Mutating data in complex list

Question

Suppose I have defined following lists:

(define a (list 1 2))
(define b (list a 3))

I defined a explicitly to show because I want to modify the list inside b. Do I need to re-construct whole data structure from scratch to add an item to inner list?

Edit: I'm not trying to change the element in the inner list, I want to add another element to the inner list.

Answer 1

It's possible to modify the list in-place using set-car! and set-cdr!, as long as you pass as parameter the exact pair within the list that needs to be modified and the corresponding value. For example, to replace an element in the inner list:

(define a (list 1 2))
(define b (list a 3))

b
=> '((1 2) 3)

(set-car! (cdar b) 'x)
b
=> ((1 x) 3)

And if you want to add another element to the inner list, do this:

(set-cdr! (cdar b) '(4))
b
=> ((1 x 4) 3)