Thanks, Daniel, this is helpful. I think that I understand your code, but
it is a still a foreign way of thinking for me. Of course, that is a big
part of why I'm learning Racket, i.e., to make programming with lists and
recursion more natural. One key gap for me is how to build up data
structures using lists and recursion. Perhaps the answer is that you don't
build up the data structure in memory but by writing to disk. In this
particular example, it is useful to have the values at every iteration for
plotting the population trajectory (by age class or summed across age
classes).
Below is a little example I was exploring that involves matrix
multiplication (col-matrix is abundance in different age classes; square
matrix is transition probabilities among age classes). It was obvious to me
how to use recursion to get the final col-matrix of abundances but not how
to build up a data structure that included the abundances at every
iteration.
(require math/matrix)
(define A (matrix [[0 0 5.905]
[0.368 0.639 0.025]
[0.001 0.152 0.051]]))
(define n (col-matrix [5 5 5]))
(define (pop-projection A n iter)
(if (zero? iter) n
(pop-projection A (matrix* A n) (- iter 1))))
(pop-projection A n 25)
On Sunday, February 10, 2019 at 3:56:25 AM UTC-8, Daniel Prager wrote:
>
> Thanks for the screenshot Travis.
>
> Just for fun, here's a version in Racket that eschews assignment, vectors
> and for loops, in favour of recursion and lists ...
>
> #lang racket
>
> (define years 30)
> (define prop-female 0.5)
> (define egg-surv 0.6)
>
> (define fecundity '(0 0 200 400 800))
> (define survival '(0.2 0.4 0.6 0.8 0))
> (define capacity '(1e6 1e5 1e4 1e3 1e2 -9999))
> (define cap0 (first capacity))
>
> (define (beverton-holt N p c) (/ N (+ (/ 1 p) (/ N c))))
>
> (define (evolve N [f fecundity] [s survival] [cap (rest capacity)] [Nt0 0]
> [Nt null])
> (if (null? f)
> (cons Nt0 (reverse Nt))
> (evolve (rest N) (rest f) (rest s) (rest cap)
> (+ Nt0 (if (= (first f) 0)
> 0
> (beverton-holt (* (first N) prop-female)
> (* (first f) egg-surv)
> (- cap0 Nt0))))
> (if (= (first s) 0)
> Nt
> (cons (beverton-holt (first N) (first s) (first cap))
> Nt)))))
>
> (define (iterate N n [i 1])
> (displayln (list i N))
> (unless (= i n) (iterate (evolve N) n (+ i 1))))
>
> (iterate (make-list (length fecundity) 10) years)
>
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