68 lines
4 KiB
HTML
68 lines
4 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>26.3. Parsing Loop Clauses</TITLE>
|
||
|
|
</HEAD>
|
||
|
|
<BODY>
|
||
|
|
<meta name="description" value=" Parsing Loop Clauses">
|
||
|
|
<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=tex2html4543 HREF="node239.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4541 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4535 HREF="node237.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4545 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4546 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html4544 HREF="node239.html"> Order of Execution</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html4542 HREF="node235.html"> Loop</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html4536 HREF="node237.html"> How the Loop </A>
|
||
|
|
<HR> <P>
|
||
|
|
<H1><A NAME=SECTION003030000000000000000>26.3. Parsing Loop Clauses</A></H1>
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
The syntactic parts of a loop construct are called <i>clauses</i>; the scope
|
||
|
|
of each clause is determined by the top-level parsing of that clause's
|
||
|
|
keyword. The following example shows a loop construct with six
|
||
|
|
clauses:
|
||
|
|
<P>
|
||
|
|
<P><pre>
|
||
|
|
(loop for i from 1 to (compute-top-value) ;First clause
|
||
|
|
while (not (unacceptable i)) ;Second clause
|
||
|
|
collect (square i) ;Third clause
|
||
|
|
do (format t "Working on ~D now" i) ;Fourth clause
|
||
|
|
when (evenp i) ;Fifth clause
|
||
|
|
do (format t "~D is a non-odd number" i)
|
||
|
|
finally (format t "About to exit!")) ;Sixth clause
|
||
|
|
</pre><P>
|
||
|
|
<P>
|
||
|
|
Each loop keyword introduces either a compound loop clause or a simple
|
||
|
|
loop clause that can consist of a loop keyword followed by a
|
||
|
|
single Lisp form. The number of
|
||
|
|
forms in a clause is determined by the loop keyword that begins the
|
||
|
|
clause and by the auxiliary keywords in the clause. The keywords
|
||
|
|
<tt>do</tt>, <tt>initially</tt>, and <tt>finally</tt> are the only loop
|
||
|
|
keywords that can take any number of Lisp forms and group them as if
|
||
|
|
in a single <tt>progn</tt> form.
|
||
|
|
<P>
|
||
|
|
Loop clauses can contain auxiliary keywords, which are sometimes
|
||
|
|
called <i>prepositions</i>. For example, the first clause in the preceding code
|
||
|
|
includes the prepositions <tt>from</tt> and <tt>to</tt>, which mark
|
||
|
|
the value from which stepping begins and the value at which stepping
|
||
|
|
ends.
|
||
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
<HR>
|
||
|
|
<UL>
|
||
|
|
<LI> <A NAME=tex2html4547 HREF="node239.html#SECTION003031000000000000000"> Order of Execution</A>
|
||
|
|
<LI> <A NAME=tex2html4548 HREF="node240.html#SECTION003032000000000000000"> Kinds of Loop Clauses</A>
|
||
|
|
<LI> <A NAME=tex2html4549 HREF="node241.html#SECTION003033000000000000000"> Loop Syntax</A>
|
||
|
|
</UL>
|
||
|
|
<BR> <HR><A NAME=tex2html4543 HREF="node239.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4541 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4535 HREF="node237.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4545 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4546 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html4544 HREF="node239.html"> Order of Execution</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html4542 HREF="node235.html"> Loop</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html4536 HREF="node237.html"> How the Loop </A>
|
||
|
|
<HR> <P>
|
||
|
|
<HR>
|
||
|
|
<P><ADDRESS>
|
||
|
|
AI.Repository@cs.cmu.edu
|
||
|
|
</ADDRESS>
|
||
|
|
</BODY>
|