eintr: Counting
1.9.3 Counting
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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.
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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.
Info Catalog
eintr: Using setq
eintr: set & setq