> > Why the following code can't extract the digit-char?
> >
> > (with-input-from-string (is "abc def 777 a b c")
> > (loop
> > :for c := (read-char is nil 'the-end)
> > :while (digit-char-p c)
> > :collect c))
> >
> > What I expect result is (#\7 #\7 #\7), however I just get NIL.
> >
> > Cannot understand why?
> >
> > Regards,
> > Jinsong
>
>
> Because you confuse :while with :when:
>
> (with-input-from-string (is "abc def 777 a b c")
> (loop
> :for c := (read-char is nil 'the-end)
> :while (not (eq c 'the-end))
> :when (digit-char-p c)
> :collect c))
>
> There is also no need to specify the end marker for read-char (the
> default nil makes for simpler and more readable code).
>
> Personally, I wouldn't even use with-input-from-string and read-char in
> this case; a simple (loop for c across ...) would be better.

Gauche Scheme and Racket:

(use srfi-42) ;; list-ec for Gauche
(use srfi-14) ;; character-sets for Gauche
or
(require srfi/42) ;; list-ec for Racket
(require srfi/14) ;; character-sets for Racket

(list-ec (:string c "abc 0 def 67 7 a 7 b c")
(if (char-set-contains? char-set:digit c))
c)

===>
(#\0 #\6 #\7 #\7 #\7)