working with empty layers

[fbyfatih]

Hello, I have a layer changing lisp but it ends with errors if I run it second time. What might be my mistake?

(defun c:sel()

(setq ss ( ssget "_A" '((8 . "S-_ANOTGRiD_T3" ))))
(setq len (sslength ss))
(if (> len 0)
(command "_chprop" ss "" "_LA" "S-ANNO-DIMS" "")
);if

)

[fbyfatih]

Ok I got my own answer. Solution is like this

(defun c:seli()

(setq ss ( ssget "_A" '((8 . "S-_ANOTGRiD_T3" ))))

(if (/= ss nil)
(command "_chprop" ss "" "_LA" "S-ANNO-DIMS" "")
);if

)

[ВeekeeCZ]

Could be used this shortened form.

  (if (setq ss ( ssget "_A" '((8 . "S-_ANOTGRiD_T3" ))))
    (command "_chprop" ss "" "_LA" "S-ANNO-DIMS" "")
    );if

[fbyfatih]

Why is this not working?

(defun c:seli()
(setq ss ( ssget "_A" '((8 . "S-_ANOTGRiD_T3" ))))
(if (ss)
(command "_chprop" ss "" "_LA" "S-ANNO-DIMS" "")
)

[ВeekeeCZ · Accepted solution]

The parents are the issue. It would work if ss was a function ....   (defun ss () whatever... )

But since it's just a variable, it's just (if ss (command ....

[john.kaulB9QW2]

With a few corrections, this is very nicely written code. Good job, @fbyfatih.

Remember: When constructing "boolean conditional constructs" (IF/COND statements) you should be using verbs to make code more readable.

Instead of typing:
(if (ssget "_A" '((8 . "S-_ANOTGRiD_T3" )))) ...

You should create a "boolean variable" to help construct a conditional (like you did). And when creating a `boolean variable` you should use positive variable names.

Examples:
full
ready
etc.

Using negative verbs results in confusing code like:
(if (not notfull) ...
(if (not notReady) ...

(defun c:seli ( / ss)
  (setq ss (ssget "_A" '((8 . "S-_ANOTGRiD_T3" ))))
  (if ss
    (command "_chprop" ss "" "_LA" "S-ANNO-DIMS" "")
    )
 (princ)
 )

[fbyfatih]

Thank you very much, you were the first people who helped me with my Autolisp learning journey

[Kent1Cooper]

A slightly shortened approach....  Instead of first setting the ss variable and then checking whether it exists to work with it, you can do the checking in the process of setting it:

(defun c:seli (/ ss)
  (if (setq ss ( ssget "_A" '((8 . "S-_ANOTGRiD_T3" ))))
    (command "_.chprop" ss "" "_LA" "S-ANNO-DIMS" "")

  )
)

[john.kaulB9QW2]

You *could* do it without setting the variable altogether but that approach is not "good coding practice"--especially when you graduate to other languages--like: (if (ssget "_A" '((8 . "S-_ANOTGRiD_T3"))).

It's considered a better habit to set the variable as I stated above and develop the habit early in learning.

This should be taken with a grain-of-salt because you should also develop the habit of constructing small functions (small "black boxes") where you may use a function return as your conditional check like:

(defun checkForSpecificThing ()
;; This function's purpose is to check for the existence of a specific thing
;; in the drawing.
;;
;; Return: True or False
(if (ssget "_A" '((8 . "S-_ANOTGRiD_T3")))) )

(if (checkForSpecificThing) ...

EDIT: Added code tags.