Question

I’m trying to learn Lisp (elisp, actually), and I tried writing the following function as an exercise Project Euler Problem 2

(defun sumfib (n fn1 fn2 sum)
  "Calculate Fibonacci numbers up to 4,000,000 and sum all the even ones"
  (if (< 4000000 (+ fn1 fn2))
      sum
    (if (equal n 3)
        (sumfib 1 (+ fn1 fn2) fn1 (+ sum (+fn1 fn2)))
      (sumfib (+ n 1) (+ fn1 fn2) fn1 sum)))

When I evaluate it I get the following error:

Debugger entered--Lisp error: (void-variable fn1)
  (+ fn1 fn2)
  (< 4000000 (+ fn1 fn2))
...

Why wouldn’t it recognize fn1? If I try to put (+ fn1 fn2) before the ‘if’ it doesn’t complain about it there, so why the error here?

Also, I realize the function may not actually be correct, but for now I’m not worried about the logic – I’ll figure that out later. For now, I’m only interested in understanding this error.

Answer 1

Not sure if this solves your problem (I don’t have an elisp interpreter handy), but you’re missing a right paren. There should be one at the end of the first ‘if’ statement, after ‘sum’.

  (if (< 4000000 (+ fn1 fn2))
      sum)

instead of:

 (if (< 4000000 (+ fn1 fn2))
      sum

I also think there might be some other issues in there.