hashes.md

Hashes

A Hash is a dictionary - like collection of key-value pairs.

Examples:

ages   = { "Tony" => 22, "KenJ" => 25 }
months = { 1 => "January", 2 => "February", 3 => "March" }
attrs  = { :font_size => 22, :color => "red" }

Create a Hash

# Create a new empty hash
profile = {} 

# Add new elements to hash
profile[:user_name] = "Tony"    
profile[:language]  = "English" 
profile[:age]       = 25     

profile
=> {:user_name => "Tony", :language => "English", :age => 25}

Accessing value by key

ages = { "Tony" => 22, "KenJ" => 25 }

ages["Tony"] #=> 22
ages["KenJ"] #=> 25

months = { 1 => "January", 2 => "February", 3 => "March" }

months[1]   #=> "January"
months[2]   #=> "February"
months[3]   #=> "March"
months[4]   #=> nil

attrs = { :font_size => 22, :color => "red" }

attrs[:font_size]  #=> 22
attrs[:color]      #=> "red"

Common Uses

profile = {:user_name => "Tony", :language => "English", :age => 25}

• Get list keys

profile.keys 
=> [:user_name, :language, :age]

• Get list values

profile.values
=> ["Tony", "English", 25]

Iteration

each

You can iterate over keys and values

{x: 1, y: 2, z: 3}.each do |key, value|
  puts "key: #{key} and value: #{value}"
end

# Output
key: x and value: 1
key: y and value: 2
key: z and value: 3

each_with_index

{x: 3, y: 4, z: 5}.each_with_index do |(key, value), index| 
  puts "Index #{index}: key is #{key} and value is #{value}"
end

# Output
Index 0: key is x and value is 3
Index 1: key is y and value is 4
Index 2: key is z and value is 5

Filtering with select, reject

• select

hash = {:a => 1, :b => 2, :c => 3}
hash.select{|key, value| value.even?}   
=> {:b=>2}

• reject

hash = {:a => 1, :b => 2, :c => 3}
hash.reject{|key, value| value.even?}   
=> {:a=>1, :c=>3}

Noted

• Remember that keys in a hash are unique.
• Recommend using Symbols as Hash Keys