Alist example not quite right about emacs-elisp-programming HOT 3 CLOSED

You can actually, have association lists as lists of two elements or list of cons cells. It works with both, your functions and mine are correct. Here na example:

ELISP> (defun plist->alist (plist)
  (if (null plist)
      '()
      (cons
       (list (car plist) (cadr plist))
       (plist->alist (cddr plist)))))
plist->alist
ELISP> 
ELISP> (plist->alist (list :x 10 :y 20 :name "point"))
((:x 10)
 (:y 20)
 (:name "point"))

ELISP> (assoc :x (plist->alist (list :x 10 :y 20 :name "point")))
(:x 10)

ELISP> (assoc :y (plist->alist (list :x 10 :y 20 :name "point")))
(:y 20)

ELISP> 

The problem with list of cons cells is that in the first code below, it becomes a single list, which makes hard to distinguish between a "list" or cons cell (Every list is a cons cell, of the head element and the rest of the list).

ELISP> (cons :x '(1 2 3 4))
(:x 1 2 3 4)

ELISP> '(:x . (1 2 3 4))
(:x 1 2 3 4)

ELISP> (list :x '(1 2 3 4))
(:x
 (1 2 3 4))

from emacs-elisp-programming.

OK, I agree: should have read the manual more carefully. However, as the
manual says, reverse lookup with rassoc will fail on your version:

ELISP> (setq my-alist '((:a . 1) (:b . 2)))
((:a . 1)
(:b . 2))

ELISP> (setq yr-alist '((:a 1) (:b 2)))
((:a 1)
(:b 2))

ELISP> (rassoc 2 my-alist)
(:b . 2)

ELISP> (rassoc 2 yr-alist)
nil

---Fran

On 7 December 2015 at 04:54, Caio Rordrigues [email protected]
wrote:

You can actually, have association lists as lists of two elements or list
of cons cells. It works with both, your functions and mine are correct.
Here a example:

ELISP> (defun plist->alist (plist)
(if (null plist)
'()
(cons
(list (car plist) (cadr plist))
(plist->alist (cddr plist)))))
plist->alist
ELISP>
ELISP> (plist->alist (list :x 10 :y 20 :name "point"))
((:x 10)
(:y 20)
(:name "point"))

ELISP> (assoc :x (plist->alist (list :x 10 :y 20 :name "point")))
(:x 10)

ELISP> (assoc :y (plist->alist (list :x 10 :y 20 :name "point")))
(:y 20)

ELISP>

The problem with list of cons cells is that in the first code below the it
becomes a single list, which makes hard to distinguish between a "list" or
cons cell (Every list is a cons cell, of the head element and the rest of
the list).

ELISP> (cons :x '(1 2 3 4))
(:x 1 2 3 4)

ELISP> '(:x . (1 2 3 4))
(:x 1 2 3 4)

ELISP> (list :x '(1 2 3 4))
(:x
(1 2 3 4))

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#12 (comment)
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from emacs-elisp-programming.

Thanks for the contribution. Ok I will add your function, but I will call this list of cons pair
of clist '((a . b) (x . y)) and the lists of two elements of alist to reduce the ambiguity '((a b) (x y)).

from emacs-elisp-programming.