Generate array of all letters and digits

[*('a'..'z'), *('0'..'9')] # doesn't work in Ruby 1.8

or

('a'..'z').to_a + ('0'..'9').to_a

or

(0...36).map{ |i| i.to_s 36 }

(the Integer#to_s method converts a number to a string representing it in a desired numeral system)