TABLE OF CONTENTS Most programming languages have a way to accept string input and convert it to a particular data type. But Visual LISP, because it runs in a CAD/CAM environment, must do more than just accept basic input in the form of numbers or strings. Visual LISP's input system must be able to accept points, entity selections, selection sets, window selections, angles, and distances, as well as numbers and strings.
Operator input in the Visual LISP environment can be accepted in three ways: in a dialog box, from the graphics screen, and from the command line. The next chapter introduces dialog boxes and their components. In this chapter, you look at command-line and graphic screen input. Virtually all applications written in Visual LISP use at least one of the subrs described in this chapter.
Like output, the input to a command function should parallel the AutoCAD system as much as possible even if you have a better method in mind. (If you are writing code only for yourself, however, improve the interface to suit your fancy.) When writing code for a large community of AutoCAD users, it is best to keep the essence of the command like AutoCAD. That means supplying a default value whenever possible and including options in the same form when prompting the user. The normal approach is to provide a prompting question, and then present the options (if any) between the [ and ] characters, with the default option following between the
