From Newsgroup: comp.lang.lisp

Ken Tilton wrote:

Steve Allan wrote:

I've been lurking here for a bit, and trying to learn lisp in my spare time. As an exercise, I'm trying to port some Perl code to lisp, and
the first bit of functionality I wrote was to read a file that looks
like this:

#------------------------------------------
# unique-label hostname home-dir #------------------------------------------
mach1 host1 /home/me
mach2 host2 /export/home/me
mach3 host3 c:\\home\\me

And split each line into a list, returning a list of lists:

(("mach1" "host1" "/home/me")
("mach2" "host2" "/export/home/me")
("mach3" "host3" "c:\\home\\me"))

The code below does this, but I'm pretty sure there's much that could
be improved. To speed up my learning, I'd love feedback in either/both
of these areas.

1) Critique on the implementation - how to improve the code as
written.

2) Suggestions for a better approach altogether.

I realize the task I'm coding is pretty mundane, but I think I could
learn a lot of the basics by really getting this right, so your
critique would be most welcome.

Thanks!

Code is below.

--
-- Steve

;;;;================================================================
(defun get-platforms (file)
(with-open-file (platforms file)
(loop for line = (read-line platforms nil)
while line
unless (position #\# line)
collect (split-on-space line))))

(defun split-on-space (string)
"Splits string on whitespace, meaning spaces and tabs"
(unless (null string)
(let ((space (or (position #\space string) (position #\tab string))))
(cond
(space (cons
(subseq string 0 space)
(split-on-space
(string-trim '(#\Space #\Tab) (subseq string space)))))
(t (list string))))))

Not bad at all. (unless (null x)...) should be (when x...) I think you
might agree. You might avoid searching the string twice, one for tab and
once for space, by using position-if. And a lot of us have little macros
akin to Paul Graham's AIF. I have (untested):

(when string
(bif (delim-pos (position-if (lambda (c)
(or (char= c #\space)(char= c #\tab)))
string))
(cons (subseq string 0 delim-pos) ...etc...)
(list string))

BIF left as an exercise. :)

Gauche Scheme

(use srfi-13) ;; string-tokenize

(with-input-from-string
"mach1 host1 /home/me
mach2 host2 /export/home/me
mach3 host3 c:\\home\\me"
(lambda()
(generator-map string-tokenize read-line)))

(("mach1" "host1" "/home/me")
("mach2" "host2" "/export/home/me")
("mach3" "host3" "c:\\home\\me"))
--
The good news is, it's not Lisp that sucks, but Common Lisp. --- Paul Graham --- Synchronet 3.21a-Linux NewsLink 1.2