Need help understanding a piece of code

Hello. I am trying improve my programming techniques, but am having a
problem understanding a code snippet provided in the AutoCAD help. I have
two questions regarding the code below. First, why is that cond function
required there? The variable has already ben ititialized and set to nil, so
why test it (assuming that is what the cond is doing)? Second, how does the
cond function work here anyway? What is the deal with the (*acad-object")?
If you just throw a variable into a list you get will get an error won't
you? I just can't make sense of this.


(setq *acad-object* nil) ; Initialize global variable
(defun acad-object ()
(cond (*acad-object*) ; Return the cached object
(t
(setq *acad-object* (vlax-get-acad-object))
)
)
)

 

*acad-object* is set to NIL at loadtime.

Subsequent calls are to the "ACAD-OBJECT" function. It is designed to run
and set *acad-object* to (vlax-get-acad-object) only once.

Don

 

As far as I can tell, you could replace the whole mess with a simple (setq
*acad-object* (vlax-get-acad-object)). It doesn't matter whether the
variable has been set previously or not, that will still work. And I agree
it seems nonsensical to null it out and then test for it.

The cond works as intended though. Each list in a cond construction has a
"test" expression and (optionally) one or more "result" expressions which
are evaluated if the test expression evaluates non-nil. As usal with lisp,
the last thing evaluated is returned. If there are no "result" expressions
in a cond list, the value of the "test" expression -- the last thing
evaluated -- is returned.

So (*acad-object*) simply means, if *acad-object* has a value, return it. An
unnecessarily verbose version of the same thing would be:

(cond
((not (null *acad-object*) )
*acad-object*
)
(t
(setq *acad-object* (vlax-get-acad-object))
)
)

 

It also might help to analyze the return values...

<CLIP>
Command: (setq *acad-object* nil)
nil

Command: (defun acad-object ()
(_> (cond (*acad-object*)
((_> (t
(((_> (setq *acad-object* (vlax-get-acad-object))
(((_> )
((_> )
(_> )
ACAD-OBJECT

Command: (acad-object)
#<VLA-OBJECT IAcadApplication 00b9b5e4>

Command: (acad-object)
#<VLA-OBJECT IAcadApplication 00b9b5e4>

<END CLIP>

Don

 

Larry,

The first statement:
(setq *acad-object* nil)

runs only when the .lsp file that contains the statement is loaded. So the
global variable will be set to nil only upon the load.

Now, when the function (acad-object) is called, it will ultimately return
the Acad Application object. The (cond) is used to test if the global
variable has been bound.

The first time the function is called, the global variable is nil, so the
cond skips the first test:
(*acad-object*)
and evaluates the 2nd test, which is T, so it executes:
(setq *acad-object* (vlax-get-acad-object))
which, of course, returns the Acad Application object

This binds the global variable to the Acad Application object. Now, the
_next_ time the function is called, without being reloaded, the global
variable _does_ evaluate to something (the Acad Application object), so the
cond's first test is used as the return value of the function itself.

--
R. Robert Bell


Hello. I am trying improve my programming techniques, but am having a
problem understanding a code snippet provided in the AutoCAD help. I have
two questions regarding the code below. First, why is that cond function
required there? The variable has already ben ititialized and set to nil, so
why test it (assuming that is what the cond is doing)? Second, how does the
cond function work here anyway? What is the deal with the (*acad-object")?
If you just throw a variable into a list you get will get an error won't
you? I just can't make sense of this.


(setq *acad-object* nil) ; Initialize global variable
(defun acad-object ()
(cond (*acad-object*) ; Return the cached object
(t
(setq *acad-object* (vlax-get-acad-object))
)
)
)

 

Thanks so much to all of you for taking the time to analyse the code and
explain it to me. I had forgotten that the first line only executes upon
loading, so that makes sense now. Also, I had previously thought a cond
test always had to have "result" expression following the test expression.
Now realizing otherwise, this cond example makes sense.

Thanks again, guys. I have learned something valuable today.

LT