Hashes in Ruby

Hashes in Ruby

A hash in Ruby is a collection of key-value pairs, where each key is unique, and the value can be of any data type. They are enclosed in curly braces {}, and each key-value pair is separated by a comma. The key and value are separated by a hash rocket (=>) or a colon (:).

Here's an example of a hash in Ruby:

In this example, person1 is a hash with three key-value pairs where the keys are symbols and person2 is a hash with three key-value pairs where the keys are strings. The keys are : name, : age, and : city for person1. Similarly, for person2, the keys are "name", "age" and "city". We can access the value of a particular key in the hash using the [] method, like this:

Creating Hashes in Ruby

There are different ways to create hashes in Ruby. One way is to use the curly braces {} and separate each key-value pair with a comma.

Here's an example:

There is another way to create hashes

Here instead of using hash rockets we're directly using name: "Rohit", but internally it's doing the same thing that is :name => "Rohit" but this way is more preferred and adopted by developers.

You can also create a hash using the Hash.new method. This method takes an optional argument that sets the default value for any key that hasn't been defined yet.

Here's an example:

In this example, we first create an empty hash using the Hash.the new method, and then we add three key-value pairs using the []= method. We pass the key as the first argument and the value as the second argument.

Using Symbols as Keys in Hash

It is common to use symbols as keys in hashes in Ruby. Symbols are lightweight objects that represent names or labels, and they're used throughout Ruby to identify method names, variable names, and other entities. When you use a symbol as a key in a hash, Ruby creates only one instance of the symbol, which saves memory and makes lookups faster.

Here's an example:

In this example, we're using the hash rocket syntax to create a hash with three key-value pairs. The keys are symbols, and their values are strings and integers.

Different Ways to Create Hashes in Ruby

Apart from using curly braces and symbols as keys, there are other ways to create hashes in Ruby.

Using a block with Hash.new:

You can create a hash with a default value using Hash.new and pass a block to it. The block will be called whenever a key is accessed that hasn't been defined yet.

Here's an example:

In this example, we create a hash with a default value of "unknown" using Hash.new. We pass a block to the method that takes two arguments, hash and key. Inside the block, we set the value of the key to "unknown" using the []= method.

Using the to_h Method

If you have an array of arrays, where each subarray contains a key-value pair, you can convert it to a hash using the to_h method.

Here's an example:

In this example, we first create an array of arrays, where each subarray contains a key-value pair. We then call the to_h method on the array to convert it to a hash.

Accessing Values from Hashes in Ruby

For accessing the values of a particular key from hashes in Ruby, we can use the [] method.

Here's an example:

In this example, we access the value of the :name key in the person hash using the [] method.

There is also a fetch method for accessing values from hashes in Ruby. It raises an exception if a key is not present. It can also accept a default value as its second argument, which is returned if the key is not found.

Here's an example:

With the help of the fetch method, the developer can control what happens if the key is not present in the hash. Hence, it is a preferred way of accessing hash values in Ruby.

Modifying Hashes in Ruby

You can modify the values of keys in a hash using the []= method.

Here's an example:

In this example, we modify the value of the key in the person hash using the []= method.

Removing Data from Hashes in Ruby

You can remove a key-value pair from a hash by passing the key as an argument to the delete function.

Here's an example:

In this example, we remove the :age key-value pair from the person hash using the delete method.

Ruby Hash Methods

The Hash class in Ruby provides many useful methods for working with hashes. These methods can be divided into two categories: class methods and instance methods.

Class Methods

[] :

Creates a new hash from a list of key-value pairs.

new:

Creates a new, empty hash.

try_convert:

Attempts to convert an object into a hash. If the object can be converted, it returns the hash; otherwise, it returns nil.

In this example, we attempt to convert the person hash into a new hash using Hash.try_convert. Since the person is already a hash, the method returns the same hash.

Instance Methods

==:

Checks if two hashes are equal.

[]:

Returns the value associated with a particular key in the hash.

In this example, we use the [] method to retrieve the value associated with the :name key in the person hash.

[]=:

Sets the value associated with a particular key in the hash.

In this example, we use the []= method to set the value associated with the :age key in the person hash to 31.

clear:

Removes all key-value pairs from the hash.

In this example, we use the clear method to remove all key-value pairs from the person hash.

default:

Returns the default value for the hash.

In this example, we create a new hash with a default value of "unknown" using Hash.new. We then use the default method to retrieve the default value.

default=:

This instance method sets the default value for the hash.

In this example, we create a new hash using Hash.new. We then use the default= method to set the default value to "unknown".

default_proc:

Returns the default proc for the hash.

In this example, we create a new hash with a default proc that sets the value of a new key to "unknown". We then use the default_proc.

delete:

Removes a key-value pair from the hash and returns the value.

In this example, we use the delete method to remove the key-value pair associated with the :age key in the person hash and retrieve the value.

delete_if:

Removes key-value pairs from the hash that meet a certain condition.

