page174.html
来自「wqeqwvrw rkjqhwrjwq jkhrjqwhrwq jkhrwq」· HTML 代码 · 共 69 行
HTML
69 行
<HTML>
<HEAD>
<TITLE>Finding the Position of an Item and Accessing by Position</TITLE>
</HEAD>
<BODY bgcolor="#FFFFFF">
<img src="cover75.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cover75.gif" alt="Logo" align=right>
<b>Data Structures and Algorithms
with Object-Oriented Design Patterns in C++</b><br>
<A NAME="tex2html4063" HREF="page175.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page175.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/next_motif.gif"></A> <A NAME="tex2html4061" HREF="page168.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page168.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/up_motif.gif"></A> <A NAME="tex2html4055" HREF="page173.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page173.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/previous_motif.gif"></A> <A NAME="tex2html4065" HREF="page9.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page9.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/contents_motif.gif"></A> <A NAME="tex2html4066" HREF="page620.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page620.html"><IMG WIDTH=43 HEIGHT=24 ALIGN=BOTTOM ALT="index" SRC="index_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/index_motif.gif"></A> <BR><HR>
<H3><A NAME="SECTION008116000000000000000">Finding the Position of an Item and Accessing by Position</A></H3>
<P>
Program <A HREF="page174.html#proglist4c" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page174.html#proglist4c"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A> defines two more member functions
of the <tt>ListAsArray</tt> class,
<tt>FindPosition</tt> and <tt>operator[]</tt>.
The <tt>FindPosition</tt> member function takes as its lone argument
a <tt>const</tt> reference to an <tt>Object</tt>.
The purpose of this function is to search the ordered list
for an item which matches the object,
and to return its position.
The result is a reference to a <tt>Position</tt>.
Since the <tt>Position</tt> class is an abstract base class,
there can be no object instances of that class.
However, there can be object instances of a concrete class
derived from the <tt>Position</tt> class,
such as the <tt>ListAsArray::Pos</tt> class.
Therefore, the <tt>FindPosition</tt> function allocates
an new instance of the <tt>ListAsArray::Pos</tt> class an
returns a reference to that instance.
<P>
<P><A NAME="9613"> </A><A NAME="proglist4c"> </A> <IMG WIDTH=575 HEIGHT=315 ALIGN=BOTTOM ALT="program9549" SRC="img795.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img795.gif" ><BR>
<STRONG>Program:</STRONG> <tt>ListAsArray</tt> Class <tt>FindPosition</tt> Member Function and Subscripting Operator Definitions<BR>
<P>
<P>
The search algorithm used in <tt>FindPosition</tt> is identical to that
used in the <tt>Find</tt> routine (Program <A HREF="page171.html#proglist2c" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page171.html#proglist2c"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>).
The <tt>FindPosition</tt> makes use of <tt>operator==</tt> to locate
a contained object which is equal to the search target.
Notice that if no match is found,
the <tt>offset</tt> is set to the value <tt>count</tt>,
which is one position to the right of the last item in the ordered list.
The running time of <tt>FindPosition</tt> is identical to that of
<tt>Find</tt>: <IMG WIDTH=172 HEIGHT=26 ALIGN=MIDDLE ALT="tex2html_wrap_inline61650" SRC="img790.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img790.gif" >, where <IMG WIDTH=72 HEIGHT=9 ALIGN=BOTTOM ALT="tex2html_wrap_inline61308" SRC="img709.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img709.gif" >.
<P>
The subscripting operator, <tt>operator[]</tt>,
defined in Program <A HREF="page174.html#proglist4c" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page174.html#proglist4c"><IMG ALIGN=BOTTOM ALT="gif" SRC="cross_ref_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/cross_ref_motif.gif"></A>
takes a <tt>const</tt> reference to a <tt>Position</tt>
and returns a reference to the item
in the ordered list at the given position.
The <tt>Position</tt> argument is dynamically cast
to an <tt>ListAsArray::Pos</tt>.
Remember that in C++,
run-time checks are made to ensure that the cast is safe.
After a simple validity check,
the <tt>offset</tt> recorded in the position is used
to index into the <tt>array</tt> variable,
to obtain the desired result.
If the <tt>offset</tt> is equal to <tt>count</tt>,
the position is invalid.
In this case, a reference to the <tt>NullObject</tt> instance is returned.
The running time of this operator is clearly <I>O</I>(1).
<P>
<HR><A NAME="tex2html4063" HREF="page175.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page175.html"><IMG WIDTH=37 HEIGHT=24 ALIGN=BOTTOM ALT="next" SRC="next_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/next_motif.gif"></A> <A NAME="tex2html4061" HREF="page168.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page168.html"><IMG WIDTH=26 HEIGHT=24 ALIGN=BOTTOM ALT="up" SRC="up_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/up_motif.gif"></A> <A NAME="tex2html4055" HREF="page173.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page173.html"><IMG WIDTH=63 HEIGHT=24 ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/previous_motif.gif"></A> <A NAME="tex2html4065" HREF="page9.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page9.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/contents_motif.gif"></A> <A NAME="tex2html4066" HREF="page620.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page620.html"><IMG WIDTH=43 HEIGHT=24 ALIGN=BOTTOM ALT="index" SRC="index_motif.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/index_motif.gif"></A> <P><ADDRESS>
<img src="bruno.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/icons/bruno.gif" alt="Bruno" align=right>
<a href="javascript:if(confirm('http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/copyright.html \n\nThis file was not retrieved by Teleport Pro, because it is addressed on a domain or path outside the boundaries set for its Starting Address. \n\nDo you want to open it from the server?'))window.location='http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/copyright.html'" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/copyright.html">Copyright © 1997</a> by <a href="javascript:if(confirm('http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/signature.html \n\nThis file was not retrieved by Teleport Pro, because it is addressed on a domain or path outside the boundaries set for its Starting Address. \n\nDo you want to open it from the server?'))window.location='http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/signature.html'" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/signature.html">Bruno R. Preiss, P.Eng.</a> All rights reserved.
</ADDRESS>
</BODY>
</HTML>
⌨️ 快捷键说明
复制代码Ctrl + C
搜索代码Ctrl + F
全屏模式F11
增大字号Ctrl + =
减小字号Ctrl + -
显示快捷键?