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From: epstein@sunc4.cs.uiuc.edu (Milt Epstein)
Newsgroups: comp.lang.lisp
Subject: Re: Accessing a field of a Lisp structure
Message-ID: <28047A1F.44CE@ibma0.cs.uiuc.edu>
Date: 11 Apr 91 15:00:47 GMT
References: <DPX_J3+@ee.eng.ohio-state.edu> <1991Apr10.215214.2827@cbnewsl.att.com>
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Organization: University of Illinois at Urbana-Champaign
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In <1991Apr10.215214.2827@cbnewsl.att.com> davel@cbnewsl.att.com (David Loewenstern) writes:

>In article <DPX_J3+@ee.eng.ohio-state.edu> mzaik@ee.eng.ohio-state.edu (Tahsin Mzaik) writes:
>>
>>I would like to write a macro that can access a particular field 
>>in a structure. 
>>
>>Example: Let the structure be:
>>      (defstruct test a b c)
>>
>>      (make-test instance)
>>
>>I would like to wite a macro 'access-field' that I can invoke
>>something like  
>>
>>      (access-field instance 'a)
>
>Try:
>(defmacro access-field (instance field)
>  `(,(intern (format nil "~A-~A" 'test field)) ,instance))
>
>Note that this will allow 
>(access-field instance a)
>to work but not, for example
>(access-field instance (get-slot-name-from-some-wierd-place))

Someone else also posted this question just a couple of days ago, to
which Barry Margolin responded suggesting a function like:

(defun access-field (instance field)
  (case field
    (a (test-a instance))
    (b (test-b instance))
    (c (test-c instance))))

Note that if you use a function, it doesn't matter where you get the
slot-name from.

I sent a message to Barry suggesting the INTERN-FORMAT version, and he
pointed out a couple of things:

1. It's better to use FIND-SYMBOL than INTERN, since the symbol (the
structure accessor function) should already exist.

2. The CASE version appeared to be much faster (he did some timing
test on a Symbolics).

-- 
Milt Epstein
Department of Computer Science
University of Illinois
epstein@cs.uiuc.edu