153 lines
6.3 KiB
HTML
153 lines
6.3 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>26.9. Variable Initializations</TITLE>
|
||
|
|
</HEAD>
|
||
|
|
<BODY>
|
||
|
|
<meta name="description" value=" Variable Initializations">
|
||
|
|
<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=tex2html4652 HREF="node248.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4650 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4644 HREF="node246.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4654 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4655 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html4653 HREF="node248.html"> Conditional Execution</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html4651 HREF="node235.html"> Loop</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html4645 HREF="node246.html"> Value Accumulation</A>
|
||
|
|
<HR> <P>
|
||
|
|
<H1><A NAME=SECTION003090000000000000000>26.9. Variable Initializations</A></H1>
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
<A NAME=LOOPVARSECTION>A</A>
|
||
|
|
local loop variable is one that exists only when the Loop Facility
|
||
|
|
is invoked. At that time, the variables are declared and are
|
||
|
|
initialized to some value. These local variables exist until loop
|
||
|
|
iteration terminates, at which point they cease to exist. Implicitly
|
||
|
|
variables are also established by iteration control clauses and the
|
||
|
|
<tt>into</tt> preposition of accumulation clauses.
|
||
|
|
<P>
|
||
|
|
The loop keyword <tt>with</tt> designates a loop clause that allows you to
|
||
|
|
declare and initialize variables
|
||
|
|
that are local to a loop. The variables are initialized one time
|
||
|
|
only; they can be initialized sequentially or in parallel.
|
||
|
|
<P>
|
||
|
|
By default, the <tt>with</tt> construct initializes variables
|
||
|
|
sequentially; that is, one variable is assigned a value before the
|
||
|
|
next expression is evaluated. However, by using the loop keyword
|
||
|
|
<tt>and</tt> to join several <tt>with</tt> clauses, you can force
|
||
|
|
initializations to occur in parallel; that is, all of the specified
|
||
|
|
expressions are evaluated, and the results are bound to the respective
|
||
|
|
variables simultaneously.
|
||
|
|
<P>
|
||
|
|
Use sequential binding for making the initialization of
|
||
|
|
some variables depend on the values of previously bound variables.
|
||
|
|
For example, suppose you want to bind the variables <tt>a</tt>, <tt>b</tt>,
|
||
|
|
and <tt>c</tt> in sequence:
|
||
|
|
<P><pre>
|
||
|
|
(loop with a = 1
|
||
|
|
with b = (+ a 2)
|
||
|
|
with c = (+ b 3)
|
||
|
|
with d = (+ c 4)
|
||
|
|
return (list a b c d))
|
||
|
|
=> (1 3 6 10)
|
||
|
|
</pre><P>
|
||
|
|
<P>
|
||
|
|
The execution of the preceding loop is equivalent to the execution of
|
||
|
|
the following code:
|
||
|
|
<P><pre>
|
||
|
|
(let* ((a 1)
|
||
|
|
(b (+ a 2))
|
||
|
|
(c (+ b 3))
|
||
|
|
(d (+ c 4)))
|
||
|
|
(block nil
|
||
|
|
(tagbody
|
||
|
|
next-loop (return (list a b c d))
|
||
|
|
(go next-loop)
|
||
|
|
end-loop)))
|
||
|
|
</pre><P>
|
||
|
|
<P>
|
||
|
|
If you are not depending on the value of previously bound variables
|
||
|
|
for the initialization of other local variables, you can use
|
||
|
|
parallel bindings as follows:
|
||
|
|
<P><pre>
|
||
|
|
(loop with a = 1
|
||
|
|
and b = 2
|
||
|
|
and c = 3
|
||
|
|
and d = 4
|
||
|
|
return (list a b c d))
|
||
|
|
=> (1 2 3 4)
|
||
|
|
</pre><P>
|
||
|
|
<P>
|
||
|
|
The execution of the preceding loop is equivalent to the execution of
|
||
|
|
the following code:
|
||
|
|
<P>
|
||
|
|
<P><pre>
|
||
|
|
(let ((a 1)
|
||
|
|
(b 2)
|
||
|
|
(c 3)
|
||
|
|
(d 4))
|
||
|
|
(block nil
|
||
|
|
(tagbody
|
||
|
|
next-loop (return (list a b c))
|
||
|
|
(go next-loop)
|
||
|
|
end-loop)))
|
||
|
|
</pre><P>
|
||
|
|
<P>
|
||
|
|
<BR><b>[Loop Clause]</b><BR>
|
||
|
|
<tt>with</tt> <em>var</em> <tt>[type-spec]</tt> <tt>[=</tt> <em>expr</em>] {<tt>and</tt> <em>var</em> <tt>[type-spec]</tt> <tt>[=</tt> <em>expr</em>]}*<P>The <tt>with</tt> construct initializes variables that are local to
|
||
|
|
a loop. The variables are initialized one time only.
