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<TITLE>Implementation</TITLE>
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<b>Data Structures and Algorithms
with Object-Oriented Design Patterns in C++</b><br>
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<H3><A NAME="SECTION0017311000000000000000">Implementation</A></H3>
<P>
Program <A HREF="page552.html#proggraph1c" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page552.html#proggraph1c"><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 the code for the two <tt>DepthFirstTraversal</tt>
routines of the <tt>Graph</tt> class.
One of them accepts two arguments, the other, three.
As indicated in Program <A HREF="page539.html#proggraph3h" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page539.html#proggraph3h"><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 two-argument routine is declared <tt>public</tt>
whereas the three-argument one is <tt>protected</tt>.
<P>
The user of the <tt>Graph</tt> class only sees the two-argument
<tt>DepthFirstTraversal</tt> routine.
This routine takes a reference to a <tt>Visitor</tt> instance
and a reference to a <tt>Vertex</tt> instance.
The idea is that the <tt>Visit</tt> function of the visitor
is called once for each vertex in the graph
and the vertices are visited in depth-first traversal order
starting from the specified vertex.
<P>
<P><A NAME="50367"> </A><A NAME="proggraph1c"> </A> <IMG WIDTH=575 HEIGHT=544 ALIGN=BOTTOM ALT="program50318" SRC="img2396.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img2396.gif" ><BR>
<STRONG>Program:</STRONG> <tt>Graph</tt> Class <tt>DepthFirstTraversal</tt> Member Function Definition<BR>
<P>
<P>
In order to ensure that each vertex is visited at most once,
an array of length <IMG WIDTH=17 HEIGHT=24 ALIGN=MIDDLE ALT="tex2html_wrap_inline71781" SRC="img2365.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img2365.gif" > of Boolean values
called <tt>visited</tt> is used (line 4).
I.e., <IMG WIDTH=128 HEIGHT=24 ALIGN=MIDDLE ALT="tex2html_wrap_inline71975" SRC="img2397.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img2397.gif" > only if vertex <I>i</I> has been visited.
All the array elements are initially <tt>false</tt> (lines 5-6).
After initializing the array,
the two-argument routine calls the three-argument one,
passing it a reference to the array as the third argument.
<P>
The three-argument routine returns immediately if the visitor is done.
Otherwise, it visits the specified node,
and then it follows all the edges emanating from that node
and recursively visits the adjacent vertices
<em>if those vertices have not already been visited</em>.
<P>
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