284 lines
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284 lines
12 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>26.8. Value Accumulation</TITLE>
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<BODY>
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<meta name="description" value=" Value Accumulation">
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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=tex2html4640 HREF="node247.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4638 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4632 HREF="node245.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4642 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4643 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4641 HREF="node247.html"> Variable Initializations</A>
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<B>Up:</B> <A NAME=tex2html4639 HREF="node235.html"> Loop</A>
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<B> Previous:</B> <A NAME=tex2html4633 HREF="node245.html"> End-Test Control</A>
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<HR> <P>
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<H1><A NAME=SECTION003080000000000000000>26.8. Value Accumulation</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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<A NAME=LOOPACCUMSECTION>Accumulating</A>
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values during iteration and returning them from a loop
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is often useful. Some of these accumulations occur so
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frequently that special loop clauses have been developed to handle them.
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<P>
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The loop keywords <tt>append</tt>, <tt>appending</tt>,
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<tt>collect</tt>, <tt>collecting</tt>,
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<tt>nconc</tt>, and <tt>nconcing</tt>
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designate clauses that
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accumulate values in lists and return them.
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<P>
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The loop keywords <tt>count</tt>, <tt>counting</tt>,
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<tt>maximize</tt>, <tt>maximizing</tt>,
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<tt>minimize</tt>, <tt>minimizing</tt>,
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<tt>sum</tt>, and <tt>summing</tt>
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designate clauses that accumulate and
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return numerical values.
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[There is no semantic difference between the ``ing'' keywords and their non-``ing''
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counterparts. They are provided purely for the sake of stylistic diversity among users.
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I happen to prefer the non-``ing'' forms-when I use <tt>loop</tt> at all.-GLS]
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<P>
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The loop preposition <tt>into</tt> can be used to name the
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variable used to hold partial accumulations.
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The variable is bound as if by the loop
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construct <tt>with</tt> (see section <A HREF="node247.html#LOOPVARSECTION">26.9</A>). If
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<tt>into</tt> is used, the construct does not provide a default return value;
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however, the variable is available
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for use in any <tt>finally</tt> clause.
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<P>
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You can combine value-returning accumulation clauses in a loop if
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all the clauses accumulate the same type of data object.
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By default, the Loop Facility returns only one value;
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thus, the data objects collected by multiple accumulation clauses
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as return values must have compatible types. For example, since both
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the <tt>collect</tt> and <tt>append</tt> constructs accumulate objects into a
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list that is returned from a loop, you can combine them safely.
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<P>
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<P><pre>
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;;; Collect every name and the kids in one list by using
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;;; COLLECT and APPEND.
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(loop for name in '(fred sue alice joe june)
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for kids in '((bob ken) () () (kris sunshine) ())
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collect name
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append kids)
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=> (FRED BOB KEN SUE ALICE JOE KRIS SUNSHINE JUNE)
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</pre><P>
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[In the preceding example, note that the items accumulated by the
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<tt>collect</tt> and <tt>append</tt> clauses are interleaved in the result list, according to
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the order in which the clauses were executed.-GLS]
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<P>
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Multiple clauses that do not accumulate the same type of data object
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can coexist in a loop only if each clause accumulates its values into
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a different user-specified variable. Any number of values can
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be returned from a loop if you use the Common Lisp function <tt>values</tt>,
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as the next example shows:
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<P><pre>
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;;; Count and collect names and ages.
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(loop for name in '(fred sue alice joe june)
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as age in '(22 26 19 20 10)
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append (list name age) into name-and-age-list
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count name into name-count
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sum age into total-age
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finally
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(return (values (round total-age name-count)
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name-and-age-list)))
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=> 19 and (FRED 22 SUE 26 ALICE 19 JOE 20 JUNE 10)
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</pre><P>
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<P>
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<BR><b>[Loop Clause]</b><BR>
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<tt>collect</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt><BR></tt><tt>collecting</tt> <em>expr</em> <tt>[into</tt> <em>var</em>]<P>During each iteration, these constructs collect the value of the specified
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expression into a list. When iteration terminates, the list is returned.
