Re: guile style

[Tim Van den Langenbergh]

On Saturday, 19 June 2021 02:55:34 CEST jerry wrote:
> I am fairly new to guile and scheme. People tell me that I should use a
> functional style.
>
> I have 3 solutions for project euler problem #1. The first is
> functional, the second is imperative and the third is written in "Little
> Schemer" style.
>
> I was hoping other guile users would comment on preferences or the
> "correct way". Sorry in advance for any wrapping problems that may occur.
>
> #!/usr/local/bin/guile  -s
> !#
> (use-modules (srfi srfi-1) (jpd stdio)) ;; for folds
> (define N 1000)
>
> (define ans
>    (fold + 0
>      (filter
>        (lambda (x) (or (= 0 (modulo x 3)) (= 0 (modulo x 5))))
>        (iota N))))
> (print ans)
>
> (define ans 0)
> (for i N
>    (if (or (= 0 (modulo i 3)) (= 0 (modulo i 5))) (set! ans (+ ans i))))
> (print ans)
>
> (define ans
>    (let loop ((i 1) (ans 0))
>      (cond
>        ((>= i N) ans)
>        ((or (= 0 (modulo i 3)) (= 0 (modulo i 5))) (loop (1+ i) (+ ans i)))
>        (else (loop (1+ i) ans)) )))

I'm not 100% sure about how Guile does it, but I know that some Scheme
implementations do some boxing for set! operations, which will make the second
variation poorly optimised. Personally I would use combine the first and third
answers by doing the divisible-by check during the fold, like this:

(use-modules (srfi srfi-1))

(define (divisible-by? divident divisor)
~~(zero? (modulo divident divisor)))

(define N 1000)

(define ans
~~(fold (lambda (i res)
~~~~~~~~~~(if (or (divisible-by? i 3)
~~~~~~~~~~~~~~~~~~(divisible-by? i 5))
~~~~~~~~~~~~(+ i res)
~~~~~~~~~~~~res))
~~~~~~~~0
~~~~~~~~(iota N)))

Vale,

-Tim