A bit of modernity yields this:
(define (project-pop.v1 y r k thetasd t [i 1])
(cond
[(= i t) (list y)]
[else (define theta (flvector-ref (flnormal-sample 0.0 thetasd 1) 0))
(define y1 (* y (- r (* r (/ y k))) (exp theta)))
(cons y (project-pop y1 r k thetasd t (+ i 1)))]))
Since this is really a fold, you could also use one of the Racket
comprehensions to bring across the intent:
(define (project-pop.v2 y r k thetasd t)
(define-values (_ l)
(for/fold ((y y) (l '())) ((i (in-range 0 t 1)))
(define theta (flvector-ref (flnormal-sample 0.0 thetasd 1) 0))
(values (* y (- r (* r (/ y k))) (exp theta)) (cons y l))))
(reverse l))
Given how short these functions are, v1 is just fine.
Or you could stick to the original R design and write an imperative vector-fill
function:
(define (project-pop.v3 y0 r k thetasd t)
(define y (make-vector t y0))
(for ((i (in-range 1 t 1)))
(define theta (flnormal-sample 0.0 thetasd 1))
(define y@i-1 (vector-ref y (- i 1)))
(vector-set! y i (* y@i-1 (- r (* r (/ y@i-1 k))) (exp (flvector-ref theta
0)))))
y)
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