Ruby hashes are similar to objects in JS, or Dictionaries in Python.
Hashes are like dining menus, the dish names are the keys and the food are the values.
Creating Hashes
Using literal ({}):
my_hash = {
"a random word" => "pineapple",
"Amanda's math test score" => 94,
"an array" => [1, 2, 3],
"an empty hash within a hash" => {}
}
Using ::new:
my_hash = Hash.new
my_hash #=> {}
Accessing Values
It's the same way that you access elements in an array.
movies = {
"horror" => "Talk To Me",
"action" => "Blue Beetle"
}
movies["horror"] #=> "Talk To Me"
Adding or Changing Data
Just call the key and set a value:
movies["drama"] = "Eternal Sunshine Of The Spotless Mind"
movies #=> {"horror" => "Talk To Me", "action" => "Blue Beetle", "drama" => "Eternal Sunshine Of The Spotless Mind"}
movies["horror"] = "Scream"
movies #=> {"horror" => "Scream", "action" => "Blue Beetle", "drama" => "Eternal Sunshine Of The Spotless Mind"}
Removing Data
Use #delete method, and it returns the value of the key-value pair that was deleted from the hash:
movies.delete("horror") #=> "Scream"
movies #=> {"action" => "Blue Beetle", "drama" => "Eternal Sunshine Of The Spotless Mind"}
Methods
Specific to hashes: #keys and #values
movies = {
"The Grand Budapest Hotel" => "Wes Anderson",
"Lost in Translation" => "Sofia Coppola",
"Lady Bird" => "Greta Gerwig"
}
movies.keys #=> ["The Grand Budapest Hotel", "Lost in Translation", "Lady Bird"]
movies.values #=> ["Wes Anderson", "Sofia Coppola", "Greta Gerwig"]
Merging two hashes
The hash2 will overwrite the values of hash1:
hash1 = { "a" => 100, "b" => 200 }
hash2 = { "b" => 254, "c" => 300 }
hash1.merge(hash2) #=> { "a" => 100, "b" => 254, "c" => 300 }
Symbols as Hash Keys
Symbols are used as keys because they are more performant than strings:
# 'Rocket' syntax
american_cars = {
:chevrolet => "Corvette",
:ford => "Mustang",
:dodge => "Ram"
}
# 'Symbols' syntax
japanese_cars = {
toyota: "Supra",
mazda: "RX-7",
subaru: "Impreza"
}
To access:
american_cars[:ford] #=> "Mustang"
japanese_cars[:mazda] #=> "RX-7"
Common Hash Methods
key?
The key? method allows you to check if a hash contains a specific key. It returns a boolean value.
irb :001 > name_and_age = { "Bob" => 42, "Steve" => 31, "Joe" => 19}
=> {"Bob"=>42, "Steve"=>31, "Joe"=>19}
irb :002 > name_and_age.key?("Steve")
=> true
irb :003 > name_and_age.key?("Larry")
=> false
select
The select method allows you to pass a block and will return any key-value pairs that evaluate to true when passed to the block.
irb :004 > name_and_age.select { |k,v| k == "Bob" }
=> {"Bob"=>42}
irb :005 > name_and_age.select { |k,v| (k == "Bob") || (v == 19) }
=> {"Bob"=>42, "Joe"=>19}
fetch
The fetch method allows you to pass a given key and it will return the value for that key if it exists. You can also specify an option for return if that key is not present.
irb :006 > name_and_age.fetch("Steve")
=> 31
irb :007 > name_and_age.fetch("Larry")
=> KeyError: key not found: "Larry"
from (irb):32:in `fetch'
from (irb):32
from /usr/local/rvm/rubies/ruby-2.5.3/bin/irb:16:in `<main>'
irb :008 > name_and_age.fetch("Larry", "Larry isn't in this hash")
=> "Larry isn't in this hash"
to_a
The to_a method returns an array version of your hash when called.
irb :009 > name_and_age.to_a
=> [["Bob", 42], ["Steve", 31], ["Joe", 19]]
irb :010 > name_and_age
=> {"Bob"=>42, "Steve"=>31, "Joe"=>19}
Top comments (1)
Great content cousin!