Add hash concept and exercise

concepts/hashes/.meta/config.json

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+{
+  "blurb": "Hashes are collections of key-value pairs, where each unique key maps to a specific value. They are useful for storing and retrieving data based on keys rather than numerical indices.",
+  "authors": ["meatball133"],
+  "contributors": ["kotp"]
+}

concepts/hashes/about.md

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+# About
+
+[Hashes][hash] are also known as dictionary or map in other languages, they are a mutable unsorted collection which maps keys to values.
+Each key is unique and is used to retrieve the corresponding value.
+The keys can be of any data type which is hashable (has a `hash` method), this includes strings, numbers, and most data types and objects in Ruby.
+
+Even though Hashes are unordered collections, [Ruby maintains the insertion order of key-value pairs][entry_order].
+This means that when you iterate over a `Hash`, the pairs will be returned in the order they were added.
+However, deleting elements may affect the order of remaining elements.
+Hashes behavior of maintaining insertion order was introduced in Ruby [1.9][ruby-1.9].
+
+## Creating Hashes
+
+You can create a `Hash` using curly braces `{}` with key-value pairs formed as `key => value` and separated by commas:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30, "city" => "New York"}
+```
+
+You can also mix and match different types of keys and values:
+
+```ruby
+my_hash = {1 => "one", :two => 2, "three" => [3, "three"]}
+```
+
+Alternatively if the keys are symbols, you can use a more the newer syntax which was introcued in Ruby 1.9:
+
+```ruby
+my_hash = {name: "Alice", age: 30, city: "New York"}
+```
+
+You can create an empty `Hash` using the `Hash.new` method:
+
+```ruby
+empty_hash = Hash.new
+```
+
+## Accessing values
+
+You can access values in a `Hash` instance using its corresponding keys, the syntax reminds of array indexing, but using the key instead of an index:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30, "city" => "New York"}
+my_hash["name"]
+# => "Alice"
+```
+
+If the key does not exist in the `Hash` instance, the `[]` method will return `nil`:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash["city"]
+# => nil
+```
+
+If the disired behavior is to not return `nil` for non-existing keys, another way of accessing values is by using the [`fetch`][fetch] method, which allows you provide a default value for non-existing keys.
+If the `fetch` method is used without a default value and the key does not exist, it will raise a `KeyError` exception.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.fetch("city", "Unknown")
+# => "Unknown"
+
+my_hash.fetch("city")
+# => KeyError: key not found: "city"
+```
+
+## Modifying
+
+You can add or update key-value pairs in a `Hash` instance by assigning a value to a key using the assignment operator `=`.
+Assigning a value to an existing key will update the value, while assigning a value to a new key will add a new key-value pair:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash["city"] = "New York"
+my_hash["age"] = 31          
+
+my_hash
+# => {"name" => "Alice", "age" => 31, "city" => "New York"}
+```
+
+## Default values
+
+When fetching a value with `[]` for a key that does not exist in the `Hash` instance, Ruby returns `nil` by default.
+
+```ruby
+my_hash = {"name" => "Alice"}
+my_hash["age"]
+# => nil
+```
+
+This might not always be desirable, so you can set a default value for the `Hash` instance using `Hash.new(default_value)`.
+See that the default value is returned only for keys that do not exist in the `Hash` instance.
+
+```ruby
+my_hash = Hash.new("unknown")
+my_hash["name"] = "Alice"
+my_hash["age"]
+# => "unknown"
+
+my_hash["name"]
+# => "Alice"
+```
+
+~~~~exercism/note
+Be cautious when using mutable objects (like Arrays or other Hashes) as default values, as they can lead to unexpected behavior.
+~~~~
+
+## Deleting key-value pairs
+
+You can delete a key-value pair from a `Hash` instance using the [`delete`][delete] method, which takes the key as an argument:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30, "city" => "New York"}
+my_hash.delete("age")
+my_hash
+# => {"name" => "Alice", "city" => "New York"}
+```
+
+## Methods
+
+There are several useful instance methods available for Hashes in Ruby.
+Here are some commonly used ones:
+
+### `has_value?` and `include?`
+
+You can check if a `Hash` instance contains a specific value using the [`has_value?`][has_value?] method.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.has_value?(30)
+# => true
+my_hash.has_value?(25)
+# => false
+```
+
+You can check if a `Hash` instance contains a specific key using the [`include?`][include?] method.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.include?("name")
+# => true
+my_hash.include?("city")
+# => false
+```
+
+### `keys` and `values`
+
+You can retrieve all the keys of a `Hash` instance using the [`keys`][keys] method, which returns an array of keys.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.keys
+# => ["name", "age"]
+```
+
+You can retrieve all the values of a `Hash` instance using the [`values`][values] method, which returns an array of values.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.values
+# => ["Alice", 30]
+```
+
+## Iterating over Hashes
+
+You can iterate over the key-value pairs in a `Hash` instance using the `each_pair` method.
+This will give you access to both the key and the value for each pair:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.each_pair do |key, value|
+  puts "#{key}: #{value}"
+end
+# Output:
+# name: Alice
+# age: 30
+```
+
+You can also iterate over just the keys or just the values using the [`each_key`][each_key] or [`each_value`][each_value] methods, respectively:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.each_key do |key|
+  puts key
+end
+# Output:
+# name
+# age
+```
+
+[entry_order]: https://docs.ruby-lang.org/en/master/Hash.html#class-Hash-label-Entry+Order
+[each_pair]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-each_pair
+[each_key]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-each_key
+[keys]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-keys
+[each_value]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-each_value
+[values]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-values
+[has_value?]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-has_value-3F
+[include?]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-include-3F
+[hash]: https://docs.ruby-lang.org/en/master/Hash.html
+[fetch]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-fetch
+[delete]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-delete
+[ruby-1.9]: https://ruby-doc.org/3.4/NEWS/NEWS-1_9_1.html

