Simple e-lisp question

Simple e-lisp question

[Eric Lilja]

Hi, I want to write an elisp-function switch2 that should switch the 
first two elements in a list. I came up with this:

(defun switch2 (x)
   (append (list (second x) (first x)) (nthcdr 2 x))
)
(switch2 '(a b c d)) ; Yields (b a c d)
(switch2 '(a b)) ; Yields (b a)
(switch2 '(a)) ; Yields (nil a)
(switch2 '()) ; Yields (nil nil)

The problem is how it handles a list with only one element and an empty 
list. I'm not sure how it should handle only one element, maybe return 
an unmodified list or an empty list? If an empty list is given the 
result should be an empty list.

How can I fix my swith2 to cope better with the last two calls above and 
can I use the more fundamental list functions if you know what I mean 
and avoid nthcdr altogether?

- Eric Lilja

PS. I don't know lisp, heh, just found this old exercise in my papers. DS.

RE: Simple e-lisp question

[Drew Adams]

> switch the first two elements in a list.
> 
> (defun switch2 (x)
>    (append (list (second x) (first x)) (nthcdr 2 x)))
>
> (switch2 '(a b c d)) ; Yields (b a c d)
> (switch2 '(a b)) ; Yields (b a)
> (switch2 '(a)) ; Yields (nil a)
> (switch2 '()) ; Yields (nil nil)
> 
> The problem is how it handles a list with only one element 
> and an empty list. I'm not sure how it should handle only
> one element, maybe return an unmodified list or an empty
> list? If an empty list is given the result should be an
> empty list.
> 
> How can I fix my swith2 to cope better with the last two 
> calls above and can I use the more fundamental list
> functions if you know what I mean and avoid nthcdr altogether?

Lisp is English. If you can describe it clearly, you can code it.

If x has at least 2 elements, then 2nd, 1st, 3rd...
Else x

at least 2 = cdr
2nd        = cadr
1st        = car
3rd...     = cddr

(defun switch2 (x)
  (if (cdr x)
      (cons (cadr x) (cons (car x) (cddr x)))
    lst))

Hope this wasn't your only homework. ;-)

Re: Simple e-lisp question

[Eric Lilja]

Drew Adams wrote:
> 
> Lisp is English. If you can describe it clearly, you can code it.
> 
> If x has at least 2 elements, then 2nd, 1st, 3rd...
> Else x
> 
> at least 2 = cdr
> 2nd        = cadr
> 1st        = car
> 3rd...     = cddr
> 
> (defun switch2 (x)
>   (if (cdr x)
>       (cons (cadr x) (cons (car x) (cddr x)))
>     lst))
> 
> Hope this wasn't your only homework. ;-)

Thanks alot Mr Adams! And it wasn't even homework, just an old exercise 
I found.

- EL

Re: Simple e-lisp question

[Pascal J. Bourguignon]

Eric Lilja <mindcooler@gmail.com> writes:

> Hi, I want to write an elisp-function switch2 that should switch the
> first two elements in a list. I came up with this:
>
> (defun switch2 (x)
>   (append (list (second x) (first x)) (nthcdr 2 x))
> )
> (switch2 '(a b c d)) ; Yields (b a c d)
> (switch2 '(a b)) ; Yields (b a)
> (switch2 '(a)) ; Yields (nil a)
> (switch2 '()) ; Yields (nil nil)
>
> The problem is how it handles a list with only one element and an
> empty list. I'm not sure how it should handle only one element, maybe
> return an unmodified list or an empty list? If an empty list is given
> the result should be an empty list.

This is not a lisp question then.  It's a make up your mind question.


> How can I fix my swith2 to cope better with the last two calls above
> and can I use the more fundamental list functions if you know what I
> mean and avoid nthcdr altogether?

So what do you want?


On my part, I'd like it to return the object unmodified if there are
less than two elements or it's not a list.

These kinds of operations are usually named "swap".  Switch means
nothing in this context.  You can switch things on or off, but here
you want to swap.


(defun swap-first-two (object)
   (cond
     ((atom object)        object)
     ((atom (rest object)) object)
     (t  (list* (second object) (first object) (rest (rest object))))))


(defun test/swap-first-two ()
    (equalp '( a
               ()
               (a)
               (a . b)
               (b a)
               (b a . c)
               (b a c)
               (b a c d) )
            (mapcar (function swap-first-two)
                    '( a
                       ()
                       (a)
                       (a . b)
                       (a b)
                       (a b . c)
                       (a b c)
                       (a b c d) ))))

(test/swap-first-two) --> t 


-- 
__Pascal Bourguignon__