Sunday, November 11, 2007

Rails: Enumerable#index_by

Documentation
Convert an enumerable to a hash. Examples:

people.index_by(&:login)
=> { "nextangle" => , "chade-" => , ...}
people.index_by { |person| "#{person.first_name} #{person.last_name}" }
=> { "Chade- Fowlersburg-e" => , "David Heinemeier Hansson" => , ...}
Usage
I've used Enumerable#index_by for 2 different reasons recently. In one instance I needed faster access to find an element in an array. We found that pulling something out of a hash was faster than using the find method of array. We did a few benchmarks similar to the contrived examples below.
array = [1..100].to_a
hash = array.index_by { |element| element }
array_bm = Benchmark.measure do
array.find { |element| element = 99 }
end
hash_bm = Benchmark.measure do
hash.include? 99
end

p array_bm.real # => 1.382
p hash_bm.real # => 1.001
note: we couldn't use the include? method for our particular instance, so I left it out of the contrived example.

The benchmarks for our project revealed even larger gains, thus it made sense to convert our array to a hash and work with that instead of the array.

The other usage we found for index_by was to utilize the fact that index_by overwrites the value instead of appending (which group_by does). We had a list of scores and wanted to group them together. The group_by method could handle that case; however, we only wanted to keep track of the highest score per group. The test below shows an example where a list of grades can be grouped by grade letter and the highest score per grade letter.

Test

require 'rubygems'
require 'active_support'
require 'test/unit'
require 'dust'

unit_tests do
test "index by highest numeric per grade level" do
grades = [50, 55, 67, 62, 71, 89, 84, 85, 99]
expected = {"A"=>99, "B"=>89, "C"=>71, "D"=>67, "F"=>55}
actual = grades.sort.index_by do |grade|
case
when grade < 60 then "F"
when grade < 70 then "D"
when grade < 80 then "C"
when grade < 90 then "B"
else "A"
end
end
assert_equal expected, actual
end
end
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