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<P>
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The argument <i>var</i> is
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set to the list of collected values; if <i>var</i> is specified, the loop
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does not return the final list automatically. If <i>var</i> is not
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specified, it is equivalent to specifying an internal name for
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<i>var</i> and returning its value in a <tt>finally</tt> clause.
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The <i>var</i> argument
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is bound as if by the construct <tt>with</tt>.
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You cannot specify a data type for <i>var</i>; it must be of type <tt>list</tt>.
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<P>
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Examples:
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<P><pre>
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;;; Collect all the symbols in a list.
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(loop for i in '(bird 3 4 turtle (1 . 4) horse cat)
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when (symbolp i) collect i)
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=> (BIRD TURTLE HORSE CAT)
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;;; Collect and return odd numbers.
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(loop for i from 1 to 10
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if (oddp i) collect i)
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=> (1 3 5 7 9)
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;;; Collect items into local variable, but don't return them.
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(loop for i in '(a b c d) by #'cddr
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collect i into my-list
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finally (print my-list)) `;Prints 1 line
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(A C)
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=> NIL
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</pre><P>
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<P>
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<BR><b>[Loop Clause]</b><BR>
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<tt>append</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt><BR></tt><tt>appending</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt><BR></tt><tt>nconc</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt><BR></tt><tt>nconcing</tt> <em>expr</em> <tt>[into</tt> <em>var</em>]<P>These constructs are similar to <tt>collect</tt> except that the
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values of the specified expression must be lists.
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<P>
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The <tt>append</tt> keyword causes its list values to be concatenated
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into a single list, as if
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they were arguments to the Common Lisp function <tt>append</tt>.
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<P>
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The <tt>nconc</tt> keyword causes its list values to be concatenated
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into a single list,
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as if they were arguments to the Common Lisp function <tt>nconc</tt>.
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Note that the <tt>nconc</tt> keyword destructively modifies its argument lists.
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<P>
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The argument <i>var</i> is
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set to the list of concatenated values; if you specify <i>var</i>, the loop
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does not return the final list automatically. The <i>var</i> argument
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is bound as if by the construct <tt>with</tt>.
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You cannot specify a data type for <i>var</i>; it must be of type <tt>list</tt>.
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<P>
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Examples:
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<P><pre>
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;;; Use APPEND to concatenate some sublists.
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(loop for x in '((a) (b) ((c)))
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append x)
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=> (A B (C))
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</pre><P>
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<P><pre>
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;;; NCONC some sublists together. Note that only lists
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;;; made by the call to LIST are modified.
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(loop for i upfrom 0
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as x in '(a b (c))
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nconc (if (evenp i) (list x) '()))
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=> (A (C))
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</pre><P>
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<P>
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<BR><b>[Loop Clause]</b><BR>
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<tt>count</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt>[type-spec]</tt> <tt><BR></tt><tt>counting</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt>[type-spec]</tt><P>The <tt>count</tt> construct counts the number of times that the specified
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expression has a non-<tt>nil</tt> value.
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<P>
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The argument <i>var</i> accumulates the number of occurrences; if
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<i>var</i> is specified, the loop
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does not return the final count automatically. The <i>var</i> argument
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is bound as if by the construct <tt>with</tt>.
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<P>
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If <tt>into</tt> <i>var</i> is used, the optional
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<i>type-spec</i> argument specifies a data type for <i>var</i>.
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If there is no <tt>into</tt> variable, the optional <i>type-spec</i>
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argument applies to the internal variable that is keeping the count.
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In either case it is an error to specify a non-numeric
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data type.
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The default type is implementation-dependent, but it must be a subtype
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of <tt>(or integer float)</tt>.
