Beginner question

Hey I just started learning lisp, and there's a problem in my text-book I just can't seem to solve. Hope someone can help out.

We have a date in the form yyyymmdd. Define 3 functions year, month and day, which from a date will give the values year, month and day. e.g:

(year 19980220)
->1998

(month 19980220)
->2

(day 19980220)
->20

thanks.

Ok I managed to figure out a way to solve it. Is there a better way?

(defun day (n) (mod n 100))
(defun month (n) (mod (/ (- n (day n)) 100) 100))
(defun year (n) (/ (- n (day n) (* (month n) 100)) 10000))

I remember some comp.lang.lisp sage mentioning floor (http://www.lispworks.com/documentation/ ... floorc.htm). Thus directed, the definition of month could be:

(defun month (n)
  (mod (floor n 100) 100))

Seeing that you now know about floor, the definition of year becomes even simpler than the above definition of month.

Are you sure they don't mean the string "yyyymmdd" then you can use subseq.

(defun year(str) (read-from-string(subseq str 0 4)))
(defun month(str) (read-from-string(subseq str 4 6)))
(defun day(str)(read-from-string(subseq str 6 8)))

Of course, it assumes the string is long enough.
Hmm, is there an read-integer-from-string and such? (year "'hax0519") -> 'HAX ; 4

Jasper wrote:]
Hmm, is there an read-integer-from-string and such? (year "'hax0519") -> 'HAX ; 4

Function PARSE-INTEGER