How do I find the index of the maximum value of an array?

Question

I tried the solution recommended here -- In Ruby, what is the cleanest way of obtaining the index of the largest value in an array?

array = [nil, nil, nil, nil, nil, 0.9655172413793104, nil, nil]
idx = array.each_with_index.max[1]

But am getting some exceptions:

ArgumentError: comparison of Array with Array failed
    from (irb):4:in `each'
    from (irb):4:in `each_with_index'
    from (irb):4:in `each'
    from (irb):4:in `max'
    from (irb):4
    from /Users/davea/.rvm/gems/ruby-2.4.0/gems/railties-5.0.3/lib/rails/commands/console.rb:65:in `start'
    from /Users/davea/.rvm/gems/ruby-2.4.0/gems/railties-5.0.3/lib/rails/commands/console_helper.rb:9:in `start'
    from /Users/davea/.rvm/gems/ruby-2.4.0/gems/railties-5.0.3/lib/rails/commands/commands_tasks.rb:78:in `console'
    from /Users/davea/.rvm/gems/ruby-2.4.0/gems/railties-5.0.3/lib/rails/commands/commands_tasks.rb:49:in `run_command!'
    from /Users/davea/.rvm/gems/ruby-2.4.0/gems/railties-5.0.3/lib/rails/commands.rb:18:in `<top (required)>'
    from bin/rails:4:in `require'
    from bin/rails:4:in `<main>'

How do you determine if `nil` is less than or greater than a float? Assuming you think `nil` is equivalent to zero, then you could squeeze `.map(&:to_f)` into the chain. — Sagar Pandya, Nov 12 '17 at 19:25
What should the result be if there are multiple maximum values? — Jörg W Mittag, Nov 12 '17 at 19:46

score 6 · Accepted Answer · answered Nov 12 '17 at 19:28

If you want to omit nils from the result, then you can use:

array.index(array.compact.max)

Or if you wish to treat nils like zeros, then first convert them to Floats:

array.index(array.map(&:to_f).max)

In the event of a tie, this will return the index of the first max value. You could also get the last index with Array#rindex.

Cary Swoveland · Answer 2 · 2017-11-12T21:26:56.417

def max_idx(array)
  mx = array.select { |e| e.kind_of?(Numeric) }.max 
  mx ? array.each_index.select { |i| array[i] == mx } : nil
end      

require 'bigdecimal'
max_idx [nil,3,1]                   #=> [1]
max_idx [nil,3.2,"cat",1]           #=> [1]
max_idx [nil,3,nil,1,3]             #=> [1,4]
max_idx [nil,3.2,nil,1,3.2]         #=> [1,4]
max_idx [nil,Rational(3),1]         #=> [1]
max_idx [nil,BigDecimal.new("3"),1] #=> [1]
max_idx [nil,nil,nil]               #=> nil

How do I find the index of the maximum value of an array?

2 Answers2