On Sun, Jul 22, 2012 at 12:50 PM, Mauricio <maufd...@gmail.com> wrote:
> About your question regarding using cdr instead of cadr since there are
> only two elements in the list, that is not quite the way it works, you have
> to remember that a list is just a bunch of cons cell, the first (car) has
> an element and the second (cdr) points to the car of the next cons cell,
> the last cdr in the last cons in the list point to nil, and that is what
> tells lisp it has reached the end of the list.

> When you do cadr, you get the element which is in the first element of the
> cons cell, in this case a list of symbols which forms the description,
> while if you do cdr you get the a list with another list inside it which is
> the list you were interested in.

> To illustrate better just evaluate the following two lines in your repl.

> [1]> (cdr '(1 '(1 3 4)))
> ('(1 3 4))
> [2]> (cadr '(1 '(1 3 4)))
> '(1 3 4)

> The above is directly copied from my CLISP repl. As you can see the
> results are different. I always try to remember that the list is just a
> construct made of cons cells, and that helps me get my mind around the
> car/cdr combinations.

> Also if you want shorthand for the second element in this case you can use
> second instead of cadr.

> [3]> (second '(1 '(1 3 4)))
> '(1 3 4)

> Hope I understood your question, and if so that my answer was clear.

> El sábado, 21 de julio de 2012 10:39:29 UTC-6, richard kappler escribió:

>> Some questions brought up by the text game in ch 5, particularly the
>> describe-location function:

>> (defun describe-location (location nodes)
>>     (cadr (assoc location nodes)))

>> As I understand it, what's happening is we're defining a function named
>> describe-location that takes location and nodes as arguments, so for
>> example if I call
>> (describe-location 'garden *nodes*) it goes to the *nodes* parameter,
>> pulls out the 'garden bit (garden (you are in a beautiful garden. there is
>> a well in front of you)) and lops off "(garden " because of the cadr
>> returning only (you are in a beautiful garden. there is a well in front of
>> you)

>> 1.  I think I get the read vs evaluate bit, but it's rather tenuous.  Am
>> I correct that first Lisp reads the whole thing, then evaluates from the
>> inside out [first finds, (assoc location nodes) then "cadr's" it then
>> returns the result as the value of describe location]?

>> 2.  cadr is the same as (car (cdr ()) which is the same as from the rest
>> take the first which is equivalent to get rid of the first symbol and from
>> the rest take the first?

>> 3.  Do we use cadr because its just less typing (more elegant and
>> downright cool?) than (first (rest())?

>> 4.  Why not just use cdr since, in this case anyway, we know there are
>> only two symbols, the location and the description.  Would cdr not do the
>> same as cadr in this instance?

>> regards, Richard