Simplified Common Lisp reference
decf
Symbol class: Mathematics, Arithmetics, Logic and Comparisons
Syntax:
Symbol type: macro
decfplacedecrement(optional) => numeric value
Argument description:
place place with numeric value
decrement numeric value

DECF macro modifies a place with numeric value. Its value is decremented by decrement number. Default decrement is 1.

(let ((a 10)) (decf a) a) => 9
(let ((a 10)) (decf a 2.3) a) => 7.7
(let ((a 10)) (decf a -2.3) a) => 12.3
(let ((a (list 10 11 12 13))) (decf (elt a 2) 2.3) a) => (10 11 9.7 13)
Function indexFull documentation for decf (HyperSpec)