User:Eriksiers/Alternate CLISP exponentiation

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I wrote this while reading the first few chapters of Practical Common Lisp. (I'm about halfway through chapter 4 right now.) It seems to work -- at least for me, using CLISP -- and I think it might be a better version than the "real" solution provided at Exponentiation operator#Common Lisp, but I'm still way too green to know for sure. (This version can handle negative values of b, whereas that version can't -- I think.)

FWIW, I wrote this before looking at the Exponentiation operator page. Yay me.

Later, I might update this to handle b as a float.

<lang lisp>; calculates a to the bth power (a^b)

b must be an integer

a can be integer or floating-point

either or both can be negative

limits determined by lisp, I suppose -- arbitrary? none? I don't know

(defun ^ (a b)

   (if
       (> b 0)
       (* a
           (^ a
               (- b 1)
           )
       )
       (if
           (< b 0)
           (/ 1
               (^ a
                   (- 0 b)
               )
           )
           (if (= b 0) 1)
       )
   )

)</lang>

Sample outputs:

CL-USER> (^ 1.1 2)
1.21
CL-USER> (^ -1 2)
1
CL-USER> (^ -1 3)
-1
CL-USER> (^ -1.1 -1)
-0.9090909
CL-USER> (^ 2 300)
2037035976334486086268445688409378161051468393665936250636140449354381299763336706183397376
CL-USER> (^ 1 .5)
; Evaluation aborted on NIL.