The lisp-way to solve Fibonnaci

Question

I wanted to try and learn Lisp, but I very quickly gave up. I figured I\'d try again. I\'m looking at Problem 2 on Project Euler - finding the sum of all even Fibonacci numbers

Answer 1

To expand on Danio's answer, the article at http://fare.tunes.org/files/fun/fibonacci.lisp presents two ways of making the code run faster. Using straight recursion (tail call or no) is O(2^n) and very slow. The difficulty is that each value gets calculated over and over again. You have to do things differently. The two recommendations are:

1. Use an iterative approach.

(defun bubble-fib (n)
  (declare (optimize (speed 3) (safety 0) (debug 0)))
  (check-type n fixnum)
  (loop repeat n
  with p = 0 with q = 1
  do (psetq p q
        q (+ p q))
  finally (return p)))

2. Use Memoization. This means remembering the values see before and recalling them rather than recalculating them. The article provides a CL package that will do this as well as some code to do it yourself.