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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_1004</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_1003.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_1004</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_1005.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 1004</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">sorted, however, swapping two of them can be time-consuming. The C++ code to swap two </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> is the same, regardless of the size of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3">an ordinary assignment statement will do. But the length of time to make the swap varies greatly depending on the size of the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3">. For example, it may take 10 times as long to swap </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> with 20 members as it does to swap </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> with 2 members.</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">If we are dealing with large </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3">, we can make the sorting operation more efficient and save memory by making the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> dynamic variables, and by sorting pointers to the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> rather than sorting the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> themselves. This way, only simple pointer variables are swapped on each iteration, rather than whole </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3">.</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">SelSort</font><font face="Times New Roman, Times, Serif" size="3"> function has to be modified somewhat to sort large </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> rather than simple variables. The </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3"> themselves are dynamically allocated, and a pointer to each is stored in an array. It is these pointers that are swapped when the algorithm calls for exchanging two values.</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">We have to declare an array </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ptrList</font><font face="Times New Roman, Times, Serif" size="3">, which holds pointers to the personnel records, to be the maximum size we might need. However, we create each </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PersonnelData</font><font face="Times New Roman, Times, Serif" size="3"> variable only when we need it. Therefore, room for 1000 pointers is set aside in memory for the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ptrList</font><font face="Times New Roman, Times, Serif" size="3"> array, but at run time there are only as many </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PersonnelData</font><font face="Times New Roman, Times, Serif" size="3"> variables in memory as there are records in the file (see Figure 17-14).</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">ptrList</font><font face="Times New Roman, Times, Serif" size="3"> array before and after sorting is shown in Figure 17-15. Note that when the algorithm wants to swap the contents of two </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">structs</font><font face="Times New Roman, Times, Serif" size="3">, we swap the pointers instead.</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">Now that we have chosen a concrete data representation for the personnel record list, we should review the ADT operations once more. We identified</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" width="100%"><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 align="center"><font face="Times New Roman, Times, Serif" size="3"><img src="a9576a7f21c631504aeb619064fbaac3.gif" border="0" alt="1004-01.gif" width="510" height="193" /></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" width="100%"><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 align="center"><font face="Times New Roman, Times, Serif" size="2">Figure聽17-14<br />Array聽of聽Pointers聽to聽</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">PersonnelData</font><font face="Times New Roman, Times, Serif" size="2">聽Structs</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_1003.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_1004</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_1005.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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