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From: Barry Fishman <barry_fish...@acm.org>
Newsgroups: comp.lang.lisp
Subject: Re: Trying to make my own reverse function
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You are making thing less readable by not putting a space before (.
Lisp can become easy to read if you don't try to make it look like
some other language.

On 2012-09-26 10:09:44 EDT, Yves S. Garret wrote:
> ; this is the initial function that will set things up for the below method.
> (defun custom-reverse2(list-to-reverse)
>   ; set things up here so that we will pass in a list of two lists.  The first
>   ;   list will store the reversed list -- after processing is done, but for
>   ;   the moment is blank -- and the second list is the one that was passed in
>   ;   in order to reverse.
>   (reverse-rec((list '() list-to-reverse))))

    (reverse-rec (list '() list-to-reverse)))

The double parens are usually wrong unless you are using some
macro like let.  (The lack of a space may be making it appear to be
a list inside a function call.)

> ; this method will do the iterating and reversing recursively.
> (defun reverse-rec(grand-list)
>   ; get the second list in the grand-list and see if it's nil.
>   (if (null (car (cdr grand-list)))
>     ; if the second list is a nil, then return the reversed list.
>     (car grand-list)
>     ; otherwise continue reversing the entire thing.
>     ((setf new-list (cons (car (car (cdr grand-list))) (car grand-list)))
>      (reverse-rec(list new-list (cdr (car (cdr grand-list))))))))

First setf is not the way to create a local variable, you need to use let
or let*.

Second, the '((setf ...', outside of a let, is another case of doubled
left parenthesis and a warning something may be wrong.  Here you are
grouping two statements by putting them in an outer parenthesis:

         ((setf newlist ...) (reverse-rec ...))

You would do that with:

         (progn (setf newlist ...) (reverse-rec ...))

Add considering that the setf is wrong:

         (let ((newlist ...)) (reverse-rec ...))

Also duplicate chains of '(cdr (car ...' are a hint that you need to
choose more meaningful local variable:

;; This method will do the iterating and reversing recursively.
(defun reverse-rec (grand-list)
  (let ((to-list (car grand-list))
        (from-list (car (cdr grand-list))))
    (if (null from-list)
        to-list
        (reverse-rec (list (cons (car from-list) to-list)
                           (cdr from-list))))))

The '(car (cdr grand-list))' could be expressed as (cadr grand-list) or
even (second grand-list).

But would be simpler to rather than create a new grand-list as an
argument to reverse-rec, just pass the lists as separate arguments.

(defun custom-reverse2 (list-to-reverse)
  (reverse-rec '() list-to-reverse))

Then:

(defun reverse-rec (to-list from-list)
    (if (null from-list)
        to-list
        (reverse-rec (cons (car from-list) to-list)
                     (cdr from-list))))

-- 
Barry Fishman