In Ruby, what is the cleanest way of obtaining the index of the largest value in an array?

If a is the array, I want a.index(a.max), but something more Ruby-like. It should be obvious, but I'm having trouble finding the answer at so and elsewhere. Obviously, I am n开发者_如何学JAVAew to Ruby.

For Ruby 1.8.7 or above:

a.each_with_index.max[1]

It does one iteration. Not entirely the most semantic thing ever, but if you find yourself doing this a lot, I would wrap it in an index_of_max method anyway.

In ruby 1.9.2 I can do this;

arr = [4, 23, 56, 7]
arr.rindex(arr.max)  #=> 2

Here is what I am thinking to answer this question :

a = (1..12).to_a.shuffle
# => [8, 11, 9, 4, 10, 7, 3, 6, 5, 12, 1, 2]
a.each_index.max_by { |i| a[i] }
# => 9

Just wanted to note a behavioral and performance difference for some of the solutions here. The "tie breaking" behavior of duplicate max elements:

a = [3,1,2,3]
a.each_with_index.max[1]
# => 3
a.index(a.max)
# => 0

Out of curiosity I ran them both in Benchmark.bm (for the a above):

user     system      total        real
each_with_index.max  0.000000   0.000000   0.000000 (  0.000011)
index.max  0.000000   0.000000   0.000000 (  0.000003)

Then I generated a new a with Array.new(10_000_000) { Random.rand } and reran the test:

user     system      total        real
each_with_index.max
  2.790000   0.000000   2.790000 (  2.792399)
index.max  0.470000   0.000000   0.470000 (  0.467348)

This makes me think unless you specifically need to choose the higher index max, a.index(a.max) is the better choice.

Here is a way to get all the index values of the max values if more than one.

Given:

> a
=> [1, 2, 3, 4, 5, 6, 7, 9, 9, 2, 3]

You can find the index of all the max values (or any given value) by:

> a.each_with_index.select {|e, i| e==a.max}.map &:last
=> [7, 8]

a = [1, 4 8]
a.inject(a[0]) {|max, item| item > max ? item : max }

At least it's Ruby-like :)

Using #each_with_index and #each_with_object. Only one pass required.

def index_of_first_max(e)
  e.each_with_index.each_with_object({:max => nil, :idx => nil}) { |x, m| 
    x, i = x

    if m[:max].nil? then m[:max] = x
    elsif m[:max] < x then m[:max] = x; m[:idx] = i
    end
  }[:idx]
end

Or combining #each_with_index with #inject:

def index_of_first_max(e)
  e.each_with_index.inject([nil, 0]) { |m, x|
    x, i = x
    m, mi = m

    if m.nil? || m < x then [x, i]
    else [m, mi]
    end
  }.last    
end