151 lines
7.3 KiB
HTML
151 lines
7.3 KiB
HTML
|
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
|
||
|
|
<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
|
||
|
|
<HEAD>
|
||
|
|
<TITLE>19.1. Introduction to Structures</TITLE>
|
||
|
|
</HEAD>
|
||
|
|
<BODY>
|
||
|
|
<meta name="description" value=" Introduction to Structures">
|
||
|
|
<meta name="keywords" value="clm">
|
||
|
|
<meta name="resource-type" value="document">
|
||
|
|
<meta name="distribution" value="global">
|
||
|
|
<P>
|
||
|
|
<b>Common Lisp the Language, 2nd Edition</b>
|
||
|
|
<BR> <HR><A NAME=tex2html3642 HREF="node170.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3640 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3634 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3644 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3645 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html3643 HREF="node170.html"> How to Use </A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html3641 HREF="node168.html"> Structures</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html3635 HREF="node168.html"> Structures</A>
|
||
|
|
<HR> <P>
|
||
|
|
<H1><A NAME=SECTION002310000000000000000>19.1. Introduction to Structures</A></H1>
|
||
|
|
<P>
|
||
|
|
<A NAME=DEFSTRUCTINTROSECTION>The</A>
|
||
|
|
structure facility is embodied in the <tt>defstruct</tt> macro,
|
||
|
|
which allows the user to create and use
|
||
|
|
aggregate data types with named elements. These are like
|
||
|
|
``structures'' in PL/I, or ``records'' in Pascal.
|
||
|
|
<P>
|
||
|
|
As an example, assume you are writing a Lisp
|
||
|
|
program that deals with space ships in a two-dimensional plane.
|
||
|
|
In your program, you need to
|
||
|
|
represent a space ship by a Lisp object of some kind. The interesting
|
||
|
|
things about a space ship, as far as your program is concerned, are
|
||
|
|
its position (represented as <i>x</i> and <i>y</i> coordinates),
|
||
|
|
velocity (represented as components along the <i>x</i> and <i>y</i> axes), and mass.
|
||
|
|
<P>
|
||
|
|
A ship might therefore be represented as a record structure with five
|
||
|
|
components: <i>x</i>-position, <i>y</i>-position, <i>x</i>-velocity, <i>y</i>-velocity, and mass.
|
||
|
|
This structure could in turn be implemented as a Lisp object in a
|
||
|
|
number of ways. It could be a list of five elements; the <i>x</i>-position
|
||
|
|
could be the <i>car</i>, the <i>y</i>-position the <i>cadr</i>, and so on. Equally
|
||
|
|
well it could be a vector of five elements: the <i>x</i>-position could be
|
||
|
|
element 0, the <i>y</i>-position element 1, and so on. The problem with either
|
||
|
|
of these representations is that the components occupy places in the
|
||
|
|
object that are quite arbitrary and hard to remember. Someone looking at
|
||
|
|
<tt>(cadddr ship1)</tt> or <tt>(aref ship1 3)</tt> in a piece of code might
|
||
|
|
find it difficult to determine that this is accessing the <i>y</i>-velocity
|
||
|
|
component of <tt>ship1</tt>. Moreover, if the representation of a ship should
|
||
|
|
have to be changed, it would be very difficult to find all the places
|
||
|
|
in the code to be changed to match (not all occurrences of <tt>cadddr</tt>
|
||
|
|
are intended to extract the <i>y</i>-velocity from a ship).
|
||
|
|
<P>
|
||
|
|
Ideally components of record structures should have names. One would
|
||
|
|
like to write something like
|
||
|
|
<tt>(ship-y-velocity ship1)</tt> instead of <tt>(cadddr ship1)</tt>.
|
||
|
|
One would also like a more mnemonic way to create a ship than this:
|
||
|
|
<P><pre>
|
||
|
|
(list 0 0 0 0 0)
|
||
|
|
</pre><P>
|
||
|
|
Indeed, one would like <tt>ship</tt> to be a new data type, just like other
|
||
|
|
Lisp data types, that one could test with <tt>typep</tt>, for example.
|
||
|
|
The <tt>defstruct</tt> facility provides all of this.
|
||
|
|
<P>
|
||
|
|
<tt>defstruct</tt> itself is a macro that defines a structure. For the
|
||
|
|
space ship example, one might define the structure by saying:
|
||
|
|
<P><pre>
|
||
|
|
(defstruct ship
|
||
|
|
x-position
|
||
|
|
y-position
|
||
|
|
x-velocity
|
||
|
|
y-velocity
|
||
|
|
mass)
|
||
|
|
</pre><P>
|
||
|
|
This declares that every <tt>ship</tt> is an object with five named components.
