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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>page_1062</title> <link rel="stylesheet" href="reset.css" type="text/css" media="all"> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> </head> <body> <table summary="top nav" border="0" width="100%"> <tr> <td align="left" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_1061.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_1062</strong></td> <td align="right" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_1063.html">next page ></a></td> </tr> <tr> <td align="left" colspan="3" style="background: #ffffff; padding: 20px;"> <table border="0" width="100%" cellpadding="0"><tr><td align="center"> <table border="0" cellpadding="2" cellspacing="0" width="100%"><tr><td align="left"></td> <td align="right"></td> </tr></table></td></tr><tr><td align="left"><p></p><table border="0" cellspacing="0" cellpadding="0" width="100%"><tr><td align="right"><font face="Times New Roman, Times, Serif" size="2" color="#FF0000">Page 1062</font></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">Creating an Empty Linked List</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">To create a linked list with no nodes, all that is necessary is to assign the external pointer the value </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">NULL</font><font face="Times New Roman, Times, Serif" size="3">. For the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">OrdList</font><font face="Times New Roman, Times, Serif" size="3"> class, the appropriate place to do this is in the class constructor:</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">OrdList::OrdList()<br /><br />//聽Constructor<br /><br />//聽Postcondition:<br />//聽聽聽聽聽head聽==聽NULL<br /><br />{<br />聽聽聽聽head聽=聽NULL;<br />}</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">One thing you will notice as we go through the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">OrdList</font><font face="Times New Roman, Times, Serif" size="3"> member functions is that the <i>implementation assertions</i> (the preconditions and postconditions appearing in the implementation file) are often stated differently from the <i>abstract assertions</i> (those located in the specification file). Abstract assertions are written in terms that are meaningful to the user of the ADT; implementation details should not be mentioned. In contrast, implementation assertions can be made more precise by referring directly to variables and algorithms in the implementation code. In the case of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">OrdList</font><font face="Times New Roman, Times, Serif" size="3"> class constructor, the abstract postcondition is simply that an empty list (not a linked list) has been created. On the other hand, the implementation postcondition</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">//聽Postcondition:<br />//聽聽聽聽聽head聽==聽NULL</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">is phrased in terms of our private data (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">head</font><font face="Times New Roman, Times, Serif" size="3">) and our particular list implementation (a dynamic linked list).</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">Testing for an Empty Linked List</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="3">The </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">OrdList</font><font face="Times New Roman, Times, Serif" size="3"> member function </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">IsEmpty</font><font face="Times New Roman, Times, Serif" size="3"> returns TRUE if the list is empty and FALSE if the list is not empty. Using a dynamic linked list representation, we return TRUE if </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">head</font><font face="Times New Roman, Times, Serif" size="3"> contains the value </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">NULL</font><font face="Times New Roman, Times, Serif" size="3">, and FALSE otherwise:</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table border="0" cellspacing="0" cellpadding="0"><tr><td rowspan="5"></td> <td colspan="3" height="12"></td> <td rowspan="5"></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">Boolean聽OrdList::IsEmpty()聽const<br /><br />//聽Postcondition:<br />//聽聽聽聽聽Function聽value聽==聽TRUE,聽if聽head聽==聽NULL<br />//聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽==聽FALSE,聽otherwise<br /><br />{<br />聽聽聽聽return聽(head聽==聽NULL);<br />}</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td></tr></table><p><font size="0"></font></p>聽 </td> </tr> <tr> <td align="left" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_1061.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_1062</strong></td> <td align="right" width="30%" style="background: #EEF3E2"><a style="color: blue; font-size: 120%; font-weight: bold; text-decoration: none; font-family: verdana;" href="page_1063.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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