In Elixir, "" and 0 are considered truthy values.

Is this idiomatic Elixir way?

Learned something new on the gsub block method!

sum += value is not needed, sum + value is enough, since the value will be returned to sum in each iteration

Doesn't need +=, s + $dict[ch] would be enough. :)

Can someone explain this one?

Nice one!

No reason. They are the same.

Here's the bench mark for both methods.

require "benchmark"

n = 1_000_000
ARR = %w(today is a good day isn't it?)
Benchmark.bmbm do |bm|
  bm.report { n.times { ARR.map(&:capitalize) } }
  bm.report { n.times { ARR.map { |i| i.capitalize } } }
end

Rehearsal ------------------------------------
   1.890000   0.000000   1.890000 (  1.893212)
   1.930000   0.010000   1.940000 (  1.928549)
--------------------------- total: 3.830000sec

       user     system      total        real
   1.900000   0.000000   1.900000 (  1.910439)
   2.050000   0.000000   2.050000 (  2.050394)

Which version of Ruby do you have locally? My Ruby 2.0.0 return false for negative numbers.

Didn't see your JS translation show up?

It's using a recursive function. It passes two arguments, n and a, and a has a default value [].

In Ruby, collatz n, a = [] is same as collatz(n, a = []), the parentesis can be omitted.

Try to debug in your local ide to see if it works. If it does work then might be some codewars issue.