CODEWITHSUNDEEP
rubyhashelement
user1854438
user1854438

Reputation: 1842

Ruby how to return an element of a dictionary?

@ dictionary = {"cat"=>"Sam"}

This a return a key 

@dictionary.key(x)

This returns a value 

@dictionary[x]

How do I return the entire element "cat"=>"Sam"

Upvotes: 0

Views: 2382

Answers (4)

steenslag
steenslag

Reputation: 80075

You can get the key and value in one go - resulting in an array:

@h = {"cat"=>"Sam", "dog"=>"Phil"}
key, value = p h.assoc("cat") # => ["cat", "Sam"]

Use rassoc to search by value ( .rassoc("Sam") )

Upvotes: 0

Cary Swoveland
Cary Swoveland

Reputation: 110695

If you wish to return one element of a hash h, you will need to specify the key to identify the element. As the value for key k is h[k], the key-value pair, expressed as an array, is [k, h[k]]. If you wish to make that a hash with a single element, use Hash[[[k, h[k]]]].

For example, if

h = { "cat"=>"Sam", "dog"=>"Diva" }

and you only wanted to the element with key "cat", that would be

["cat", h["cat"]]         #=> ["cat", "Sam"]

or

Hash[[["cat", h["cat"]]]] #=> {"cat"=>"Sam"}

With Ruby 2.1 you could alternatively get the hash like this:

[["cat", h["cat"]]].to_h  #=> {"cat"=>"Sam"}

Let's look at a little more interesting case. Suppose you have an array arr containing some or all of the keys of a hash h. Then you can get all the key-value pairs for those keys by using the methods Enumerable#zip and Hash#values_at:

arr.zip(arr.values_at(*arr))

Suppose, for example,

h = { "cat"=>"Sam", "dog"=>"Diva", "pig"=>"Petunia", "owl"=>"Einstein" }

and

arr = ["dog", "owl"]

Then:

arr.zip(h.values_at(*arr))
  #=> [["dog", "Diva"], ["owl", "Einstein"]]

In steps:

a = h.values_at(*arr)
  #=> h.values_at(*["dog", "owl"])
  #=> h.values_at("dog", "owl")
  #=> ["Diva", "Einstein"]
arr.zip(a)
  #=> [["dog", "Diva"], ["owl", "Einstein"]]

To instead express as a hash:

Hash[arr.zip(h.values_at(*arr))]
  #=> {"dog"=>"Diva", "owl"=>"Einstein"}

Upvotes: 0

D-side
D-side

Reputation: 9485

Your example is a bit (?) misleading in a sense it only has one pair (while not necessarily), and you want to get one pair. What you call a "dictionary" is actually a hashmap (called a hash among Rubyists).

A hashrocket (=>) is a part of hash definition syntax. It can't be used outside it. That is, you can't get just one pair without constructing a new hash. So, a new such pair would look as: { key => value }.

So in order to do that, you'll need a key and a value in context of your code somewhere. And you've specified ways to get both if you have one. If you only have a value, then:

{ @dictionary.key(x) => x }

...and if just a key, then:

{ x => @dictionary[x] }

...but there is no practical need for this. If you want to process each pair in a hash, use an iterator to feed each pair into some code as an argument list:

@dictionary.each do |key, value|
  # do stuff with key and value  
end

This way a block of code will get each pair in a hash once.

If you want to get not a hash, but pairs of elements it's constructed of, you can convert your hash to an array:

@dictionary.to_a
# => [["cat", "Sam"]]
# Note the double braces! And see below.
# Let's say we have this:
@dictionary2 = { 1 => 2, 3 => 4}
@dictionary2[1]
# => 2
@dictionary2.to_a
# => [[1, 2], [3, 4]]
# Now double braces make sense, huh?

It returns an array of pairs (which are arrays as well) of all elements (keys and values) that your hashmap contains.

Upvotes: 0

aelor
aelor

Reputation: 11116

@dictionary

should do the trick for you

whatever is the last evaluated expression in ruby is the return value of a method. If you want to return the hash as a whole. the last line of the method should look like the line I have written above

Upvotes: 1

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