Functions with optional arguments are slow. They are expanded to a
case-lambda and an if. This version with an auxiliary function is
faster:
#lang racket
(define as (build-list 1000000 (λ (n) (random 100))))
(define bs (build-list 1000000 (λ (n) (random 100))))
(define (f/opt as bs [acc 0])
(if (or (null? as) (null? bs))
acc
(f/opt (rest as) (rest bs) (+ acc (* (first as) (first bs))))))
(collect-garbage)
(collect-garbage)
(collect-garbage)
(time (f/opt as bs))
(define (f/acc as bs acc)
(if (or (null? as) (null? bs))
acc
(f/acc (rest as) (rest bs) (+ acc (* (first as) (first bs))))))
(define (f/no-acc as bs acc)
(f/acc as bs 0))
(collect-garbage)
(collect-garbage)
(collect-garbage)
(time (f/no-acc as bs 0))
;cpu time: 281 real time: 268 gc time: 0
;2447735069
;cpu time: 94 real time: 93 gc time: 0
;2447735069
Gustavo
On Thu, Jul 27, 2017 at 10:07 PM, Jon Zeppieri <zeppi...@gmail.com> wrote:
> You can get better performance out of the recursive function by using
> car/cdr instead of first/rest; first/rest require their arguments to
> be lists, whereas car/cdr require theirs to be pairs, which is a lot
> cheaper to check. Also, using an optional argument (in a loop,
> especially) makes a difference.
>
> Even so, for/sum wins, because it uses ops like unsafe-car and
> unsafe-cdr, which it can do safely, since it first establishes that
> it's working with a list (and then doesn't have to check again).
>
> - Jon
>
> On Thu, Jul 27, 2017 at 8:55 PM, Daniel Prager
> <daniel.a.pra...@gmail.com> wrote:
>> Revised to collect garbage before each timing shows the recursive function
>> isn't too bad (but not great):
>>
>> cpu time: 405 real time: 404 gc time: 0
>> 2452578471
>>
>> cpu time: 368 real time: 368 gc time: 0
>> 2452578471
>>
>> cpu time: 50 real time: 50 gc time: 0
>> 2452578471
>>
>> cpu time: 194 real time: 195 gc time: 75
>> 2452578471
>>
>>
>> Dan
>>
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