# How to Use Pattern Matching to Locate Elements in a Hash Array

Having the following structure of a Hash that includes an Array of Hashes, for example, and you want the email of a moderator:

```ruby
system = {
  users: [
    { username: 'alice', role: 'admin', email: 'alice@example.com' },
    { username: 'bob', role: 'user', email: 'bob@example.com' },
    { username: 'charlie', role: 'moderator', email: 'charlie@example.com' }
  ]
}

moderator = system[:users].find { |u| u[:role] == 'moderator' }
email = moderator[:email] 

puts email # charlie@example.com
```

Now here is doing the same thing in Ruby using pattern matching:

```ruby
system = {
  users: [
    { username: 'alice', role: 'admin', email: 'alice@example.com' },
    { username: 'bob', role: 'user', email: 'bob@example.com' },
    { username: 'charlie', role: 'moderator', email: 'charlie@example.com' }
  ]
}

system => {users: [*, { role: 'moderator', email: }, *]}
puts email # charlie@example.com
```

Let’s put the two lines one above the other:

```ruby
# Using Enumerator#find
moderator = system[:users].find { |u| u[:role] == 'moderator' }
email = moderator[:email] 

# Using Pattern Matching
system => {users: [*, { role: 'moderator', email: }, *]}
```

There is another difference between them: in case of Pattern Matching it will throw a `NoMatchingPatternError` when not found, while the two lines that use `Enumerator#find` will throw a `NoMethodError`:

```ruby
moderator = system[:users].find { |u| u[:role] == 'THIS_ROLE_DOES_NOT_EXISTS' }
email = moderator[:email] 
# undefined method '[]' for nil (NoMethodError)

system => {users: [*, { role: 'THIS_ROLE_DOES_NOT_EXISTS', email: }, *]}
# {users: [{username: "alice", role: "admin", email: "alice@example.com"}, 
# {username: "bob", role: "user", email: "bob@example.com"}, 
# {username: "charlie", role: "moderator", email: "charlie@example.com"}]}: 
# [{username: "alice", role: "admin", email: "alice@example.com"}, 
# {username: "bob", role: "user", email: "bob@example.com"}, 
# {username: "charlie", role: "moderator", email: "charlie@example.com"}] 
# does not match to find pattern (NoMatchingPatternError)does not match 
# to find pattern (NoMatchingPatternError)
```

## How does pattern matching work in this example

If `email` does *not exist* either as a variable or method, matching it will create a local variable with the value from the matched row's key.

```ruby
system = {
  users: [
    { username: 'charlie', role: 'moderator', email: 'charlie@example.com' }
  ]
}
system => {users: [*, { role: 'moderator', email: }, *]}
puts binding.local_variables
# => [:system, :email]
```

If a variable of method labeled `email` does exists, it will just try to match it against its value:

```ruby
system = {
  users: [
    { username: 'charlie', role: 'moderator', email: 'charlie@example.com' }
  ]
}

email = 'charlie@example.com'
system => {users: [*, { role: 'moderator', email: }, *]}
puts email # charlie@example.com
```

If the variable already has a value, it will be overriden if the matching is succesful:

```ruby
system = {
  users: [
    { username: 'charlie', role: 'moderator', email: 'charlie@example.com' }
  ]
}
email = 'bob@example.com'
system => {users: [*, { role: 'moderator', email: }, *]}
puts email # charlie@example.com
```

You can force if you want to search for the content of that variable with the `^` operator:

```ruby
system = {
  users: [
    { username: 'charlie', role: 'moderator', email: 'charlie@example.com' }
  ]
}
email = 'bob@example.com'
system => {users: [*, { role: 'moderator', email: ^email}, *]}
# {users: [{username: "charlie", role: "moderator", email: "charlie@example.com"}]}: [{username: "charlie", role: "moderator", email: "charlie@example.com"}] does not match to find pattern (NoMatchingPatternError)
```

---

👉 If you like this article and want it in your inbox each week, [subscribe to my newsletter](https://newsletter.lucianghinda.com). You’ll find **ideas on Ruby, software development, software testing, building products and workshops**, plus notes on creativity, tech trends, and whatever else sparks my curiosity.

👐 Want to improve your **developer testing skills**? Visit [goodenoughtesting.com/articles](https://goodenoughtesting.com/articles) to discover resources on testing for developers.

👉 [Join my Short Ruby Newsletter](https://newsletter.shortruby.com) for weekly Ruby updates and visit rubyandrails.info, a directory of Ruby learning content.

🤝 Connect with me on [Linkedin](https://linkedin.com/in/lucianghinda), [Bluesky](https://bsky.app/profile/lucianghinda.com), [Ruby.social](https://ruby.social/@lucian), , and [Twitter](https://x.com/lucianghinda), where I mostly post about Ruby and Ruby on Rails.

🎥 Follow [my YouTube channel](https://www.youtube.com/@shortruby) for short videos about Ruby and Rails.