best way to get the list of unique keys from 2 alists

best way to get the list of unique keys from 2 alists

[Kevin Rodgers]

I've got 2 alists of (SYMBOL . "STRING") pairs, and I need to get the
list of unique symbol names to pass to completing-read as its TABLE
argument: (("SYMBOL-NAME") ...)

I know about remove-duplicates and union, but I'd like to avoid using
cl*.el functions.

Since completing-read seems to ignore nil entries in TABLE, this is what
I've got now:

(nconc (mapcar (lambda (assoc)
                  (list (symbol-name (car assoc))))
                alist-1)
        (mapcar (lambda (assoc)
                  (or (assq (car assoc) alist-1)
                      (list (symbol-name (car assoc)))))
                alist-2))

Is there a cleaner way?

Thanks,
-- 
Kevin

RE: best way to get the list of unique keys from 2 alists

[Drew Adams]

    I've got 2 alists of (SYMBOL . "STRING") pairs, and I need to get the
    list of unique symbol names to pass to completing-read as its TABLE
    argument: (("SYMBOL-NAME") ...)

    I know about remove-duplicates and union, but I'd like to avoid using
    cl*.el functions.

    Since completing-read seems to ignore nil entries in TABLE, this is what
    I've got now:

    (nconc (mapcar (lambda (assoc)
                      (list (symbol-name (car assoc))))
                    alist-1)
            (mapcar (lambda (assoc)
                      (or (assq (car assoc) alist-1)
                          (list (symbol-name (car assoc)))))
                    alist-2))

    Is there a cleaner way?

That looks good to me.

You could always define union or remove-duplicates, but removing duplicates
would be less efficient than what you're doing (consing up the second list
completely, just to perhaps remove stuff). Anyway, here are non-destructive
versions of remove-duplicates and union, FWIW:

(defun remove-duplicates (list)
  "Copy of LIST with duplicate elements removed.
Tested with `equal'."
  (let ((tail list)
        new)
    (while tail
      (unless (member (car tail) new)
         (push (car tail) new))
      (pop tail))
    (nreverse new)))

(defun union (list1 list2)
  "Combine LIST1 and LIST2 using a set-union operation.
The result list contains all items that appear in either LIST1 or
LIST2.  This is a non-destructive function; it copies the data if
necessary."
  (cond ((null list1) list2)
        ((null list2) list1)
	((equal list1 list2) list1)
	(t
	 (or (>= (length list1) (length list2))
	     (setq list1 (prog1 list2 (setq list2 list1)))) ; Swap them.
	 (while list2
           (unless (member (car list2) list1)
               (setq list1 (cons (car list2) list1)))
	   (setq list2 (cdr list2)))
	 list1)))

Re: best way to get the list of unique keys from 2 alists

[Kevin Rodgers]

Kevin Rodgers wrote:
> I've got 2 alists of (SYMBOL . "STRING") pairs, and I need to get the
> list of unique symbol names to pass to completing-read as its TABLE
> argument: (("SYMBOL-NAME") ...)
> 
> I know about remove-duplicates and union, but I'd like to avoid using
> cl*.el functions.
> 
> Since completing-read seems to ignore nil entries in TABLE, this is what
> I've got now:
> 
> (nconc (mapcar (lambda (assoc)
>                  (list (symbol-name (car assoc))))
>                alist-1)
>        (mapcar (lambda (assoc)
>                  (or (assq (car assoc) alist-1)
>                      (list (symbol-name (car assoc)))))
>                alist-2))
> 
> Is there a cleaner way?

Since completing-read also accepts an obarray for its TABLE, I think
this might be a little better:

(let ((table (make-vector 83 0))) ; 83 is a suitable prime
   (mapc (lambda (assoc)
           (intern (symbol-name (car assoc)) table))
         alist-1)
   (mapc (lambda (assoc)
           (intern (symbol-name (car assoc)) table))
         alist-2)
   table)

-- 
Kevin Rodgers

Re: best way to get the list of unique keys from 2 alists

[Stefan Monnier]

> I've got 2 alists of (SYMBOL . "STRING") pairs, and I need to get the
> list of unique symbol names to pass to completing-read as its TABLE
> argument: (("SYMBOL-NAME") ...)

I'd just do:

    (completing-read "foo" (mapcar (lambda (x) (symbol-name (car x)))
                                   (append list1 list2)))

You don't need to remove duplicates.


        Stefan

Re: best way to get the list of unique keys from 2 alists

[Kevin Rodgers]

Stefan Monnier wrote:
>>I've got 2 alists of (SYMBOL . "STRING") pairs, and I need to get the
>>list of unique symbol names to pass to completing-read as its TABLE
>>argument: (("SYMBOL-NAME") ...)
> 
> 
> I'd just do:
> 
>     (completing-read "foo" (mapcar (lambda (x) (symbol-name (car x)))
>                                    (append list1 list2)))
> 
> You don't need to remove duplicates.

In 21.4 the TABLE elements must be lists, and duplicates are displayed
in the *Completions* buffer.

-- 
Kevin Rodgers

Re: best way to get the list of unique keys from 2 alists

[Stefan Monnier]

> In 21.4 the TABLE elements must be lists,

I trust you'll know how to adjust the code for that.

> and duplicates are displayed in the *Completions* buffer.

Please report it as a bug.


        Stefan