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User:Eriksiers/Alternate CLISP exponentiation

From Rosetta Code

This is a better example of exponent (it doesn't solve for negatives however so I will leave in tact the other example):

 
(defun ^ (a b)
(let ((r 1))
(dotimes (x b r)
(setf r (* r a)))))
 

(The above example is by Ahungry.)

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.

; 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)
)
)
)

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.