Re: How to get plist properties list?

[Jean Louis <bugs@gnu.support>]

* Stefan Monnier <monnier@iro.umontreal.ca> [2021-01-09 20:03]:
> > If I wish to get the element like number 17th I do not know what I
> 
> If you need that, it's "too late" to do something else.

I see. Example is here:

(defun mutt-identity-signature (identity)
  (let* ((sql (format "SELECT * FROM identities WHERE identities_id = %s" identity))
	 (list (first (rcd-sql-list sql *cf*)))
	 (identity-id (elt list 0))
	 (identity-name (elt list 1))
	 (identity-first-name (elt list 2))
	 (identity-last-name (elt list 3))
...snip...

As if I do not do like that above, then to construct hash from SQL, I
would need to make little more complex SQL query like:

SELECT 'identities_id', identities_id,
 'identities_name', identities_name,
 'identities_firstname', identities_firstname,
 'identities_lastname', identities_lastname
 AND SO ON.

I could automate it, and now I see I have a function to automate it.

(defun rcd-db-table-id-hash-values (table id pg)
  "Returns hash for TABLE by its table ID with its expanded foreign key values."
  (let* ((oid (rcd/table-oid table pg)) 
	 (attributes (rcd-table-full-attributes oid pg))
	 (foreign-keys (rcd/table-foreign-keys table pg))
	 (hash (make-hash-table)))
      (dolist (col attributes hash)
	(let* ((column (aref col 0))
	       (foreign-key (rcd/column-foreign-key column foreign-keys))
	       (bvalue (rcd-db-get-entry table column id pg))
	       (value (if (and foreign-key bvalue) (rcd/combo-query (car foreign-key) bvalue pg) bvalue)))
	  (puthash (intern column) value hash)))))

then instead of the initial SQL query that requires using `elt' to
fetch values, I get bunch of various functions that need to construct
a hash.

Then I can use instead of:

(format "SELECT * FROM identities WHERE identities_id = %s" identity)

with `elt' complexity, the new complexity:

(rcd-db-table-id-hash-values "identities" identity *cf*)

but underlying functions are many. I get the hash but I doubt I get
the speed up. In this example it is selection of the identity that
selects signature for the email being composed. I cannot visually see
any difference.

You were very right that it is probably too late, so I can put
attention in future. But not that I need to change much of
functionality for sake of speed when no delay is sensible.

> The question is "if I wish to do something over all (or most of) the
> elements of a list".  And for that the answer is "don't use dotimes +
> nth" but use `dolist` or `mapcar` or `while + cdr`.

That is my conclusion now as well.

> Needing the N'th element of a list is actually rare (just like it's
> rare to need to go to kilometer N of a road compared to the frequency
> of having to walk along that road doing this along its way).

I do not know if it is rare as I cannot count and see easily what
other people are doing

I know that I often use it like this, 

(defun mapping/map-locations-point-kml-placemark (p)
  (mapping/kml-placemark (elt p 3) (elt p 4) (list (elt p 0) (elt p 1) (elt p 2))))

(defun mapping/map-locations-point-marble (p)
  (mapping/kml-marble-bookmark (elt p 3) (elt p 4) (list (elt p 0) (elt p 1) (elt p 2))))

using it in different manner would complicate the code more I think.

If I do some grep in ~/.emacs.d/elpa/* I see many packages using it as
well.

In general I understood that one could think of speed in certain
situations, but in other situations where speed is not important using
lists is fine.

Jean