Tài liệu Programming AutoCAD

Often you want to execute a procedure based on a certain condition. One way of
doing this is with the IF statement, which does one thing if a statement is true and
another thing if it is false. In other words, the operation is conditioned on the truth
of a certain statement.
Looping is an important part of programming. Frequently you want to execute a
procedure over and over until the routine has finished operating on all the target
objects or items. Looping sets up the condition that determines when the operation
starts, the number of objects upon which the routine operates, and when the operation
ends.
Conditional structures
Conditional structures enable program flow to be determined based on the outcome
of a given decision. These decisions result in the return of either T, meaning
true, or nil, meaning false. To try out the following statements, type them in the
Visual LISP Console window. For instance, for the statement