On Thursday, April 11, 2019 at 1:50:23 PM UTC+8, travis....@gmail.com wrote:
>
> Hi All,
>
> I'm trying to better understand recursion. I decided to rewrite the
> logmodr function from this blog post
> <http://www.seascapemodels.org/rstats/2017/02/26/speeding-up-sims.html> as
> a recursive function. I was easily able to write the recursive functions if
> all but one of the arguments were set to default values. But I was
> completely stumped by how to handle the situation without using default
> argument values. After a considerable amount of flailing about, I finally
> realized that I could accomplish this task by passing the arguments as a
> struct (see code below). I find the struct approach relatively intuitive
> but it strikes me as quite verbose. Are there other (better?) ways to
> handle this task?
>
The trick is to use nested functions, like so:
(define (project-pop-1 y r k thetasd t)
(define (logmod y)
(define theta (flvector-ref (flnormal-sample 0.0 thetasd 1) 0))
(* y (- r (* r (/ y k))) (exp theta)))
(define (loop y i)
(if (= i t)
(list y)
(cons y (loop (logmod y) (add1 i)))))
(loop y 1))
(project-pop-1 1.0 1.8 20.0 0.1 25)
Also, multiplication accepts any number of arguments, instead of (* a (* b
c)) when you can write (* a b c)
Also, if you want to use structures, you can use `struct-copy` and
`match-define` and the code looks much nicer. Your implementation of
logmod can be written as:
(define (logmod-1 args-struct)
(match-define (args y r k thetasd) args-struct)
(define theta (flvector-ref (flnormal-sample 0.0 thetasd 1) 0))
(define new-y (* y (- r (* r (/ y k))) (exp theta)))
(struct-copy args args-struct [y new-y]))
Alex.
>
> Thanks,
>
> Travis
>
>
> #lang racket
>
> (require math)
>
> (struct args (y r k thetasd) #:transparent) ; only 'y' changes in
> project-pop function below
>
> (define (logmod args-struct)
> (define theta (flvector-ref (flnormal-sample 0.0 (args-thetasd
> args-struct) 1) 0))
> (args (* (args-y args-struct) (* (- (args-r args-struct) (* (args-r
> args-struct) (/ (args-y args-struct) (args-k args-struct)))) (exp theta)))
> (args-r args-struct)
> (args-k args-struct)
> (args-thetasd args-struct)))
>
> (define (project-pop args-struct t [i 1])
> (define y (args-y args-struct))
> (if (= i t)
> (list y)
> (cons y (project-pop (logmod args-struct) t (+ i 1)))))
>
> (project-pop (args 1.0 1.8 20.0 0.1) 25)
>
>
>
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