在Ruby中，如何从具有该值的Hash中提取密钥

Question

i thought i'm a Ruby giant when i wrote this oneliner:

# having this hash
hash = { 'Portugal' => 1, 'France' => 2, 'USA' => 3 }

# country_id comes from input
country_name = (hash.select { |k,v| v == country_id.to_i }.first || []).first

it does correctly extract the country name and does Not fail if country not found.

i'm totally pleased with it.

however my tutor says it can/should be optimized in terms of readability, length and performance!

what can be clearer/faster than this?

please advise

Answer 1

well, seems your tutor is right :)

you can do like this:

hash.invert[ country_id.to_i ] # will work on all versions

or, as suggested by @littlecegian

hash.key( country_id.to_i )    # will work on 1.9 only

or, as suggested by @steenslag

hash.index( country_id.to_i )  # will work on 1.8 and 1.9, with a warning on 1.9

Full example:

hash = { 'Portugal' => 1, 'France' => 2, 'USA' => 3 }

%w[2 3 1 blah].each do |country_id|

  # all versions
  country_name = hash.invert[ country_id.to_i ]

  # 1.9 only
  country_name = hash.key( country_id.to_i )

  # 1.8 and 1.9, with a warning on 1.9
  country_name = hash.index( country_id.to_i )


  printf "country_id = %s, country_name = %s\n", country_id, country_name
end

will print:

country_id = 2, country_name = France
country_id = 3, country_name = USA
country_id = 1, country_name = Portugal
country_id = blah, country_name =

see it running here

Answer 2

如果它是ruby 1.9.3，你可以使用hash.key（country_id.to_i）

Answer 3

hash = { 'Portugal' => 1, 'France' => 2, 'USA' => 3 }
puts hash.invert[3] # "USA"

Answer 4

hash = { 'Portugal' => 1, 'France' => 2, 'USA' => 3 }
hash.index(2) # => "France"

is the Ruby 1.8.x way. The index method is deprecated in 1.9 and being replaced with the key method.