Expressions and arithemetics in common lisp functions

Question

I'm trying to write a function such that P(x) for any integer x there is a list of three elements, namely the square, cube, and fourth power of n but I'm stuck on how i can combine then to make one function for example i have the square, cube and the power 4 function here are my functions below

(defun p(x) (* x x))

(defun p(x) (* x x x))

(defun p(x) (expt x x) //thought i am not sure about this one

is there a way i can make my result(i.e function p(x) ) list look like (4 27 256) if I have a list of ( 2 3 4) after executing the program? Was thinking of mapcar function though I'm not sure on how I can go about it. Any suggestions?

if you want to create a list, how about the function LIST? — Rainer Joswig
– Rainer Joswig, Commented Oct 8, 2012 at 18:49
i thought about it but im just a newbie dont know how to manipulate the functions — Mildred Shimz
– Mildred Shimz, Commented Oct 8, 2012 at 19:07

score 4 · Accepted Answer · 2012-10-08 20:07:31Z

4

Yes, mapcar could be a way to go.

;; a simple solution.
(defun expt-list (n powers)
  (mapcar #'(lambda (x) (expt n x)) powers))

;; a bit more complex, so you  don't compute powers of the same 
;; base multiple times.
(defun expt-list-memoize (n powers)
  (let ((hash (make-hash-table)))
    (mapcar #'(lambda (x)
                (let ((y (gethash x hash)))
                  (if y y (setf (gethash x hash) (expt n x)))))
            powers)))

edited Oct 8, 2012 at 20:07

answered Oct 8, 2012 at 19:58

user797257

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2 Comments

Mildred Shimz Over a year ago

how do i enter the numbers or arguments its giving some error i do not know why

Mildred Shimz Over a year ago

im getting expt-list at the end but when i enter (expt-list 2 3 5) its not working,am i missing something?

Patrick · Accepted Answer · 2012-10-09 08:07:44Z

1

There is one oddity in Common Lisp, that one has to recognize: If you want to refer to a function via a symbol, you will have to quote it with (function p) or with the abbrevation #'p

Given

(defun p(x) (* x x))

you can compute a list of squares with

CL-USER> (mapcar #'p (list 1 2 3 4))

(1 4 9 16)

answered Oct 9, 2012 at 8:07

Patrick

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Comments

peter · Accepted Answer · 2012-10-09 12:31:27Z

0

The requirements here are a little fuzzy but if the intent is, given a call such as (expt-list 2 3 4) to have returned a list of each number raised to its own power, eg:

(expt-list 2 3 4)
=> (4 27 256)

would something along the lines of:

(defun expt-list (&rest list-of-n)
  "return a list of each 'n' in LIST-OF-N raised to its own power"
  (loop for n in list-of-n collect (expt n n)))

do the trick?

answered Oct 9, 2012 at 12:31

peter

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