# Set

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.