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<P>
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Example:
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<P><pre>
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(loop for i in '(a b nil c nil d e)
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count i)
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=> 5
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</pre><P>
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<P>
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<BR><b>[Loop Clause]</b><BR>
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<tt>sum</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt>[type-spec]</tt> <tt><BR></tt><tt>summing</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt>[type-spec]</tt><P>The <tt>sum</tt> construct forms a cumulative sum of the values of the
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specified expression at each iteration.
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<P>
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The argument <i>var</i> is used to accumulate
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the sum; if <i>var</i> is specified, the loop
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does not return the final sum automatically. The <i>var</i> argument
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is bound as if by the construct <tt>with</tt>.
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<P>
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If <tt>into</tt> <i>var</i> is used, the optional
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<i>type-spec</i> argument specifies a data type for <i>var</i>.
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If there is no <tt>into</tt> variable, the optional <i>type-spec</i>
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argument applies to the internal variable that is keeping the sum.
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In either case it is an error to specify a non-numeric
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data type.
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The default type is implementation-dependent, but it must be a subtype
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of <tt>number</tt>.
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<P>
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Examples:
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<P><pre>
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;;; Sum the elements of a list.
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(loop for i fixnum in '(1 2 3 4 5)
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sum i)
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=> 15
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;;; Sum a function of elements of a list.
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(setq series
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'(1.2 4.3 5.7))
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=> (1.2 4.3 5.7)
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(loop for v in series
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sum (* 2.0 v))
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=> 22.4
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</pre><P>
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<P>
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<BR><b>[Loop Clause]</b><BR>
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<tt>maximize</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt>[type-spec]</tt> <tt><BR></tt><tt>maximizing</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt>[type-spec]</tt> <tt><BR></tt><tt>minimize</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt>[type-spec]</tt> <tt><BR></tt><tt>minimizing</tt> <em>expr</em> <tt>[into</tt> <em>var</em>] <tt>[type-spec]</tt><P>The <tt>maximize</tt> construct compares the value of the specified expression
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obtained during the first iteration with values obtained in successive
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iterations. The maximum value encountered is determined and returned. If the
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loop never executes the body, the returned value is not meaningful.
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<P>
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The <tt>minimize</tt> construct is similar to <tt>maximize</tt>; it
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determines and returns the minimum value.
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<P>
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The argument <i>var</i> accumulates the maximum or
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minimum value; if <i>var</i> is specified, the loop
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does not return the maximum or minimum automatically. The <i>var</i> argument
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is bound as if by the construct <tt>with</tt>.
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<P>
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If <tt>into</tt> <i>var</i> is used, the optional
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<i>type-spec</i> argument specifies a data type for <i>var</i>.
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If there is no <tt>into</tt> variable, the optional <i>type-spec</i>
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argument applies to the internal variable that is keeping the intermediate result.
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In either case it is an error to specify a non-numeric
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data type.
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The default type is implementation-dependent, but it must be a subtype
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of <tt>(or integer float)</tt>.
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<P>
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Examples:
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<P><pre>
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(loop for i in '(2 1 5 3 4)
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maximize i)
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=> 5
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</pre><P>
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<P><pre>
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(loop for i in '(2 1 5 3 4)
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minimize i)
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=> 1
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;;; In this example, FIXNUM applies to the internal
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;;; variable that holds the maximum value.
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(setq series '(1.2 4.3 5.7))
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=> (1.2 4.3 5.7)
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(loop for v in series
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maximize (round v) fixnum)
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=> 6
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;;; In this example, FIXNUM applies to the variable RESULT.
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(loop for v float in series
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minimize (round v) into result fixnum
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finally (return result))
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=> 1
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</pre><P>
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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=tex2html4640 HREF="node247.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4638 HREF="node235.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4632 HREF="node245.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4642 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4643 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4641 HREF="node247.html"> Variable Initializations</A>
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<B>Up:</B> <A NAME=tex2html4639 HREF="node235.html"> Loop</A>
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<B> Previous:</B> <A NAME=tex2html4633 HREF="node245.html"> End-Test Control</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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