gistfile1.rb

def reduce(arr, amount) 
   sample = [] 
   ratio = (arr.count / amount).floor 
   while sample.count < amount 
     index = sample.count 
     inf = index * ratio 
     sup = (index + 1) * ratio - 1.0 
     sum = arr[inf..sup].reduce(:+)
     sample << sum / sup - inf + 1.0
   end 
   sample 
 end

arr = (1..10).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
reduce(arr, 10)
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

arr = (1..20).to_a
=> [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
reduce(arr, 5)
=> [2.5, 6.5, 10.5, 14.5, 18.5]