ruby hash

Hash organizes data in key-value pairs. Hash is so fast that it has an O(1) constant lookup time characteristic. Meaning, the time to look up for the value associated to a key is constant regardless of the collection’s size.Hash is created using curly braces.

Creating a Hash

	detail = {}
detail.class # Hash

detail[:name] = "Alice"
detail[:age] = 19

detail # => { :name=>"Alice", :age=>19 }
	

			
	
			

		
	

We can also initialize a hash when creating it. Again, the keys are each a symbol.

Creating and Initializing a Hash

	detail = { name: "Alice", age: 19 }
detail # => { :name=>"Alice", :age=>19 }
	

			
	
			

		
	

To get the value of a hash, we can use the [] (bracket) operator and then specify the key. If the value does not exist, nil is returned.

Retrieving a Value from a Hash by Using Its Key

	detail[:name] # => "Alice"
detail[:age] # => 19
detail[:number] # => nil
	

			
	
			

		
	

We can instantiate hash using the older notation (key => value) instead of the new one (key: value).

Defining a Symbol Using Older Notation

Initiating a hash in this way feels like something from the past. It’s normally preferred to use the new notation, especially if all the keys are symbols.

	 checkMe = {
  true => "Yes!",
  false => "No!",
  1 => "Yes!",
  0 => "No!",
 }

  [checkMe[false], checkMe[1]] # => ["No!", "Yes!"]
	

			
	
			

		
	

Using a Conditional Assignment Operator

	detail # => { :name=>"Alice", :age=>19 }
detail[:age] ||= 19

detail # => {:name=>"Alice", :age=>19}

detail[:email] ||= "meAlice@example.com"

detail # => { :name=>"Alice", :age=>19, :email=>"meAlice@example.com" }
	

			
	
			

		
	

To delete a mapping in a hash, we use the Hash#delete method. On top of that, invoking Hash#delete also returns the associated value.

Deleting a Key in a Hash

	>> detail # => { :name=>"Alice", :age=>19, :email=>"meAlice@example.com" }

>> detail.delete :email # => "meAlice@example.com"

>> detail # => { :name=>"Alice", :age=>19 }

>> detail.delete :nothing # => nil
	

			
	
			

		
	

To merge a hash with another one, we can use the Hash#merge method. This method returns a new hash instead of modifying in place.

Using Merge to Merge Two Hashes

	detail # => { :name=>"Alice", :age=>19 }
detail.merge(email: "meAlice@example.com") # => { :name=>"Alice", :age=>19, :email=>"meAlice@example.com" }
	

			
	
			

		
	

If we want to modify in place, we can use Hash #merge!

Converting a Hash to an Array, and Vice Versa

	detail = [[:name, "Alice"], [:age, 19]].to_h # => { :name=>"Adam", :age=>27 }
detail.to_a # => [[:name, "Alice"], [:age, 19]]
	

			
	
			

		
	

Just like an array, a hash is enumerable. As such, it has the each method to iterate its key-value pair