recursion2.rb

# Fibonacci sequence with recursion
def fib(number)
  if number < 0
    raise ArgumentError, 'fib(n) defined for number>=0'
  end

  return fib(number - 1) + fib(number - 2) if number > 1
  number
end

# Fibonacci sequence without recursion
def fib2(number)
  raise ArgumentError, 'fib(n) defined for number>=0' if number < 0
  n0, n1 = 0, 1
  (0...number).each do
    n0, n1 = n1, n0 + n1
  end
  n0
end