Sum ints on stdin

Question

I have been meaning to learn LISP and for a small task I wanted to find out the sum of all integers on STDIN. So for

> clisp a.lisp
1
2
3^D
6

My code is as follows. I have a small helper function, my core function and the invocation of it.

(defun read-int()
  (parse-integer (read-line)))

(defun sum-stdin()
    (handler-case
      ; recurse
      (+ (read-int) (sum-stdin))
      ; base case: if eof
      (error(c)
        (values 0))))

(write (sum-stdin))

Is this according to "the lisp way"?

One thing I see is that it feels weird to basically have the base case of my recursive function what would otherwise be the catch block in a non-functional language. I don't think there is a rule against it, but it just seems very unusual and hacky.

Rainer Joswig · Accepted Answer · 2019-06-27 07:29:03Z

Dealing with end of file in I/O operations:

There are two basic ways to deal with EOF in functions like read-line:

1) Return a value

           ; stream     ; signal eof error     ; return value if EOF
(read-line stream       nil                    nil)

2) Signal an error

           ; stream     ; signal eof error
(read-line stream       t)

Typical way:

use a loop, like SDS said
don't signal an error, but deal with an explicit EOF value

Less typical:

use a loop, like SDS said
signal an error and handle it -> might need slightly more tricky error handling code

Possible:

use a loop, like SDS said
hide the error handling code behind a macro...

sds · Accepted Answer · 2019-06-26 21:50:39Z

3

No, this is not lispy at all.

Lispers use recursion only when necessary, not as a generic substitute for iteration. Moreover, even in scheme they use tail recursion instead of iteration, and your code is not tail recursive.

A typical lisp solution would be

(defun sum-stream(&optional (s *standard-input*))
  (loop for line = (read-line s nil nil) while line
      sum (parse-integer line)))
(print (sum-stream))

answered Jun 26, 2019 at 21:50

sds

1,42510 silver badges17 bronze badges

\$\begingroup\$ I would really liked to have given this the answer checkmark -- because it shows the better solution. But the other answer starts to explain the proper way, so it seems more "complete" \$\endgroup\$

user45891
– user45891

2019-06-27 22:25:38 +00:00
Commented Jun 27, 2019 at 22:25

Add a comment |

Stack Exchange Network

Sum ints on stdin

2 Answers 2

You must log in to answer this question.

Hot Network Questions

Sum ints on stdin

2 Answers 2

You must log in to answer this question.

Related

Hot Network Questions