Get index of array element faster than O(n)

Given I have a HUGE array, and a value from it. I want to get index of the value in array. Is there any other way, rather then call Array#index to get it? The problem comes from the need of keeping really huge array and calling Array#index enormous amount of times.

After a couple of tries I found that caching indexes inside elements by storing structs with (value, index) fields instead of the value itself gives a huge step in performance (20x times win).

Still I wonder if there's a more convenient way of finding index of en element without caching (or there's a good caching technique that will boost up the performance).

Solution 1:

Why not use index or rindex?

array = %w( a b c d e)
# get FIRST index of element searched
puts array.index('a')
# get LAST index of element searched
puts array.rindex('a')

index: http://www.ruby-doc.org/core-1.9.3/Array.html#method-i-index

rindex: http://www.ruby-doc.org/core-1.9.3/Array.html#method-i-rindex

Solution 2:

Convert the array into a hash. Then look for the key.

array = ['a', 'b', 'c']
hash = Hash[array.map.with_index.to_a]    # => {"a"=>0, "b"=>1, "c"=>2}
hash['b'] # => 1