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  ANSI Common Lisp   12 Numbers   12.2 Dictionary of Numbers

12.2.33 incf, decf Macro

incf place [delta-form]    new-value

decf place [delta-form]    new-value

Arguments and Values:
place - a place.

delta-form - a form; evaluated to produce a delta. The default is 1.

delta - a number.

new-value - a number.

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 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

Side Effects:
Place is modified.

See Also:
+, -, 1+, 1-, setf

Allegro CL Implementation Details:

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