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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="SECTION007113000000000000000"><tt>Push</tt>, <tt>Pop</tt>, and <tt>Top</tt> Member Functions</A></H3>
<P>
Program&nbsp;<A HREF="page135.html#progstack2c" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page135.html#progstack2c"><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 the
<tt>Push</tt>, <tt>Pop</tt>, and <tt>Top</tt>,
member functions of the <tt>StackAsArray</tt> class.
The first of these, <tt>Push</tt>,
adds an element to the stack.
It takes as its lone argument a reference to the <tt>Object</tt>
to be pushed onto the stack.
<P>
<P><A NAME="6282">&#160;</A><A NAME="progstack2c">&#160;</A> <IMG WIDTH=575 HEIGHT=391 ALIGN=BOTTOM ALT="program5873" SRC="img710.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img710.gif"  ><BR>
<STRONG>Program:</STRONG> <tt>StackAsArray</tt> Class <tt>Push</tt>, <tt>Pop</tt>,     and <tt>Top</tt> Member Function Definitions<BR>
<P>
<P>
The <tt>Push</tt> function first checks to see
if there is room left in the stack.
If no room is left, it throws a <tt>domainerror</tt> exception.
Otherwise, it simply puts a pointer to the object into the array,
and then increases the <tt>count</tt> variable by one.
In a correctly functioning program,
stack overflow should not occur.
If we assume that overflow does not occur,
the running time of the <tt>Push</tt> function is clearly <I>O</I>(1).
<P>
The <tt>Pop</tt> function removes an item from the stack
and returns a reference to that item.
If the stack is the owner of its contained objects,
then when an item is removed from the stack,
that item ceases to be owned by the stack.
Consequently, the <tt>Pop</tt> function never invokes <tt>operator delete</tt>.
The <tt>Pop</tt> function first checks if the stack is empty.
If the stack is empty, it throws a <tt>domainerror</tt> exception.
Otherwise, it simply decreases <tt>count</tt> by one
and returns a reference to the item found at the top of the stack.
In a correctly functioning program,
stack underflow will not occur normally.
The running time of the <tt>Pop</tt> function is <I>O</I>(1).
<P>
Finally, the <tt>Top</tt> function is a stack accessor
which returns a <tt>const</tt> reference to the top item in the stack.
The <tt>Top</tt> function is a <tt>const</tt> member function
since it does not modify the stack.
In particular, it does <em>not</em> remove the top item from the stack.
The <tt>Top</tt> function first checks if the stack is empty.
If the stack is empty, it throws a <tt>domainerror</tt> exception.
Otherwise, it returns a reference to the top item,
which is found at position  <IMG WIDTH=68 HEIGHT=20 ALIGN=MIDDLE ALT="tex2html_wrap_inline61316" SRC="img711.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img711.gif"  > in the array.
Assuming stack underflow does not occur normally,
the running time of the <tt>Top</tt> function is <I>O</I>(1).
<P>
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