Reading Delimited Input in Lisp

Question

In Common Lisp, I need to capture user input in the form of a list of reals. This list is terminated by two zeros in a row. For example, 5.0 0 6.0 3 2.0 5 1.0 7 13.0 8 0 0 should return the list (5.0 0 6.0 3 2.0 5 1.0 7 13.0 8).

I've come across the function read-delimited-list, which takes a single character as an ending point. I can stop at a single zero, but I'm not sure how to expand this function, and I don't know of another function that takes a delimited string instead of a char.

EDIT Thanks to the accepted answer, I've developed the following:

(butlast (loop for y = -1 then x for x = (read stream) until (and (= 0 x) (= 0 y))
    collect x)))

It seems a bit more readable to me (and at this point I'm openly admiting that my Lisp eyes are only 3 days old), and I'm not sure about performance implications (does collect append or reverse the list after the loop exits scope? is there an easier way to chop off the last element, or simply not append it?).

Vsevolod Dyomkin · Accepted Answer · 2012-09-06 03:53:52Z

4

For such simple case you can just use read in a loop, collect the results read so far and stop, when the last two are zeros.

(defun read-double-zero-terminated-list (stream)
  (do* ((prev -1 val)
        (val (read stream) (read stream))
        list)
       ((and (zerop prev) (zerop val)) (reverse (cdr list)))
     (push val list)))

read-delimited-list is intended for the cases of creating specialized syntax, so it's probably overkill here.

answered Sep 6, 2012 at 3:53

Vsevolod Dyomkin

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3 Comments

6502 Over a year ago

Not overkilling, just plain wrong. read-delimited-list works at a character level input... I bet efritz doesn't want to stop on a 0.17.

yilmazhuseyin Over a year ago

Why are you using do*? It seems to me like it would also work with do loop.

Svante Over a year ago

Remember to always set *read-eval* to nil when using read like this, to protect against #..

Svante · Accepted Answer · 2012-09-07 18:50:51Z

0

The best way to parse numbers in Common Lisp is to use the library parse-number. If your user input can be read line by line, you can simply split the lines on whitespace and use parse-number on the results.

answered Sep 7, 2012 at 18:50

Svante

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