Steel Bank Common Lisp

On Thu, Oct 15, 2009 at 4:38 AM, Carlos Konstanski
<ckonstanski@...> wrote:
> I don't think I understand REDUCE fully. Take the following two
> examples. The first one REDUCEs a list of two items. It works as
> expected. the second one REDUCEs a list of one item. It does not give
> the result I was hoping for.
>
> I guess I should ask for clarification: what is REDUCE supposed to do
> when given a list of one element?
>
>
> SVBRONZE> (let ((roles '(roles_dealer roles_architect)))
>             (reduce #'(lambda (&optional x y)
>                         (cond ((and x y) (format nil "~a = true OR ~a = true" x y))
>                               ((and x (not y)) (format nil "~a = true" x))
>                               (t "")))
>                     (mapcar #'(lambda (role)
>                                 (string-downcase (symbol-name role)))
>                             roles)))
> "roles_dealer = true OR roles_architect = true"
>
> SVBRONZE> (let ((roles '(roles_dealer)))
>             (reduce #'(lambda (&optional x y)
>                         (cond ((and x y) (format nil "~a = true OR ~a = true" x y))
>                               ((and x (not y)) (format nil "~a = true" x))
>                               (t "")))
>                     (mapcar #'(lambda (role)
>                                 (string-downcase (symbol-name role)))
>                             roles)))
> "roles_dealer"
"If the subsequence contains exactly one element and no initial-value
is given, then that element is returned
and function is not called." from
http://www.lispworks.com/reference/HyperSpec/Body/f_reduce.htm


-- 
With best regards, Stas.

On Oct 14, 2009, at 20:38, Carlos Konstanski wrote:

> SVBRONZE> (let ((roles '(roles_dealer)))
>             (reduce #'(lambda (&optional x y)
>                         (cond ((and x y) (format nil "~a = true OR  
> ~a = true" x y))
>                               ((and x (not y)) (format nil "~a =  
> true" x))
>                               (t "")))
>                     (mapcar #'(lambda (role)
>                                 (string-downcase (symbol-name role)))
>                             roles)))
> "roles_dealer"

This is an improper use of REDUCE even if the behavior with one  
argument were what you wanted. If you give it more than two arguments,  
say (a b c), you will get the result:

   a = true OR b = true = true OR c = true

Instead, use the :key option to REDUCE or move the addition of "~a =  
true" into the MAPCAR.

If you only wanted this to work for 0, 1, or 2 arguments, then  
replacing REDUCE with APPLY will make the above code work.

Also, STRING-DOWNCASE takes a string designator, so the use of SYMBOL- 
NAME is unnecessary.

-- 
Kevin Reid                                  <http://switchb.org/kpreid/>