Here is an example that shows how to use setq in a counter. You
might use this to count how many times a part of your program repeats
itself. First set a variable to zero; then add one to the number each
time the program repeats itself. To do this, you need a variable that
serves as a counter, and two expressions: an initial setq
expression that sets the counter variable to zero; and a second
setq expression that increments the counter each time it is
evaluated.
(setq counter 0) ; Let’s call this the initializer. (setq counter (+ counter 1)) ; This is the incrementer. counter ; This is the counter.
(The text following the ‘;’ are comments. See Change a Function Definition.)
If you evaluate the first of these expressions, the initializer,
(setq counter 0), and then evaluate the third expression,
counter, the number 0 will appear in the echo area. If
you then evaluate the second expression, the incrementer, (setq
counter (+ counter 1)), the counter will get the value 1. So if you
again evaluate counter, the number 1 will appear in the
echo area. Each time you evaluate the second expression, the value of
the counter will be incremented.
When you evaluate the incrementer, (setq counter (+ counter 1)),
the Lisp interpreter first evaluates the innermost list; this is the
addition. In order to evaluate this list, it must evaluate the variable
counter and the number 1. When it evaluates the variable
counter, it receives its current value. It passes this value and
the number 1 to the + which adds them together. The sum
is then returned as the value of the inner list and passed to the
setq which sets the variable counter to this new value.
Thus, the value of the variable, counter, is changed.