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<TITLE>Head, Enqueue and Dequeue Member Functions</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="SECTION007223000000000000000"><tt>Head</tt>, <tt>Enqueue</tt> and <tt>Dequeue</tt> Member Functions</A></H3>
<P>
The <tt>Head</tt>, <tt>Enqueue</tt> and <tt>Dequeue</tt>
member functions of the <tt>QueueAsLinkedList</tt> class are
given in Program&nbsp;<A HREF="page155.html#progqueue4c" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page155.html#progqueue4c"><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>.
<P>
<P><A NAME="7448">&#160;</A><A NAME="progqueue4c">&#160;</A> <IMG WIDTH=575 HEIGHT=429 ALIGN=BOTTOM ALT="program7353" SRC="img747.gif" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/img747.gif"  ><BR>
<STRONG>Program:</STRONG> <tt>QueueAsLinkedList</tt> Class <tt>Enqueue</tt>, <tt>Head</tt> 	and <tt>Dequeue</tt> Member Function Definitions<BR>
<P>
<P>
The <tt>Head</tt> member function 
returns a <tt>const</tt> reference to the object at the head of the queue.
The head of the queue is in the first element of the linked list.
In Chapter&nbsp;<A HREF="page79.html#chapfds" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page79.html#chapfds"><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 saw that the running time of
<tt>LinkedList&lt;T&gt;::First</tt> is a constant,
Therefore, the normal running time for the <tt>Head</tt> function is <I>O</I>(1).
<P>
The <tt>Enqueue</tt> function takes a single argument--a reference to the object to be added to the tail of the queue.
The function simply calls the <tt>LinkedList&lt;T&gt;::Append</tt> function.
Since the running time for <tt>Append</tt> is <I>O</I>(1),
the running time of <tt>Enqueue</tt> is also <I>O</I>(1).
<P>
The <tt>Dequeue</tt> function removes an object from the head
of the queue and returns a reference to that object.
First, it verifies that the queue is not empty
and throws an exception when it is.
If the queue is not empty,
<tt>Dequeue</tt> saves a reference to the first item in the linked list
in the local variable <tt>result</tt>.
Then the item of the linked list is removed from the list.
When using the <tt>LinkedList&lt;T&gt;</tt> class from Chapter&nbsp;<A HREF="page79.html#chapfds" tppabs="http://dictator.uwaterloo.ca/Bruno.Preiss/books/opus4/html/page79.html#chapfds"><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 time required to delete the first item from a list
is <I>O</I>(1) regardless of the number of items in the list.
As a result,
the running time of <tt>Dequeue</tt> is also <I>O</I>(1).
<P>
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