**Module Set**

To make a set of strings:

module SS = Set.Make(String);;

To create a set you need to start somewhere so here is the empty set

let s = SS.empty;;

Alternatively if we know an element to start with we can create a set like

let s = SS.singleton "hello";;

To add some elements to the the set we can do.

let s = List.fold_right SS.add ["hello"; "world"; "community"; "manager"; "stuff"; "blue"; "green"] s;;

Now if we are playing around with sets we will probably want to see what is in the set that we have created. To do this we can write a function that will print the set out.

(* Prints a new line "\n" after each string is printed *) let print_set s = SS.iter print_endline s;;

If we want to remove a specific element of a set there is a remove function. However if we want to remove several elements at once we could think of it as doing a 'filter'. Let's filter out all words that are longer than 5 characters.

This can be written as:

let my_filter str = String.length str <= 5;; let s2 = SS.filter my_filter s;;

or using an anonymous function:

let s2 = SS.filter (fun str -> String.length str <= 5) s;;

If we want to check and see if an element is in the set it might look like this.

SS.exists (fun str -> str = "hello") s2;;

or even:

SS.exists (( = ) "hello") s2;;

The Set module also provides the set theoretic operations union, intersection and difference. For example, the difference of the original set and the set with short strings (<=5 characters) is the set of long strings:

print_set (SS.diff s s2);;

Note that the Set module provides a purely functional data structure: removing an element from a set does not alter that set but, rather, returns a new set that is very similar to (and shares much of its internals with) the original set.