project_euler_43.rb

# Sub-string divisibility
# Problem 43
# The number, 1406357289, is a 0 to 9 pandigital number because it is made up of each of the digits 0 to 9 in some order, but it also has a rather interesting sub-string divisibility property.
#
# Let d1 be the 1st digit, d2 be the 2nd digit, and so on. In this way, we note the following:
#
# d2d3d4=406 is divisible by 2
# d3d4d5=063 is divisible by 3
# d4d5d6=635 is divisible by 5
# d5d6d7=357 is divisible by 7
# d6d7d8=572 is divisible by 11
# d7d8d9=728 is divisible by 13
# d8d9d10=289 is divisible by 17
# Find the sum of all 0 to 9 pandigital numbers with this property.

puts (0..9).to_a.permutation(10).inject(0) { |sum, permutation|
  s = permutation.join
  if(s[1..3].to_i % 2 == 0 && s[2..4].to_i % 3 == 0 &&
     s[3..5].to_i % 5 == 0 && s[4..6].to_i % 7 == 0 &&
     s[5..7].to_i % 11 == 0 && s[6..8].to_i % 13 == 0 &&
     s[7..9].to_i % 17 == 0)
    sum + s.to_i
  else
    sum
  end
}