*Slightly too significant, since I wrote g incorrectly. This is right:
#lang racket
(collect-garbage)
(collect-garbage)
(collect-garbage)
(define as (build-list 1000000 (λ (n) (random 100))))
(define bs (build-list 1000000 (λ (n) (random 100))))
(define (f as bs [acc 0])
(if (or (null? as) (null? bs))
acc
(f (rest as) (rest bs) (+ acc (* (first as) (first bs))))))
(time (f as bs))
(collect-garbage)
(collect-garbage)
(collect-garbage)
(define (g as bs)
(let g ([as as]
[bs bs]
[acc 0])
(cond
[(or (null? as) (null? bs))
acc]
[else
(match-define (cons this-a more-as)
as)
(match-define (cons this-b more-bs)
bs)
(g more-as more-bs (+ acc (* this-a this-b)))])))
(time (g as bs))
;; timed in DrRacket w/ debugging
;cpu time: 386 real time: 385 gc time: 45
;2448129646
;cpu time: 122 real time: 121 gc time: 0
;2448129646
-Philip
On Thu, Jul 27, 2017 at 8:13 PM, Philip McGrath <phi...@philipmcgrath.com>
wrote:
> Eliminating the optional argument and using match-define to avoid any
> redundant checking on first and rest gets me a significant speedup:
>
> #lang racket
>
> (collect-garbage)
> (collect-garbage)
> (collect-garbage)
>
> (define as (build-list 1000000 (λ (n) (random 100))))
> (define bs (build-list 1000000 (λ (n) (random 100))))
>
> (define (f as bs [acc 0])
> (if (or (null? as) (null? bs))
> acc
> (f (rest as) (rest bs) (+ acc (* (first as) (first bs))))))
>
> (time (f as bs))
>
> (collect-garbage)
> (collect-garbage)
> (collect-garbage)
>
> (define (g as bs)
> (let g ([as as]
> [bs bs]
> [acc 0])
> (cond
> [(or (null? as) (null? bs))
> acc]
> [else
> (match-define (cons this-a more-as)
> as)
> (match-define (cons this-b more-bs)
> bs)
> (g more-as more-bs (+ acc this-a this-b))])))
>
> (time (g as bs))
>
> ; timed in DrRacket w/ debugging
> ;cpu time: 326 real time: 326 gc time: 0
> ;2451374037
> ;cpu time: 68 real time: 69 gc time: 0
> ;98991285
>
> -Philip
>
> On Thu, Jul 27, 2017 at 8:08 PM, Gustavo Massaccesi <gust...@oma.org.ar>
> wrote:
>
>> 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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>
>
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