|
||
|
|
The evaluation of this form does several things:
|
||
|
|
<UL><LI>
|
||
|
|
It defines <tt>ship-x-position</tt> to be a function
|
||
|
|
of one argument, a ship, that returns the <i>x</i>-position
|
||
|
|
of the ship; <tt>ship-y-position</tt>
|
||
|
|
and the other components are given similar function definitions.
|
||
|
|
These functions are called the <i>access functions</i>, as they
|
||
|
|
are used to access elements of the structure.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
The symbol <tt>ship</tt> becomes the name of a data type of which instances
|
||
|
|
of ships are elements. This name becomes acceptable to <tt>typep</tt>,
|
||
|
|
for example; <tt>(typep x 'ship)</tt> is true if <tt>x</tt> is a ship
|
||
|
|
and false if <tt>x</tt> is any object other than a ship.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
A function named <tt>ship-p</tt> of one argument is defined; it is a predicate
|
||
|
|
that is true if its argument is a ship and is false otherwise.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
A function called <tt>make-ship</tt> is defined that, when invoked,
|
||
|
|
will create a data structure with five components, suitable for use with
|
||
|
|
the access functions. Thus executing
|
||
|
|
<P><pre>
|
||
|
|
(setq ship2 (make-ship))
|
||
|
|
</pre><P>
|
||
|
|
sets <tt>ship2</tt> to a newly created <tt>ship</tt> object.
|
||
|
|
One can specify the initial values of any desired component in the call
|
||
|
|
to <tt>make-ship</tt> by using keyword arguments in this way:
|
||
|
|
<P><pre>
|
||
|
|
(setq ship2 (make-ship <tt>:mass</tt> *default-ship-mass*
|
||
|
|
<tt>:x-position</tt> 0
|
||
|
|
<tt>:y-position</tt> 0))
|
||
|
|
</pre><P>
|
||
|
|
This constructs a new ship and initializes three of its components.
|
||
|
|
This function is called the <i>constructor function</i>
|
||
|
|
because it constructs a new structure.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
The <tt>#S</tt> syntax can be used to read instances of <tt>ship</tt>
|
||
|
|
structures, and a printer function is provided for printing
|
||
|
|
out ship structures. For example, the value of the
|
||
|
|
variable <tt>ship2</tt> shown above might be printed as
|
||
|
|
<P><pre>
|
||
|
|
#S(ship x-position 0 y-position 0 x-velocity nil
|
||
|
|
y-velocity nil mass 170000.0)
|
||
|
|
</pre><P>
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
A function called <tt>copy-ship</tt> of one argument
|
||
|
|
is defined that, when given a <tt>ship</tt> object,
|
||
|
|
will create a new <tt>ship</tt> object that is a copy of the given one.
|
||
|
|
This function is called the <i>copier function</i>.
|
||
|
|
<P>
|
||
|
|
<LI>
|
||
|
|
One may use <tt>setf</tt> to alter the components of a <tt>ship</tt>:
|
||
|
|
<P><pre>
|
||
|
|
(setf (ship-x-position ship2) 100)
|
||
|
|
</pre><P>
|
||
|
|
This alters the <i>x</i>-position of <i>ship2</i> to be <tt>100</tt>.
|
||
|
|
This works because <tt>defstruct</tt> behaves as if
|
||
|
|
it generates an appropriate <tt>defsetf</tt>
|
||
|
|
form for each access function.
|
||
|
|
</UL>
|
||
|
|
<P>
|
||
|
|
This simple example illustrates the power of <tt>defstruct</tt> to provide
|
||
|
|
abstract record structures in a convenient manner.
|
||
|
|
<tt>defstruct</tt> has many other features as well for specialized purposes.
|
||
|
|
<P>
|
||
|
|
<BR> <HR><A NAME=tex2html3642 HREF="node170.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3640 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3634 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3644 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3645 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html3643 HREF="node170.html"> How to Use </A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html3641 HREF="node168.html"> Structures</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html3635 HREF="node168.html"> Structures</A>
|
||
|
|
<HR> <P>
|
||
|
|
<HR>
|
||
|
|
<P><ADDRESS>
|
||
|
|
AI.Repository@cs.cmu.edu
|
||
|
|
</ADDRESS>
|
||
|
|
</BODY>
|