goto function?

is there a way to jump to a certain line of the program if the "if" proves true? My "if" is if the user selects the same entity which shoudn't be done, ie.
(if (equal ent1 ent2)
(alert "You've picked the same line")
How would i get that to allow the user to select again?

TIA, Shawn

 

There is no goto in lisp.
To loop and check conditions, use while
or set up the problem recursively.

 

(setq ent1 (car (entsel "\nSelect first line: ")))
(while (equal (setq ent2 (car (entsel "\nSelect second line: "))) ent1)
(alert "You've picked the same line!")
);;while
(do stuff with ent1 & ent2 here)
___

true? My "if" is if the user selects the same entity which shoudn't be
done, ie.

 

Tanx Paul.

 

another quest. If the user selects the first line but then just hits enter w/out selecting another line, how would I get it to print "second line not selected "
Something like this, but it doesn't work.

(defun c:ce (/)
(setq ent1 (car (entsel "\nSelect first line: ")))

(while (equal (setq ent2 (car (entsel "\nSelect second line: "))) ent1)
(alert "You've picked the same line!")
(if
(= ent2 nil
(alert "You've selected nothing")))
) ;while

(if (/= ent1 ent2)
(print "Entities are different")
(print "Entities are alike")
)
(princ)
)

 

(while (not (setq ent1 (car (entsel "\nSelect first line: ")))))
(while
(or
(equal (setq ent2 (car (entsel "\nSelect second line: "))) ent1)
(not ent2)
);; or
(alert
(cond
((null ent2) "Nothing selected!")
(T "You've picked the same line!")
);;cond
);;alert
);;while
(do stuff here)
___

enter w/out selecting another line, how would I get it to print "second line
not selected "