concepts/hashes/introduction.md

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+# About
+
+[Hashes][hash] are also known as dictionary or map in other languages, they are a mutable unsorted collection which maps keys to values.
+Each key is unique and is used to retrieve the corresponding value.
+The keys can be of any data type which is hashable (has a `hash` method), this includes strings, numbers, and most data types and objects in Ruby.
+
+Even though Hashes are unordered collections, [Ruby maintains the insertion order of key-value pairs][entry_order].
+This means that when you iterate over a `Hash`, the pairs will be returned in the order they were added.
+However, deleting elements may affect the order of remaining elements.
+Hashes behavior of maintaining insertion order was introduced in Ruby [1.9][ruby-1.9].
+
+## Creating Hashes
+
+You can create a `Hash` using curly braces `{}` with key-value pairs formed as `key => value` and separated by commas:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30, "city" => "New York"}
+```
+
+You can also mix and match different types of keys and values:
+
+```ruby
+my_hash = {1 => "one", :two => 2, "three" => [3, "three"]}
+```
+
+Alternatively if the keys are symbols, you can use a more the newer syntax which was introcued in Ruby 1.9:
+
+```ruby
+my_hash = {name: "Alice", age: 30, city: "New York"}
+```
+
+You can create an empty `Hash` using the `Hash.new` method:
+
+```ruby
+empty_hash = Hash.new
+```
+
+## Accessing values
+
+You can access values in a `Hash` instance using its corresponding keys, the syntax reminds of array indexing, but using the key instead of an index:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30, "city" => "New York"}
+my_hash["name"]
+# => "Alice"
+```
+
+If the key does not exist in the `Hash` instance, the `[]` method will return `nil`:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash["city"]
+# => nil
+```
+
+If the disired behavior is to not return `nil` for non-existing keys, another way of accessing values is by using the [`fetch`][fetch] method, which allows you provide a default value for non-existing keys.
+If the `fetch` method is used without a default value and the key does not exist, it will raise a `KeyError` exception.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.fetch("city", "Unknown")
+# => "Unknown"
+
+my_hash.fetch("city")
+# => KeyError: key not found: "city"
+```
+
+## Modifying
+
+You can add or update key-value pairs in a `Hash` instance by assigning a value to a key using the assignment operator `=`.
+Assigning a value to an existing key will update the value, while assigning a value to a new key will add a new key-value pair:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash["city"] = "New York"
+my_hash["age"] = 31          
+
+my_hash
+# => {"name" => "Alice", "age" => 31, "city" => "New York"}
+```
+
+## Deleting key-value pairs
+
+You can delete a key-value pair from a `Hash` instance using the [`delete`][delete] method, which takes the key as an argument:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30, "city" => "New York"}
+my_hash.delete("age")
+my_hash
+# => {"name" => "Alice", "city" => "New York"}
+```
+
+## Methods
+
+There are several useful instance methods available for Hashes in Ruby.
+Here are some commonly used ones:
+
+### `has_value?` and `include?`
+
+You can check if a `Hash` instance contains a specific value using the [`has_value?`][has_value?] method.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.has_value?(30)
+# => true
+my_hash.has_value?(25)
+# => false
+```
+
+You can check if a `Hash` instance contains a specific key using the [`include?`][include?] method.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.include?("name")
+# => true
+my_hash.include?("city")
+# => false
+```
+
+### `keys` and `values`
+
+You can retrieve all the keys of a `Hash` instance using the [`keys`][keys] method, which returns an array of keys.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.keys
+# => ["name", "age"]
+```
+
+You can retrieve all the values of a `Hash` instance using the [`values`][values] method, which returns an array of values.
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.values
+# => ["Alice", 30]
+```
+
+## Iterating over Hashes
+
+You can iterate over the key-value pairs in a `Hash` instance using the `each_pair` method.
+This will give you access to both the key and the value for each pair:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.each_pair do |key, value|
+  puts "#{key}: #{value}"
+end
+# Output:
+# name: Alice
+# age: 30
+```
+
+You can also iterate over just the keys or just the values using the [`each_key`][each_key] or [`each_value`][each_value] methods, respectively:
+
+```ruby
+my_hash = {"name" => "Alice", "age" => 30}
+my_hash.each_key do |key|
+  puts key
+end
+# Output:
+# name
+# age
+```
+
+[entry_order]: https://docs.ruby-lang.org/en/master/Hash.html#class-Hash-label-Entry+Order
+[each_pair]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-each_pair
+[each_key]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-each_key
+[keys]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-keys
+[each_value]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-each_value
+[values]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-values
+[has_value?]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-has_value-3F
+[include?]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-include-3F
+[hash]: https://docs.ruby-lang.org/en/master/Hash.html
+[fetch]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-fetch
+[delete]: https://docs.ruby-lang.org/en/master/Hash.html#method-i-delete
+[ruby-1.9]: https://ruby-doc.org/3.4/NEWS/NEWS-1_9_1.html

concepts/hashes/links.json

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+[
+  {
+    "url": "https://docs.ruby-lang.org/en/4.0/Hash.html",
+    "description": "Ruby docs: Hash"
+  },
+  {
+    "url": "https://www.geeksforgeeks.org/ruby/ruby-hash-class/",
+    "description": "GeeksforGeeks: Ruby Hash Class"
+  }
+]