How to check whether an optional param was input?

Question

I have a command with an optional argument whose value is a number. I need:

If no input argument then use default value (1000) else use input arg.

(random t) 
(defun insert-random-number-at-point(&optional to)
  "Insert at point a random number (default from 0 to 1000)."
  (interactive "nInsert at point a random number (default from 0 to 1000): ")
  (if (= (to) nil)
      (insert (number-to-string (random 1000))) ;; then use default value
    (insert (number-to-string to)))) ;; else

Start :

Insert at point a random number (default from 0 to 1000): 15

Error:

Symbol’s function definition is void: to

Answer 1

In Lisp, the term/word that follows an open parenthesis (if the open parenthesis is not quoted) is interpreted to be a function, not a variable. In the context of this question, to is a variable. Because the word to follows an open parenthesis that is not quoted, Emacs thinks it was meant to be interpreted as a function instead of a variable. Try instead using (if to (insert .... -- which tests whether the variable to has been assigned a value.

When using the function =, Emacs expects that two integers will be compared. Inasmuch as nil is not a number, Emacs will throw an error: (wrong-type-argument number-or-marker-p nil). However, using = here appears to not be needed as explained in the preceding paragraph.

Another way to test whether a variable has been assigned a value is to use the function null:

null is a built-in function in `C source code'.

(null OBJECT)

Return t if OBJECT is nil, and return nil otherwise.

So we could use (if (not (null to)) ...