alists with precomputed variables

alists with precomputed variables

[Erik Iverson]

Dear all,

I'm an Elisp beginner, just learning over the last few days. Very naive 
question here, with hopefully a simple answer.

I would like to create an alist where one of the values is something 
that I compute earlier.  Example,

I naively thought that

(setq my-var 20)

(setq my-alist '((p1 . 10) (p2 . my-var)))

would give me

((p1 . 10) (p2 . 20))

, but it instead results in

((p1 . 10) (p2 . my-var))

How can I get what I expected?

If this is a wrong-headed question, please let me know.

Thanks,
Erik

RE: alists with precomputed variables

[Drew Adams]

> (setq my-var 20)
> (setq my-alist '((p1 . 10) (p2 . my-var)))
> 
> I naively thought that would give me
> ((p1 . 10) (p2 . 20))
> 
> but it instead results in
> ((p1 . 10) (p2 . my-var))
> 
> How can I get what I expected?

(setq my-alist `((p1 . 10) (p2 ,@my-var)))

or (setq my-alist (list '(p1 . 10) (cons 'p2 my-var)))

or (setq my-alist (list (cons 'p1 10) (cons 'p2 my-var)))

See the Elisp manual, node Backquote for info about ` and ,@.
See also nodes Quoting and Building Lists.

Re: alists with precomputed variables

[Barry Margolin]

In article <mailman.5420.1232514814.26697.help-gnu-emacs@gnu.org>,
 "Drew Adams" <drew.adams@oracle.com> wrote:

> > (setq my-var 20)
> > (setq my-alist '((p1 . 10) (p2 . my-var)))
> > 
> > I naively thought that would give me
> > ((p1 . 10) (p2 . 20))
> > 
> > but it instead results in
> > ((p1 . 10) (p2 . my-var))
> > 
> > How can I get what I expected?
> 
> (setq my-alist `((p1 . 10) (p2 ,@my-var)))

I guess that might work, but ,@ is supposed to be followed by an 
expression that returns a list, so that it can be spliced into the 
containing list.  It would be better to write:

(setq my-alist `((p1 . 10) (p2 . ,my-var)))

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***

RE: alists with precomputed variables

[Drew Adams]

> > (setq my-alist `((p1 . 10) (p2 ,@my-var)))
> 
> I guess that might work, but ,@ is supposed to be followed by an 
> expression that returns a list, so that it can be spliced into the 
> containing list.  It would be better to write:
> 
> (setq my-alist `((p1 . 10) (p2 . ,my-var)))

I don't know about "supposed to be" (not only lists are spliced into list
structure), but yes, `.' followed by `,' might be clearer for an atom. But if I
didn't know the value of `my-var', I would probably use `,@', personally.

Anyway, the important thing is to read up on lists in the doc.
There are several ways to get something like what was requested.

The variants I gave are not all equivalent, in any case. In particular, '(p1 .
10) is typically not the same as (cons 'p1 10), in many contexts.