incf place [delta-form] => new-value
decf place [delta-form] => new-value
delta-form---a form; evaluated to produce a delta. The default is 1.
incf and decf are used for incrementing and decrementing the value of place, respectively.
The delta is added to (in the case of incf) or subtracted from (in the case of decf) the number in place and the result is stored in place.
Any necessary type conversions are performed automatically.
For information about the evaluation of subforms of places, see Section 22.214.171.124 (Evaluation of Subforms to Places).
(setq n 0) (incf n) => 1 n => 1 (decf n 3) => -2 n => -2 (decf n -5) => 3 (decf n) => 2 (incf n 0.5) => 2.5 (decf n) => 1.5 n => 1.5
Place is modified.
+, -, 1+, 1-, setf