(Scheme) Changing value in an array of booleans using a list

Question

I need to make an array of booleans (0 or 1) with size 800000. I also need to be able to check/change the value at an index whenever I can. I cannot use vectors or the command set!.

I was looking at the documentation and found build-list [ 1 ]. So I made an array of zeros like this:

(define arrBool (build-list 800000 (lambda (x) (* x 0))))

I know I can access an index with list-ref [ 2 ]. However, I cannot find anything in the documentation on how to change a value at that index. For example, if I want to change the 0 at index 27392 to a 1, how would I go about doing this without making a whole new list?

Any help would be appreciated, thanks!

Answer 1

You could use boxes although it's awkward:

> (define arrBool (build-list 20 (lambda (x) (box 0))))
> arrBool
'(#&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0)
> (set-box! (list-ref arrBool 2) 1)
> (set-box! (list-ref arrBool 9) 1)
> arrBool
'(#&0 #&0 #&1 #&0 #&0 #&0 #&0 #&0 #&0 #&1 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0 #&0)
> (unbox (list-ref arrBool 0))
0
> (unbox (list-ref arrBool 2))
1

EDIT

To set some indices to 1 when building the list, do

(define (make-list size indices-to-set)
  (build-list size (lambda (i) (if (member i indices-to-set) 1 0))))

then

> (make-list 20 '(2 9))
'(0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0)