Arithmetic in

About Arithmetic

Common Lisp uses the standard arithmetic operators for most operations but is somewhat unique in using a "prefix-notation" as opposed to the more familiar "infix-notion".

More visually:

;; Infix-notation (non-lisp languages)
1 + 2 + 3 + 4 + 5 ; => 15
;; Prefix-notation (lisp languages)
(+ 1 2 3 4 5)     ; => 15

While prefix notion turns some operations like 2 + 2 into the somewhat unfamiliar (+ 2 2) form, it makes it much easier to operate on more than one number at a time.

Arithmetic Operators With a Single Argument

As a small quirk, the - and / operators have a special meanings when applied to only one number:

;; A single number passed to `-` is simply negated
(- 4)   ; => -4
(- -32) ; => 32
;; A single number passed to `/` returns the reciprocal
(/ 8)   ; => 1/8
(/ 0.1) ; => 10.0

Comparing Numbers

Finally, you may find it useful to compare different numbers using functions like = (equal), /= (not equal to), and >= (greater than or equal to). When these comparisons are true (as in (= 1 1)), they return T and when they aren't (as in (> 0 1)), they return NIL.

Numeric predicates

The language also includes some useful predicates such as zerop, evenp and oddp which check for the obvious conditions.

Reference

;; Addition
(+ 1 2 3 4 5) ; => 15

;; Subtraction
(- 15 3 2)    ; => 10
(- 42)        ; => -42

;; Multiplication
(* 4 3 2 1)   ; => 24

;; Division
(/ 64 16 2)   ; => 2
(/ 5)         ; => 1/5

;; Exponentiation
(expt 2 8)    ; => 256

;; Square Root
(sqrt 25)     ; => 25

;; Modulo (Similar to remainder)
(mod 10 3)    ; => 1

;; Equality
(= 2 2)       ; => T
(= 3 5)       ; => NIL