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191 lines
8.3 KiB
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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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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<HEAD>
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<TITLE>26.3.2. Kinds of Loop Clauses</TITLE>
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<BODY>
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<meta name="description" value=" Kinds of Loop Clauses">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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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=tex2html4570 HREF="node241.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4568 HREF="node238.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4562 HREF="node239.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4572 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4573 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4571 HREF="node241.html"> Loop Syntax</A>
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<B>Up:</B> <A NAME=tex2html4569 HREF="node238.html"> Parsing Loop Clauses</A>
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<B> Previous:</B> <A NAME=tex2html4563 HREF="node239.html"> Order of Execution</A>
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<HR> <P>
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<H2><A NAME=SECTION003032000000000000000>26.3.2. Kinds of Loop Clauses</A></H2>
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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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<A NAME=LOOPKINDSSECTION>Loop</A>
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clauses fall into one of the following categories:
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<P>
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<UL> <LI>
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variable initialization and stepping
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<P>
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<UL> <LI>
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The <tt>for</tt> and <tt>as</tt> constructs provide iteration control clauses
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that establish a variable to be initialized.
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You can combine <tt>for</tt> and <tt>as</tt> clauses with the loop
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keyword <tt>and</tt> to get parallel initialization and stepping.
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<P>
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<LI>
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The <tt>with</tt> construct is similar to a single <tt>let</tt> clause.
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You can combine <tt>with</tt> clauses using
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<tt>and</tt> to get parallel initialization.
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<P>
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<LI>
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The <tt>repeat</tt> construct causes iteration to terminate after a specified
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number of times. It uses an internal variable to keep track of the
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number of iterations.
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</UL>
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<P>
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You can specify data types for loop variables (see
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section <A HREF="node251.html#LOOPTYPESSECTION">26.12.1</A>).
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It is an error to bind the same variable twice in any variable-binding
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clause of a single loop expression. Such variables include
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local variables, iteration control variables, and variables found by
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destructuring.
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<P>
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<LI> value accumulation
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<P>
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<UL> <LI> The <tt>collect</tt> construct takes one form in its clause
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and adds the value of that form to the end of a list of values. By
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default, the list of values is returned when the loop finishes.
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<P>
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<LI>
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The <tt>append</tt> construct takes one form in its clause
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and appends the value of that form to the end of a list of values. By
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default, the list of values is returned when the loop finishes.
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<P>
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<LI> The <tt>nconc</tt> construct is similar to <tt>append</tt>, but
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its list values are concatenated as if by the Common Lisp function
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<tt>nconc</tt>. By
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default, the list of values is returned when the loop finishes.
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<P>
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<LI> The <tt>sum</tt> construct takes one form in its clause that
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must evaluate to a number and adds that number into a running total.
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By default, the cumulative sum is returned when the loop finishes.
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<P>
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<LI>
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The <tt>count</tt> construct takes one form in its clause and counts the
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number of times that the form evaluates to a non-<tt>nil</tt> value. By
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default, the count is returned when the loop finishes.
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<P>
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<LI>
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The <tt>minimize</tt> construct takes one form in its clause and determines
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the minimum value obtained by evaluating that form. By default, the
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minimum value is returned when the loop finishes.
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<P>
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<LI>
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The <tt>maximize</tt> construct takes one form in its clause and
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determines the maximum value obtained by evaluating that form. By
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default, the maximum value is returned when the loop finishes.
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</UL>
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<P>
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<LI>
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termination conditions
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<P>
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<UL> <LI>
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The <tt>loop-finish</tt> Lisp macro terminates iteration and returns any
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accumulated result. If specified, any <tt>finally</tt> clauses are evaluated.
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<P>
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<LI>
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The <tt>for</tt> and <tt>as</tt> constructs provide a termination test
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that is determined by the iteration control clause.
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<P>
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<LI>
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The <tt>repeat</tt> construct causes termination after a specified
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number of iterations.
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<P>
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<LI>
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The <tt>while</tt> construct takes one form, a condition, and terminates
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the iteration if
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the condition evaluates to <tt>nil</tt>. A <tt>while</tt> clause is
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equivalent to the expression <tt>(if (not <i>condition</i>) (loop-finish))</tt>.
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<P>
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<LI>
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The <tt>until</tt> construct is the inverse of <tt>while</tt>;
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it terminates the iteration if the condition evaluates to any non-<tt>nil</tt>
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value. An <tt>until</tt> clause is equivalent to the expression
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<tt>(if <i>condition</i> (loop-finish))</tt>.
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<P>
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<LI>
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The <tt>always</tt> construct takes one form and terminates the loop
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if the form ever evaluates to <tt>nil</tt>; in this case, it returns
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<tt>nil</tt>. Otherwise, it provides a default return value of <tt>t</tt>.
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<P>
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<LI>
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The <tt>never</tt> construct takes one form and terminates the loop
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if the form ever evaluates to non-<tt>nil</tt>; in this case, it returns
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<tt>nil</tt>. Otherwise, it provides a default return value of <tt>t</tt>.
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<P>
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<LI>
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The <tt>thereis</tt> construct takes one form and terminates the loop
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if the form ever evaluates to non-<tt>nil</tt>; in this case, it returns
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that value.
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</UL>
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<P>
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<LI> unconditional execution
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<P>
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<UL> <LI>
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The <tt>do</tt> construct simply evaluates all forms in its clause.
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<P>
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<LI>
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The <tt>return</tt> construct takes one form and returns its value. It is
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equivalent to the clause <tt>do (return <i>value</i>)</tt>.
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</UL>
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<P>
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<LI> conditional execution
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<P>
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<UL> <LI>
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The <tt>if</tt> construct takes one form as a predicate and a clause that
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is executed when the predicate is true. The clause can be a value
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accumulation, unconditional, or another conditional clause; it can also
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be any combination of such clauses connected by the loop keyword <tt>and</tt>.
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<P>
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<LI>
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The <tt>when</tt> construct is a synonym for <tt>if</tt>.
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<P>
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<LI>
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The <tt>unless</tt> construct is similar to <tt>when</tt> except that it complements
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the predicate; it executes the following clause if the predicate is false.
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<P>
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<LI>
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The <tt>else</tt> construct provides an optional component of <tt>if</tt>,
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<tt>when</tt>, and <tt>unless</tt> clauses that is executed when the
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predicate is false. The component is one of the clauses described under
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<tt>if</tt>.
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<P>
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<LI>
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The <tt>end</tt> construct provides an optional component to mark the
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end of a conditional clause.
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</UL>
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<P>
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<LI> miscellaneous operations
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<P>
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<UL> <LI> The <tt>named</tt> construct assigns a name to a loop construct.
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<P>
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<LI> The <tt>initially</tt> construct causes its forms to be evaluated
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in the loop prologue, which precedes all loop code except for initial
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settings specified by the constructs <tt>with</tt>, <tt>for</tt>, or <tt>as</tt>.
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<P>
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<LI> The <tt>finally</tt> construct causes its forms to be evaluated
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in the loop epilogue after normal iteration terminates. An unconditional
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clause can also follow the loop keyword <tt>finally</tt>.
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</UL>
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</UL>
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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=tex2html4570 HREF="node241.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4568 HREF="node238.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4562 HREF="node239.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4572 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4573 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4571 HREF="node241.html"> Loop Syntax</A>
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<B>Up:</B> <A NAME=tex2html4569 HREF="node238.html"> Parsing Loop Clauses</A>
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<B> Previous:</B> <A NAME=tex2html4563 HREF="node239.html"> Order of Execution</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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</BODY>
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