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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: iterative-version for computing a Fibonacci number
Date: Thu, 26 Jun 2025 01:56:40 -0000 (UTC)
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B. Pym wrote:

> Raffael Cavallaro wrote:
> 
> > > Hi, perhaps someone can help me translate my Python version?
> > 
> > (defun fib (x)
> >   (do ((a 0 b) ;; do loop variable a is initially 0, then b
> >        (b 1 (+ a b)) ;;loop variable b is initially 1 then a + b
> >        (i 1 (1+ i))) ;;loop index incremented by 1 each go round
> >       ((> i x) a))) ;;termination condition - when index passes x stop
> >                      ;; and return a
> > 
> > (defun fib-evens (limit)
> >   "find the sum of all the even fibonacci numbers less than 1 million"
> >   (loop for i from 1   ;;loop index starts at 1 implicitly incremented by 1
> >      as current = (fib i) ;;compute fib of the current index
> >      while (< current limit) ;;stop if index exceeds limit
> >      when (evenp current) sum current)) ;;sum all the even fibs and
> > return this sum
> > 
> > CL-USER> (time (fib-evens 1000000)) ;; time the sum of all the even
> > fibs less than a million
> > Evaluation took:
> >   0.0 seconds of real time
> >   8.e-6 seconds of user run time ;;; took about one
> > onehundredthousandth of a second
> >   1.e-6 seconds of system run time
> >   0 page faults and
> >   0 bytes consed.
> > 1089154  ;; here's your answer

It seems that in CL (COBOL-Like), one has to use a macro whose
source measures more than 60 kilobytes.

In a Lispy language, one can program a much shorter solution by
simply using recursion.

Gauche Scheme

(define (sum-even-fibs limit)
  (let go ((a 0) (b 1))
    (if (>= b limit)
      0
      (+ (if (even? b) b 0) (go b (+ a b))))))

(sum-even-fibs 999999)
  ===>
1089154