How to Push Keys and Values into an Empty Hash W/ Ruby

How to push keys and values into an empty hash w/ Ruby?

Instantiate your hash in Dictionary's initialize:

class Dictionary
  def initialize
    @hash = {}
  end

  def add(defs)
    defs.each do |word, definition|
      @hash[word] = definition
    end
  end
end

Right now you have no hash until you call entries, which you don't.

entries should return the existing hash, not create a new one.

keywords should return the hash's keys.

You do not need accessors for keyword and definition. Such singular items are meaningless in a dictionary class. You might want something like lookup(word) that returned a definition.

Also, you convert words to symbols, but I don't know why–particularly since you use string keys in your spec. Pick one, although I'm not convinced this is a case where symbols add value.

Please consider variable naming carefully to provide as much context as possible.

Pushing keys and values into a hash

Did you notice that your resulting hash also contains keys which should be values? That's because header_elements is an array and you're trying to iterate it as a hash (which it isn't). In fact, you don't need to iterate it.

key, val = line.split(': ')
@headers[key] = val

Append key/value pair to hash with in Ruby

There is merge!.

h = {}
h.merge!(key: "bar")
# => {:key=>"bar"}

How to add new item to hash

Create the hash:

hash = {:item1 => 1}

Add a new item to it:

hash[:item2] = 2

How do I append a key value pair to an existing hash in Ruby?

There are 3 ways:

.merge(other_hash) Returns a new hash containing the contents of other_hash and the contents of hsh.

hash = { a: 1 }
puts hash.merge({ b: 2, c: 3 }) # => {:a=>1, :b=>2, :c=>3}

.merge!(other_hash) Adds the contents of other_hash to hsh.

hash = { a: 1 }
puts hash.merge!({ b: 2, c: 3 }) # => {:a=>1, :b=>2, :c=>3}

And most efficient way is to modify existing hash, setting new values directly:

hash = { a: 1 }
hash[:b] = 2
hash[:c] = 3
puts hash # => {:a=>1, :b=>2, :c=>3}

Corresponding Benchmarks for these methods:

       user     system      total        real
   0.010000   0.000000   0.010000 (  0.013348)
   0.010000   0.000000   0.010000 (  0.003285)
   0.000000   0.000000   0.000000 (  0.000918)

Creating a Hash with values as arrays and default value as empty array

Lakshmi is right. When you created the Hash using Hash.new([]), you created one array object.

Hence, the same array is returned for every missing key in the Hash.

That is why, if the shared array is edited, the change is reflected across all the missing keys.

Using:

Hash.new { |h, k| h[k] = [] }

Creates and assigns a new array for each missing key in the Hash, so that it is a unique object.

How to add to an existing hash in Ruby

If you have a hash, you can add items to it by referencing them by key:

hash = { }
hash[:a] = 'a'
hash[:a]
# => 'a'

Here, like [ ] creates an empty array, { } will create a empty hash.

Arrays have zero or more elements in a specific order, where elements may be duplicated. Hashes have zero or more elements organized by key, where keys may not be duplicated but the values stored in those positions can be.

Hashes in Ruby are very flexible and can have keys of nearly any type you can throw at it. This makes it different from the dictionary structures you find in other languages.

It's important to keep in mind that the specific nature of a key of a hash often matters:

hash = { :a => 'a' }

# Fetch with Symbol :a finds the right value
hash[:a]
# => 'a'

# Fetch with the String 'a' finds nothing
hash['a']
# => nil

# Assignment with the key :b adds a new entry
hash[:b] = 'Bee'

# This is then available immediately
hash[:b]
# => "Bee"

# The hash now contains both keys
hash
# => { :a => 'a', :b => 'Bee' }

Ruby on Rails confuses this somewhat by providing HashWithIndifferentAccess where it will convert freely between Symbol and String methods of addressing.

You can also index on nearly anything, including classes, numbers, or other Hashes.

hash = { Object => true, Hash => false }

hash[Object]
# => true

hash[Hash]
# => false

hash[Array]
# => nil

Hashes can be converted to Arrays and vice-versa:

# Like many things, Hash supports .to_a
{ :a => 'a' }.to_a
# => [[:a, "a"]]

# Hash also has a handy Hash[] method to create new hashes from arrays
Hash[[[:a, "a"]]]
# => {:a=>"a"} 

When it comes to "inserting" things into a Hash you may do it one at a time, or use the merge method to combine hashes:

{ :a => 'a' }.merge(:b => 'b')
# {:a=>'a',:b=>'b'}

Note that this does not alter the original hash, but instead returns a new one. If you want to combine one hash into another, you can use the merge! method:

hash = { :a => 'a' }

# Returns the result of hash combined with a new hash, but does not alter
# the original hash.
hash.merge(:b => 'b')
# => {:a=>'a',:b=>'b'}

# Nothing has been altered in the original
hash
# => {:a=>'a'}

# Combine the two hashes and store the result in the original
hash.merge!(:b => 'b')
# => {:a=>'a',:b=>'b'}

# Hash has now been altered
hash
# => {:a=>'a',:b=>'b'}

Like many methods on String and Array, the ! indicates that it is an in-place operation.

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