In this example, we use the delete_if method to remove key-value pairs from the person hash where the value is a string.

each:

Iterates over each key-value pair in the hash.

In this example, we use each method to iterate over each key-value pair in the person hash and print them out.

each_key:

Iterates over each key in the hash.

In this example, we use the each_key method to iterate over each key in the person hash and print them out.

each_pair:

Iterates over each key-value pair in the hash.

In this example, we use the each_pair method to iterate over each key-value pair in the person hash and print them out.

each_value:

Iterates over each value in the hash.

In this example, we use the each_value method to iterate over each value in the person hash and print them out.

empty?:

Returns true if the hash is empty.

In this example, we use the empty? method to check if the person's hash is empty.

fetch:

Retrieves the value associated with a particular key in the hash. If the key does not exist, it returns the default value or raises an error if no default value is specified.

Here's an example:

In this example, we use the fetch method to retrieve the value associated with the key in the person hash. We also use the fetch method to retrieve the value associated with the non-existent key, providing a default value of "India". Finally, we use the fetch method to retrieve the value associated with the non-existent key without providing a default value, which raises a KeyError exception.

has_key?:

Return true if the hash contains a particular key.

In this example, we use the has_key? method to check if the person hash contains the :name and :country keys.

has_value?:

Return true if the hash contains a particular value.

In this example, we use the has_value? method to check if the person hash contains the value 30 and the string "India".

include?:

Returns true if the hash contains a particular key.

In this example, we use the include? method to check if the person hash contains the :name and :country keys.

index:

Returns the key associated with a particular value in the hash. If the value is not found, it returns nil.

In this example, we use the index method to retrieve the key associated with the value "Rohit" in the person hash. We also use the index method to try to retrieve the key associated with the non-existent value "India", which returns nil.

invert:

Returns a new hash with the keys and values of the original hash swapped.

In this example, we use the invert method to create a new hash where the keys and values of the person hash.

key?:

Return true if the hash contains a particular key.

In this example, we use the key? method to check if the person hash contains the :name and :country keys.

keys:

This instance method returns an array of all the keys of the hash.

In this example, we use the length, size, and count methods to get the number of key-value pairs in the person's hash.

member?

Returns true if the hash contains a particular key. It's an alias for the has_key? method.

In this example, we use the member? method to check if the person hash contains the :name and :country keys.

merge:

Merges two hashes together, creating a new hash that contains the combined key-value pairs.

In this example, we use the merge method to combine the person and more_info hashes into a new hash called merged_person.

merge!

Merges two hashes together, modifying the original hash to contain the combined key-value pairs.

In this example, we use the merge! method to modify the person hash to contain the combined key-value pairs from the more_info hash.

rehash:

Rebuilds the internal hash table, which can improve performance when the hash has been modified in a way that affects its hash codes.

In this example, we use the rehash method to rebuild the internal hash table for the person hash.

reject:

Returns a new hash that contains only the key-value pairs for which the block returns false.

In this example, we use the reject method to create a new hash called filtered_person that contains only the key-value pairs from the person hash where the key is not :name.

reject!:

Removes the key-value pairs for which the block returns true, modifying the original hash.

In this example, we use the reject! method to modify the person hash by removing the key-value pair where the key is :name.

replace:

Replaces the contents of one hash with the contents of another hash.

In this example, we use the replace method to replace the contents of the person hash with the contents of the more_info hash.

select or filter:

Return a new hash that contains only the key-value pairs for which the block returns true.

In this example, we use the select method to create a new hash called filtered_person that contains only the key-value pairs from the person hash where the value is a string.

shift:

Removes the first key-value pair from the hash and returns it as an array.

In this example, we use the shift method to remove the first key-value pair from the person hash and store it in a variable called removed_pair.

sort:

Returns an array of key-value pairs sorted by key.

In this example, we use the sort method to create a new array called sorted_person that contains the key-value pairs from the person hash sorted by key.

store:

Set the value of a particular key in the hash.

In this example, we use the store method to set the value of the :country key in the person hash.

to_a:

Converts the hash to a nested array of key-value pairs.

In this example, we use the to_a method to convert the person's hash to a nested array of key-value pairs.

to_s:

Converts the hash to a string.

In this example, we use the to_s method to convert the person's hash to a string.

update:

Merge another hash into the current hash, modifying the original hash.

In this example, we use the update method to merge the more_info hash into the person hash.

value?:

Returns true if the specified value exists in the hash, and false otherwise.

In this example, we use the value? method to check if the person hash contains the values "Rohit" and "London".

values:

Returns an array of all the values in the hash.

In this example, we use the values method to get an array of all the values in the person hash.

value_at:

Returns an array of the values associated with the specified keys.

In this example, we use the value_at method to get an array of the values associated with the keys :name and :city in the person hash.