Hashes–Another Data Structure!!! [Ruby Programming Language Notes]

Hashes! (Another data structure)

An array allows us to have a single variable, or location, to store & group a lot of data. (Allows for organization!)

An array was the first data structure we learned about.

[ ]–An array is made up of elements, organized by indices.

But sometimes we may need a different organization when building a program. That’s why we have HASHES!!!

A hash is made up of values stored by keys. (A key “unlocks” the corresponding value.)

{ }–curly braces are used to represent a hash in Ruby. (This can be assigned to a variable.)

In a hash, data comes in a pair, (a ‘key value’ pair).

• my_hash = { “name” => “Programming School”, “color” => “red”, “age” => 5 }
• In this line directly above, ‘my_hash‘ represents the “variable“, the “hash” is within the {}‘s, the set [“name“, “color“, “age“] are the “keys“, ‘=>‘ denotes a “rocket” symbol, and the set [“Programming School“, “red“, “5“] are the “values“.

• Similarly to arrays,
  • my_hash[“key”] will return the “value”! (Similar to indexing [] into an array.)
  • .length will return the number of ‘key-value’ pairs in the hash. So using the above example: ‘my_hash.length‘ would return ‘3’, for 3 pairs.

Hash Methods

*Many array methods can be used on hashes.

Other Methods:

• hash.length–return number of key-value pairs
• hash.has_key?(“keyword”)–returns a boolean (note the question mark); can be used to check for existence inside a hash.
• hash.has_value?(Value example)–returns a boolean (we know this b/c of the question mark).
• hash.keys–returns an array of the hashes keys.
  • Can be used to index into like:
    • hash.keys[index]
    • …because ‘.keys‘ returns an array, (so we can ‘index’ into it!)

Similarly, with:

• hash.values–returns an array of all the values;

As we get these data types we can use any other operations we want using previously learned info!

Why Hashes?/Use Cases

• Arrays are good to use in order to represent a collection of similar things. (i.e.–multiple things)
  • Ex: animals = [“dog”, “cat”, “fish”, “bird”]
• Hashes are good to represent many facets, or characteristics of a single thing.
  • Ex: person = { “name” => “Aaron”, “age” => 100, “location” => “NYC” }

Hash Enumerables

• .each–We can iterate thru a hash using the ‘.each‘ method. It passes each key & value to the block-parameter. (Gives us access to the key & also the value!)
  • hash.each do |key, value|
    • puts key
    • puts value
  • end
• each_key–iterate over just keys; passing keys;
  • hash.each_key do |thing| #note–Remember that these parameter variables can be named anything! Use what names make sense.
    • puts thing
  • end

• each_value–iterate thru each value; passing the values to the block.
  • hash.each_value do |val|
    • puts val
  • end

Assigning Key & Values

Adding Keys & Values to an Empty Hash

• my_hash = { } #empty hash
• my_hash [“key”] = 4 #create a “key” word and specify or assign it a value.
• puts my_hash
• This will return –> { “key” 4 }

Or we can make it dynamic by using variables:

• hash = { } #Empty hash created
• word = “this” #Variable created
• hash[word] = word.length #Keyword created & assigned to a value! ‘word.length’ will evaluate to a value.
• puts hash
• { “this” => 4 }

More Hash Properties

• A hashes default value is nil (if nothing is assigned).
  • Ex:
    • my_hash = { “a” => 28 }
    • puts my_hash [“a”] will return 28.
    • puts my_hash [“b”] will return nothing (blank) but it’s corresponding value is actually nil.

• We can initialize a hash with a different default value than nil.
  • Ex: my_hash = Hash.new(0) #default value we want to assign.
  • Ex: my_hash = Hash.new(“hello”) #now “hello” is the default value.

Counter Iterate

Let’s just change the variable name to counter.

Ex: counter = Hash.new(0)

If we do:

• str = “kitchen prep”
• str.each_char { |char| counter[char] += 1 }
• puts counter
• Output: { “k” => 1, “i” => 1, “t” => 1, “c” => 1, “h” => 1, “e” => 2, “n” => 1, ” ” => 1, “p” => 2, “r” => 1, }

• The code Hash.new() is the only way to initialize the default value of a hash.
  • ex: count = Hash.new(0)
  • –sets the default value to 0.

Sort by

my_hash.sort_by { |k, v| k }

‘Sort by‘ passes in a block of the key & the value. Then we specify which of these criteria we want to sort by, the k or the v.

*Ask yourself what data you have right now, & that will dictate what to do next.