📄 page103.html
字号:
<HTML>
<HEAD>
<TITLE>Multi-Dimensional Subscripting in C++</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="tex2html3180" HREF="page104.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page104.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="tex2html3178" HREF="page100.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page100.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="tex2html3172" HREF="page102.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page102.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="tex2html3182" 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="tex2html3183" 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>
<H2><A NAME="SECTION005330000000000000000">Multi-Dimensional Subscripting in C++</A></H2>
<P>
When accessing the elements of a multi-dimensional array,
programmers usually prefer to use the C++ array subscripting operator
rather than call a member function explicitly.
This is because it is much more convenient to write <tt>a[i][j]</tt>
than to write <tt>a.Select(i,j)</tt>.
However, C++ does not directly support the overloading of
multi-dimensional array subscripting.
E.g., an experienced Fortran programmer would expect to be able to write
<tt>a[i,j]</tt> and to overload <code>operator[](int,int)</code>.
Alas, neither of these things is valid in C++.
<P>
The solution to this problem is to do the subscripting in two steps.
Consider the reference <tt>a[i][j]</tt>.
In C++ this is equivalent to
<PRE>a.operator[] (i).operator[] (j)</PRE>
In effect, the first subscripting operator selects the <IMG WIDTH=17 HEIGHT=13 ALIGN=BOTTOM ALT="tex2html_wrap_inline58387" SRC="img77.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img77.gif" > row,
and then the second subscripting operator
picks the <IMG WIDTH=19 HEIGHT=27 ALIGN=MIDDLE ALT="tex2html_wrap_inline60619" SRC="img514.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img514.gif" > element of that row.
<P>
Program <A HREF="page102.html#progarray2dh" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page102.html#progarray2dh"><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> gives a framework
for implementing the two-step process in C++.
Two object classes are declared--<tt>Array2D<T></tt>
and the nested class <tt>Row</tt>.
The latter class
is used to represent a reference to a particular row of a given
two-dimensional array.
<P>
In Program <A HREF="page102.html#progarray2dh" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page102.html#progarray2dh"><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> we see that <tt>operator[]</tt>
for <tt>Array2D<T></tt> objects returns an object of type <tt>Array2D<T>::Row</tt>.
The purpose of a <tt>Row</tt> object is to ``remember''
both the array and the row of that array that is being accessed--its data members are a reference to the accessed array,
and the row number.
<P>
There is also a subscripting operator
defined for <tt>Row</tt> class objects.
This one uses the remembered row number together with the given
column number to call the <tt>Select</tt> function on the appropriate array.
<P>
The definition of the <tt>Array2D<T></tt>
subscripting member function, <tt>operator[]</tt>,
is given in Program <A HREF="page102.html#progarray2dc" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page102.html#progarray2dc"><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>.
This function simply constructs an instance of the <tt>Array2D<T>::Row</tt> class.
Clearly, the running time of this function is <I>O</I>(1).
<P>
Given the <tt>Array2D<T></tt> class we are able to write
code such as the following:
<PRE>Array2D<int> a (4, 5);
for (int i = 0; i < 4; ++i)
for (int j = 0; j < 5; ++j)
a[i][j] = 0;</PRE>
which declares a <IMG WIDTH=34 HEIGHT=22 ALIGN=MIDDLE ALT="tex2html_wrap_inline61183" SRC="img660.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img660.gif" > array of integers,
and initializes all of the elements with the value zero.
<P>
<HR><A NAME="tex2html3180" HREF="page104.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page104.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="tex2html3178" HREF="page100.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page100.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="tex2html3172" HREF="page102.html" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page102.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="tex2html3182" 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="tex2html3183" 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
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -