Ruby Hashes 2020

Hashes (aka associative arrays, maps, or dictionaries) are similar to arrays because they can be indexed collection of object references. However, while we index arrays with integers, we can index a hash with objects of any types. When we store a value in a hash, we're actually supplying two objects: the index (key) and the value pair. We can retrieve the value by indexing the hash with the same key.

The following example shows the use of hash literals: a list of key=>value pair between braces:

#!/usr/bin/env ruby

# hash1.rb

number = {'uno' => 'one', 'dos' => 'two', 'tres' => 'three'}
puts number.length
puts number['uno']
puts number

Output:

3
one
{"uno"=>"one", "dos"=>"two", "tres"=>"three"}

Hashes have one significant advantage over arrays: they can use any object as an index. Hashes have a default value. This value is returned when an attempt is made to access keys that do not exist in the hash. By default this value is nil.

We may want to use a symbol for a key if not a quoted string.

#!/usr/bin/env ruby

# hash2.rb

number = Hash.new
number[:uno] = 'One'
number[:dos] = 'Two'
number[:tres] = 'Three'
puts number[:tres]

number2 = {:uno=>'one', :dos=>'two', :tres=>'three'}
puts number2

Output:

Three
{:uno=>"one", :dos=>"two", :tres=>"three"}

We can use the following which gives us the same output:

#!/usr/bin/env ruby

# hash3.rb

number3 = {uno:'one', dos:'two', tres:'three'}
puts number3

Output:

{:uno=>"one", :dos=>"two", :tres=>"three"}

However, for a numeric key, sym: syntax will not work:

not_ok= {1: 'uno'}   # will not work
ok = {1 => 'uno'}    # will work

Here is a code doing loop with the hash:

#!/usr/bin/ruby
# hash4.rb

numbers = {uno:'one', dos:'two', tres:'three'}
numbers.each do |k,v|
   puts k.to_s + ':' + v
end

Output:

uno:one
dos:two
tres:three

Other functions of hash:

#!/usr/bin/ruby
# hash5.rb

numbers = {'uno'=>'one', 'dos'=>'two', 'tres'=>'three'}
puts "Has numers 'uno' key? : " + numbers.has_key?('uno').to_s
puts "Has numers 'two' value? : " + numbers.has_value?('two').to_s
puts "Is numers is empty? : " + numbers.empty?.to_s
puts "Size of numers? : " + numbers.size.to_s

numbers.delete('tres')
puts "Size of numers? : " + numbers.size.to_s

Output:

Has numers 'uno' key? : true
Has numers 'two' value? : true
Is numers is empty? : false
Size of numers? : 3
Size of numers? : 2

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