21.rb

# Problem 21
# https://projecteuler.net/problem=21

# Let d(n) be defined as the sum of proper divisors of n 
# (numbers less than n which divide evenly into n).
# If d(a) = b and d(b) = a, where a ≠ b, then a and b are 
# an amicable pair and each of a and b are called amicable numbers.

# For example, the proper divisors of 220 are 
# 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; 
# therefore d(220) = 284. The proper divisors of 
# 284 are 1, 2, 4, 71 and 142; so d(284) = 220.

# Evaluate the sum of all the amicable numbers under 10000.


def sum_of_dividors(num)
  sum = 0
  (1...num).each do |i|
    sum += i if (num % i).zero?
  end
  sum
end

numbers = {}
(1...10_000).each do |i|
  numbers[i] = sum_of_dividors(i)
end

sum = 0
numbers.each do |k, v|
  sum += k if numbers[v] == k and k != v
end

puts sum