gistfile1.rb

def get_random_char
  (r = rand(36)) < 26 ? (?a+r).chr : (?0+r-26).chr
end

# building a string using the above method
def generate_string(len)
  raise ArgumentError if len < 1
  (1..len).map { get_random_char }.join
end

generate_string(7) # => "tf3c38b"