Checking whether an input element is a number or not with using DO macro. (Infinite Loop)

(defun read-a-number ()

   (do ((answer nil))

       (nil)      ; This is necessary to make Infinite Loop function.

     (format t "~&Please type a number: ")

     (setf answer (read))

     (if (numberp answer) (return answer)  

       (format t t "~&Sorry, ~S is not a number. Try again." answer))))

A Program which checks whether the input number is a number or not by using DO macro.

If the input number is not a number, the program prints "Sorry, ~ is not a number. Try Again. " "Please type a number: " repetitively until you input a number.

 

The point is that (nil) is used in the test part of DO macro.

 

It is interesting that this program can get your input element by puting (setf answer (read)).