|
||
|
|
<P>
|
||
|
|
If the optional <i>type-spec</i> argument is specified for any variable
|
||
|
|
<i>var</i>, but there is no related expression <i>expr</i> to be evaluated, <i>var</i>
|
||
|
|
is initialized to an appropriate default value for its data type.
|
||
|
|
For example, for the data types <tt>t</tt>, <tt>number</tt>, and <tt>float</tt>,
|
||
|
|
the default values are <tt>nil</tt>, <tt>0</tt>, and <tt>0.0</tt>, respectively.
|
||
|
|
It is an error to specify a <i>type-spec</i> argument for <i>var</i> if
|
||
|
|
the related expression returns a value that is not of the specified type.
|
||
|
|
The optional <tt>and</tt> clause forces parallel rather than sequential
|
||
|
|
initializations.
|
||
|
|
<P>
|
||
|
|
Examples:
|
||
|
|
<P><pre>
|
||
|
|
;;; These bindings occur in sequence.
|
||
|
|
(loop with a = 1
|
||
|
|
with b = (+ a 2)
|
||
|
|
with c = (+ b 3)
|
||
|
|
with d = (+ c 4)
|
||
|
|
return (list a b c d))
|
||
|
|
=> (1 3 6 10)
|
||
|
|
|
||
|
|
;;; These bindings occur in parallel.
|
||
|
|
(setq a 5 b 10 c 1729)
|
||
|
|
(loop with a = 1
|
||
|
|
and b = (+ a 2)
|
||
|
|
and c = (+ b 3)
|
||
|
|
and d = (+ c 4)
|
||
|
|
return (list a b c d))
|
||
|
|
=> (1 7 13 1733)
|
||
|
|
|
||
|
|
;;; This example shows a shorthand way to declare
|
||
|
|
;;; local variables that are of different types.
|
||
|
|
(loop with (a b c) (float integer float)
|
||
|
|
return (format nil "~A ~A ~A" a b c))
|
||
|
|
=> "0.0 0 0.0"
|
||
|
|
|
||
|
|
;;; This example shows a shorthand way to declare
|
||
|
|
;;; local variables that are of the same type.
|
||
|
|
(loop with (a b c) float
|
||
|
|
return (format nil "~A ~A ~A" a b c))
|
||
|
|
=> "0.0 0.0 0.0"
|
||
|
|
</pre><P>
|
||
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
<BR> <HR><A NAME=tex2html4652 HREF="node248.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4650 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4644 HREF="node246.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4654 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4655 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html4653 HREF="node248.html"> Conditional Execution</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html4651 HREF="node235.html"> Loop</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html4645 HREF="node246.html"> Value Accumulation</A>
|
||
|
|
<HR> <P>
|
||
|
|
<HR>
|
||
|
|
<P><ADDRESS>
|
||
|
|
AI.Repository@cs.cmu.edu
|
||
|
|
</ADDRESS>
|
||
|
|
</BODY>
|