Edward Fagan wrote:

> Something like this function will tell you the position of
> the last comma in a string.
>
> (defun find-last-comma (s)
> (loop for n from 0 below (length s)
> if (char= (aref s n) #\,)
> do
> (if (and (loop for n downfrom (1- (length s)) to 0
> if (char= (aref n s) #\,)
> return n
> finally (return nil))
> (= n (loop for n downfrom (1- (length s)) to 0
> if (char= (aref s n) #\,)
> return n
> finally (return nil))))
> (return-from find-last-coma n))
> finally (return nil)))

Testing in SBCL:

* (FIND-LAST-COMMA "hi, hi, ho")

debugger invoked on a SIMPLE-TYPE-ERROR in thread
#<THREAD "main thread" RUNNING {23EAC201}>:
Value of N in (AREF N S) is 9, not a ARRAY.

Gauche Scheme

(string-scan-right "hi, hi, ho" #\,)
===>
6

(string-scan-right "hi, hi, ho" #\, 'before)
===>
"hi, hi"

(string-scan-right "hi, hi, ho" #\, 'after)
===>
" ho"