Re: Q: Function to convert 'abc+def = (abc def)

Subject: Re: Q: Function to convert 'abc+def = (abc def)
From: Erik Naggum <erik@naggum.net>
Date: 2000/11/20
Newsgroups: comp.lang.lisp
Message-ID: <3183715001067568@naggum.net>

* "Sandeep Koranne" <sandeep.koranne@nospam.philips.com>
| I want a function that takes input a symbol and returns a list made up
| of the two components of the input symbol separated by a character '+'.
| We can safely assume that there will be one and only one of these special
| characters embedded inside.
| Also assume that the character is always there.
| 
| I was hoping that
| (defun split-on-plus (insym)
| (ler ((ret-list nil)
|         (sym-name (symbol-name insym)))
| (push (string-left-trim "+" sym-name) ret-list)
| (push (string-right-trim "+" sym-name) ret-list)
| ret-list))

  Please make an effort to post both tested and readable code.

| I hoped this would do the trick, but I found out that the example
| given in CLt:L2 (for string-left-trim) dont work on CLISP and "cmucl".
| I tried typing them in and all they were doing was throwing the input
| string back at me.

  I highly doubt that.

| E.g.  (string-left-trim "abc" "labcabcabc") => "labcabc"
| (It should give that according to the book)

  Really?  Why do you believe that?  Please note that the _left_ edge of
  this text is in this direction <---, while the _right_ edge of the
  text is in that direction --->.  You need to read the specification of
  these functions, too.  The first argument is not a _string_, but a
  _collection_ of characters.  E.g,, string-right-trim applied to the
  same arguments would simply return "l".

| So what am I doing wrong ????

  You are _guessing_ and you fail to be aware of it.  That's a cardinal
  epistemological _sin_, dude.  Repent now.

| I had to write 'split-on-plus' using (aref ) on the string.
| I am sure there must be a better way of getting at sub-strings.

  Well, position will find the character for you.

(defun silly-function-#398 (symbol)
  (let* ((symbol-name (symbol-name symbol))
	 (delimiter (position #\+ symbol-name)))
    (list (intern (subseq symbol-name 0 delimiter))
	  (intern (subseq symbol-name (1+ delimiter))))))

  Not that this necessarily does what you expect, unless you never
  change which package is current in your system.

#:Erik
-- 
  ALGORITHM: a procedure for solving a mathematical problem in a finite
  number of steps that frequently involves repetition of an operation.
  ALGOREISM: a procedure for solving an electoral problem in a finite
  number of steps that frequently involves repetition of an operation.