# Ruby Array Set Operators

Have you ever had an array that you only want to include **unique** elements?

```ruby
list_1 = ['apple', 'orange', 'grape']
list_2 = ['strawberry', 'apple']
combined_list = (list_1 + list_2).uniq
combined_list #=> ["apple", "orange", "grape", "strawberry"]
```

In this example we're combining `list_1` and `list_2`, then removing duplicate entries using the `#uniq` method before assigning to the `combined_list` variable.

Another approach would be to not include duplicate elements all together using one of the array set operators: the union operator `|`.

---

## Union `|`

The union operator **combines** the unique values of two sets.  Here we're using this set operator to generate the same result using two arrays.

```ruby
list_1 = ['apple', 'orange', 'grape']
list_2 = ['strawberry', 'apple']
combined_list = list_1 | list_2
combined_list #=> ["apple", "orange", "grape", "strawberry"]
```

## Difference `-`

The difference operator returns the **difference** between two sets.  Just like the previous example, we'll use arrays.

```ruby
list_1 = ['a', 'b', 'c']
list_2 = ['c', 'd', 'e']
list_1_difference = list_1 - list_2
list_1_difference #=> ["a", "b"]
```

What's on the **left** side of the difference operator is important.  It takes elements on the left (in our case the elements in the array) and compares them to the elements on the right; and the elements that are **different** are returned.

So in the example above, `list_1` is on the left (`['a', 'b', 'c']`) and `list_2` is on the right `['c', 'd', 'e']`.

The elements in `list_1` that are **different** (not included in) from the elements in `list_2` are the ones that are returned `["a", "b"]`.

#### list_2 difference

We could easily see what elements are in `list_2` that are not included in `list_1` by placing `list_2` on the left side of the difference operator.

```ruby
list_1 = ['a', 'b', 'c']
list_2 = ['c', 'd', 'e']
list_1_difference = list_2 - list_1
list_1_difference #=> ["d", "e"]
```

## Intersection `&`

The intersection operator returns the elements that are **common** in both sets.  It's easy to think of `intersection` and `difference` as opposites.

```ruby
list_1 = ['a', 'b', 'c']
list_2 = ['c', 'd', 'e']
common_elements = list_1 & list_2
common_elements #=> ["c"]
```