let compare a b = compare (a mod 10) (b mod 10)
end)

let ae exp got _test_ctxt = assert_equal exp got
let aec exp got _test_ctxt = assert_equal true (CSet.equal (CSet.of_list exp) got)

let tests = [
(* Returns true if the set contains no elements *)
"sets with no elements are empty" >::
ae true (CSet.is_empty (CSet.of_list []));
"sets with elements are not empty" >::
ae false (CSet.is_empty (CSet.of_list ));
(* Sets can report if they contain an element *)
"nothing is contained in an empty set" >::
ae false (CSet.is_member (CSet.of_list []) 1);
"when the element is in the set" >::
ae true (CSet.is_member (CSet.of_list [1; 2; 3]) 1);
"when the element is not in the set" >::
ae false (CSet.is_member (CSet.of_list [1; 2; 3]) 4);
(* A set is a subset if all of its elements are contained in the other set *)
"empty set is a subset of another empty set" >::
ae true (CSet.is_subset (CSet.of_list []) (CSet.of_list []));
"empty set is a subset of non-empty set" >::
ae true (CSet.is_subset (CSet.of_list []) (CSet.of_list ));
"non-empty set is not a subset of empty set" >::
ae false (CSet.is_subset (CSet.of_list ) (CSet.of_list []));
"set is a subset of set with exact same elements" >::
ae true (CSet.is_subset (CSet.of_list [1; 2; 3]) (CSet.of_list [1; 2; 3]));
"set is a subset of larger set with same elements" >::
ae true (CSet.is_subset (CSet.of_list [1; 2; 3]) (CSet.of_list [4; 1; 2; 3]));
"set is not a subset of set that does not contain its elements" >::
ae false (CSet.is_subset (CSet.of_list [1; 2; 3]) (CSet.of_list [4; 1; 3]));
(* Sets are disjoint if they share no elements *)
"the empty set is disjoint with itself" >::
ae true (CSet.is_disjoint (CSet.of_list []) (CSet.of_list []));
"empty set is disjoint with non-empty set" >::
ae true (CSet.is_disjoint (CSet.of_list []) (CSet.of_list ));
"non-empty set is disjoint with empty set" >::
ae true (CSet.is_disjoint (CSet.of_list ) (CSet.of_list []));
"sets are not disjoint if they share an element" >::
ae false (CSet.is_disjoint (CSet.of_list [1; 2]) (CSet.of_list [2; 3]));
"sets are disjoint if they share no elements" >::
ae true (CSet.is_disjoint (CSet.of_list [1; 2]) (CSet.of_list [3; 4]));
(* Sets with the same elements are equal *)
"empty sets are equal" >::
ae true (CSet.equal (CSet.of_list []) (CSet.of_list []));
"empty set is not equal to non-empty set" >::
ae false (CSet.equal (CSet.of_list []) (CSet.of_list [1; 2; 3]));
"non-empty set is not equal to empty set" >::
ae false (CSet.equal (CSet.of_list [1; 2; 3]) (CSet.of_list []));
"sets with the same elements are equal" >::
ae true (CSet.equal (CSet.of_list [1; 2]) (CSet.of_list [2; 1]));
"sets with different elements are not equal" >::
ae false (CSet.equal (CSet.of_list [1; 2; 3]) (CSet.of_list [1; 2; 4]));
"set is not equal to larger set with same elements" >::
ae false (CSet.equal (CSet.of_list [1; 2; 3]) (CSet.of_list [1; 2; 3; 4]));
(* Unique elements can be added to a set *)
"add to empty set" >::
aec  (CSet.add (CSet.of_list []) 3);
"add to non-empty set" >::
aec [1; 2; 3; 4] (CSet.add (CSet.of_list [1; 2; 4]) 3);
"adding an existing element does not change the set" >::
aec [1; 2; 3] (CSet.add (CSet.of_list [1; 2; 3]) 3);
(* Intersection returns a set of all shared elements *)
"intersection of two empty sets is an empty set" >::
aec [] (CSet.intersect (CSet.of_list []) (CSet.of_list []));
"intersection of an empty set and non-empty set is an empty set" >::
aec [] (CSet.intersect (CSet.of_list []) (CSet.of_list [3; 2; 5]));
"intersection of a non-empty set and an empty set is an empty set" >::
aec [] (CSet.intersect (CSet.of_list [1; 2; 3; 4]) (CSet.of_list []));
"intersection of two sets with no shared elements is an empty set" >::
aec [] (CSet.intersect (CSet.of_list [1; 2; 3]) (CSet.of_list [4; 5; 6]));
"intersection of two sets with shared elements is a set of the shared elements" >::
aec [2; 3] (CSet.intersect (CSet.of_list [1; 2; 3; 4]) (CSet.of_list [3; 2; 5]));
(* Difference (or Complement) of a set is a set of all elements that are only in the first set *)
"difference of two empty sets is an empty set" >::
aec [] (CSet.difference (CSet.of_list []) (CSet.of_list []));
"difference of empty set and non-empty set is an empty set" >::
aec [] (CSet.difference (CSet.of_list []) (CSet.of_list [3; 2; 5]));
"difference of a non-empty set and an empty set is the non-empty set" >::
aec [1; 2; 3; 4] (CSet.difference (CSet.of_list [1; 2; 3; 4]) (CSet.of_list []));
"difference of two non-empty sets is a set of elements that are only in the first set" >::
aec [1; 3] (CSet.difference (CSet.of_list [3; 2; 1]) (CSet.of_list [2; 4]));
(* Union returns a set of all elements in either set *)
"union of empty sets is an empty set" >::
aec [] (CSet.union (CSet.of_list []) (CSet.of_list []));
"union of an empty set and non-empty set is the non-empty set" >::
aec  (CSet.union (CSet.of_list []) (CSet.of_list ));
"union of a non-empty set and empty set is the non-empty set" >::
aec [1; 3] (CSet.union (CSet.of_list [1; 3]) (CSet.of_list []));
"union of non-empty sets contains all unique elements" >::
aec [3; 2; 1] (CSet.union (CSet.of_list [1; 3]) (CSet.of_list [2; 3]));
]

let () =
run_test_tt_main ("custom_set tests" >::: tests)``````
``````module type ELEMENT = sig
type t
val compare : t -> t -> int
end

module Make(El: ELEMENT) = struct
type t = El.t list
type el = El.t

let el_equal a b =  El.compare a b = 0

let is_empty list = List.length list = 0

let is_member list e = List.fold_left (fun p x -> p || (El.compare x e) = 0) false list

let is_subset a b =
let rec sub = function
| []-> true
| hd :: tl -> if is_member b hd then sub tl else false
in sub a

let is_disjoint a b =
let rec sub = function
| []-> true
| hd :: tl -> if is_member b hd then false else sub tl
in sub a

let equal a b =
if (List.length a = List.length b) |> not then false else is_subset a b

let of_list (list:el list) = list

let add list e = if is_member list e |> not then (e :: list) else list

type status = [
| `OnlyA
| `OnlyB
| `Both
]

let diff_filter _ _ _ =
failwith "'diff_filter' is missing"

let difference a b =
let rec sub = function
| []-> []
| hd :: tl -> if (is_member b hd) |> not then hd :: (sub tl ) else sub tl
in sub a

let intersect a b =
let rec sub = function
| []->[]
| hd :: tl -> if (is_member b hd) then hd :: (sub tl) else sub tl
in sub a

let union a b =
let rec sub = function
| []-> b
| hd :: tl -> if (is_member b hd) then (sub tl) else hd :: (sub tl)
in sub a

end``````

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