Question

Using set! I want to be able to modify a local state list variable lst, but only a part of it

For example, I want to insert values inside an inner list:

((1 2) (4 5))

becomes

((1 2 3) (4 5))

I want to be able to do something like set! (car lst) (append (car lst) 3)

But that seems to only modify the temporary variable generated by (car lst).

The only way I can think of is to create a new list with the new desired values and set lst to be the new list, but that seems wasteful and unnecessary. Is there a better way to do it?

Answer 1

try this:

(define lst (list (list 1 2) (list 4 5)))
lst
> ((1 2) (4 5))
(set-cdr! (cdar lst) (list 3))
lst
> ((1 2 3) (4 5))

when modifying cons/lists you should use set-cdr! and set-car!

EDIT: for racket

use mutable list:

(require racket/mpair)
(define lst (mlist (mlist 1 2) (mlist 4 5)))
lst
> (mcons (mcons 1 (mcons 2 '())) (mcons (mcons 4 (mcons 5 '())) '()))
(set-mcdr! (mcdr (mcar lst)) (list 3))
> (mcons (mcons 1 (mcons 2 #<promise:temp2>)) (mcons (mcons 4 (mcons 5 '())) '()))
lst