[ruby] How to find a hash key containing a matching value

Given I have the below clients hash, is there a quick ruby way (without having to write a multi-line script) to obtain the key given I want to match the client_id? E.g. How to get the key for client_id == "2180"?

clients = {
  "yellow"=>{"client_id"=>"2178"}, 
  "orange"=>{"client_id"=>"2180"}, 
  "red"=>{"client_id"=>"2179"}, 
  "blue"=>{"client_id"=>"2181"}
}

try this:

clients.find{|key,value| value["client_id"] == "2178"}.first

Ruby 1.9 and greater:

hash.key(value) => key

Ruby 1.8:

You could use hash.index

hsh.index(value) => key

Returns the key for a given value. If not found, returns nil.

h = { "a" => 100, "b" => 200 }
h.index(200) #=> "b"
h.index(999) #=> nil

So to get "orange", you could just use:

clients.key({"client_id" => "2180"})

You can invert the hash. clients.invert["client_id"=>"2180"] returns "orange"

The best way to find the key for a particular value is to use key method that is available for a hash....

gender = {"MALE" => 1, "FEMALE" => 2}
gender.key(1) #=> MALE

I hope it solves your problem...

Another approach I would try is by using #map

clients.map{ |key, _| key if clients[key] == {"client_id"=>"2180"} }.compact 
#=> ["orange"]

This will return all occurences of given value. The underscore means that we don't need key's value to be carried around so that way it's not being assigned to a variable. The array will contain nils if the values doesn't match - that's why I put #compact at the end.

From the docs:

• (Object?) detect(ifnone = nil) {|obj| ... }
• (Object?) find(ifnone = nil) {|obj| ... }
• (Object) detect(ifnone = nil)
• (Object) find(ifnone = nil)

Passes each entry in enum to block. Returns the first for which block is not false. If no object matches, calls ifnone and returns its result when it is specified, or returns nil otherwise.

If no block is given, an enumerator is returned instead.

(1..10).detect  {|i| i % 5 == 0 and i % 7 == 0 }   #=> nil
(1..100).detect {|i| i % 5 == 0 and i % 7 == 0 }   #=> 35

This worked for me:

clients.detect{|client| client.last['client_id'] == '2180' } #=> ["orange", {"client_id"=>"2180"}] 

clients.detect{|client| client.last['client_id'] == '999999' } #=> nil 

See: http://rubydoc.info/stdlib/core/1.9.2/Enumerable#find-instance_method

According to ruby doc http://www.ruby-doc.org/core-1.9.3/Hash.html#method-i-key key(value) is the method to find the key on the base of value.

ROLE = {"customer" => 1, "designer" => 2, "admin" => 100}
ROLE.key(2)

it will return the "designer".

You could use hashname.key(valuename)

Or, an inversion may be in order. new_hash = hashname.invert will give you a new_hash that lets you do things more traditionally.

Heres an easy way to do find the keys of a given value:

    clients = {
      "yellow"=>{"client_id"=>"2178"}, 
      "orange"=>{"client_id"=>"2180"}, 
      "red"=>{"client_id"=>"2179"}, 
      "blue"=>{"client_id"=>"2181"}
    }

    p clients.rassoc("client_id"=>"2180")

...and to find the value of a given key:

    p clients.assoc("orange") 

it will give you the key-value pair.