Lisp: How to get the reference (not the value) of an element in a 2D Matrix represented as a List of Lists?

Question

I am trying to reference a specific element in a 2D Matrix represented as list of Lists so I can set that specific element to 0. However, when I run the code below:

(defvar listOfLists '( (1 3) (2 6) ))
(defvar referenceToTopLeftCorner (nth 0 (nth 0 listOfLists)))
(setq referenceToTopLeftCorner 0)
(print (format nil "listsOfLists = ~a" listOfLists))

The following output is:

""listsOfLists = ((1 3) (2 6))"

This seems strange, as I thought that the nth method can be used to get references within a list?

Answer 1

The variable you called reference is not a reference.

You need to do

(setf (nth 0 (nth 0 list-of-lists)) 0)

Note that you could also do

(defvar 1st-list (first list-of-lists))
(setf (first 1st-list) 0)

for the same effect, because 1st-list refers to the 1st list in list-of-lists.

You could also use define-symbol-macro:

(define-symbol-macro reference-to-top-left-corner
  (first (first list-of-lists)))
(setf reference-to-top-left-corner 0)

because now reference-to-top-left-corner really is a reference.

I strongly advise you not to use symbol macros yet. Such advanced tools should be used sparingly and cautiously.