80 lines
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80 lines
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<TITLE>26.2. How the Loop Facility Works</TITLE>
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<meta name="description" value=" How the Loop Facility Works">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html4531 HREF="node238.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4529 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4523 HREF="node236.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4533 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4534 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4532 HREF="node238.html"> Parsing Loop Clauses</A>
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<B>Up:</B> <A NAME=tex2html4530 HREF="node235.html"> Loop</A>
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<B> Previous:</B> <A NAME=tex2html4524 HREF="node236.html"> Introduction</A>
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<HR> <P>
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<H1><A NAME=SECTION003020000000000000000>26.2. How the Loop Facility Works</A></H1>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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The driving element of the Loop Facility is the <tt>loop</tt> macro.
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When Lisp encounters a <tt>loop</tt> macro call
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form, it invokes the Loop Facility and passes to it the loop clauses
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as a list of unevaluated forms, as with any macro.
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The loop clauses contain Common Lisp forms and loop keywords. The
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loop keywords are recognized by their symbol name, regardless of the
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packages that contain them. The <tt>loop</tt> macro translates the
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given form into Common Lisp code and returns the expanded form.
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<P>
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The expanded loop form is
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one or more lambda-expressions for the local binding of loop variables
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and a block and a tagbody that express a looping control structure.
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The variables established in the loop construct are bound as
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if by using <tt>let</tt> or <tt>lambda</tt>. Implementations can interleave the
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setting of initial values with the bindings. However, the assignment
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of the initial values is always calculated in the order specified by
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the user. A variable is thus sometimes bound to a harmless value of the
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correct data type, and then later in the prologue it is set to the true
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initial value by using <tt>setq</tt>.
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<P>
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The expanded form consists of three basic parts in the tagbody:
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<P>
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<UL><LI>
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The <i>loop prologue</i> contains forms that are executed before iteration begins,
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such as initial settings of loop variables and possibly an initial
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termination test.
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<P>
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<LI>
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The <i>loop body</i> contains those forms that are executed during iteration,
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including application-specific calculations, termination tests,
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and variable stepping. <i>Stepping</i> is the process of assigning a
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variable the next item in a series of items.
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<P>
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<LI>
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The <i>loop epilogue</i> contains forms that are executed after iteration
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terminates,
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such as code to return values from the loop.
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</UL>
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<P>
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Expansion of the <tt>loop</tt> macro produces an implicit block
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(named <tt>nil</tt>).
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Thus, the Common Lisp macro <tt>return</tt> and the special form
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<tt>return-from</tt> can be
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used to return values from a loop or to exit a loop.
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<P>
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Within the executable parts of loop clauses and around the entire
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loop form, you can still bind variables by using the Common Lisp
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special form <tt>let</tt>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html4531 HREF="node238.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4529 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4523 HREF="node236.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4533 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4534 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4532 HREF="node238.html"> Parsing Loop Clauses</A>
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<B>Up:</B> <A NAME=tex2html4530 HREF="node235.html"> Loop</A>
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<B> Previous:</B> <A NAME=tex2html4524 HREF="node236.html"> Introduction</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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