Another common way to write a while loop is to write the test
so that it determines whether a counter is greater than zero. So long
as the counter is greater than zero, the loop is repeated. But when
the counter is equal to or less than zero, the loop is stopped. For
this to work, the counter has to start out greater than zero and then
be made smaller and smaller by a form that is evaluated
repeatedly.
The test will be an expression such as (> counter 0) which
returns t for true if the value of counter is greater
than zero, and nil for false if the value of counter is
equal to or less than zero. The expression that makes the number
smaller and smaller can be a simple setq such as (setq
counter (1- counter)), where 1- is a built-in function in
Emacs Lisp that subtracts 1 from its argument.
The template for a decrementing while loop looks like this:
(while (> counter 0) ; true-or-false-test body… (setq counter (1- counter))) ; decrementer