From Newsgroup: comp.lang.lisp

B. Pym wrote:

Kent M. Pitman wrote:

... What i want to do is start at the beginning of the
file( which I can do) and search for the word "entity".
Once I find this, I want to assign the next word to a
variable. ...

Use WITH-OPEN-FILE to open the stream. It will let you specify
a variable to which the stream is bound. The I/O routines all
take a stream as argument. e.g.,

(defun find-word-association-in-file (word file)
(with-open-file (stream file :direction :input)
(loop
(let ((line (read-line stream nil nil)))
(declare (type string line))
(unless line (return nil))
(let ((pos1 (position #\Space line :test #'char-equal)))
(when (and pos1 (string-equal word line :end2 pos1))
(let ((pos2 (position #\Space line
:test (complement #'char-equal)
:start pos1)))
(when pos2
(return (subseq line pos2
(position #\Space line
:test #'char-equal
:start pos2)))))))))))

Given a data file "delete-me.text" containing:

FOO OOF
BAR RAB XYZZY
BAZ ZAB PLOVER PLUGH
NUL NIL

I find that:

(find-word-association-in-file "FOO" "delete-me.text") => "OOF"
(find-word-association-in-file "BAZ" "delete-me.text") => "ZAB"
(find-word-association-in-file "NUL" "delete-me.text") => "NIL"
(find-word-association-in-file "GEE" "delete-me.text") => NIL

Scheme

(define (find-word-association-in-file word file)
(with-input-from-file file
(lambda()
(let go ((s (read)))
(cond ((eof-object? s) #f)
((equal? s word) (read))
(#t (go (read))))))))

(find-word-association-in-file 'FOO "delete-me.text")
===>
OOF

(find-word-association-in-file 'BAZ "delete-me.text")
===>
ZAB

(find-word-association-in-file 'PLOVER "delete-me.text")
===>
PLUGH

(find-word-association-in-file 'OOF "delete-me.text")
===>
BAR

(find-word-association-in-file 'NOWHERE "delete-me.text")
===>
#f


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