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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_259</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_258.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_259</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_260.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 259</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="Courier New, Courier, Mono New, Courier, Mono" size="2">loopCount聽=聽loopCount聽+聽1;</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">When designing loops, it is the programmer's responsibility to see that the condition to be tested is set correctly (initialized) before the While statement begins. The programmer also must make sure the condition changes within the loop so that it becomes FALSE at some point; otherwise, the loop is never exited.</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="3">loopCount聽=聽1;</font><font face="Times New Roman, Times, Serif" size="3">聽聽聽聽聽聽聽聽聽聽聽聽聽</font><font face="Symbol" size="3">录</font><font face="Times New Roman, Times, Serif" size="3">聽Variable聽loopCount聽must聽be聽initialized<br /></font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">while聽(loopCount聽<=聽10)<br />{<br />聽聽聽聽.<br />聽聽聽聽.<br />聽聽聽聽.<br />聽聽聽聽loopCount++;</font><font face="Times New Roman, Times, Serif" size="3">聽聽聽聽聽聽聽聽聽聽聽</font><font face="Symbol" size="3">录</font><font face="Times New Roman, Times, Serif" size="3">聽loopCount聽must聽be聽incremented<br /></font><font face="Courier New, Courier, Mono New, Courier, Mono" 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">A loop that does not exit is called an <i>infinite loop</i> because, in theory, the loop executes forever. In the code above, omitting the incrementation of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">loopCount</font><font face="Times New Roman, Times, Serif" size="3"> at the bottom of the loop leads to an infinite loop; the While expression is always TRUE because the value of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">loopCount</font><font face="Times New Roman, Times, Serif" size="3"> is forever 1. If your program goes on running for much longer than you expect it to, chances are that you've created an infinite loop. You may have to issue an operating system command to stop the program.</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">How many times does the loop in our example execute9 or 10? To determine this, we have to look at the initial value of the loop control variable and then at the test to see what its final value is. Here we've initialized </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">loopCount</font><font face="Times New Roman, Times, Serif" size="3"> to 1, and the test indicates that the loop body is executed for each value of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">loopCount</font><font face="Times New Roman, Times, Serif" size="3"> up through 10. If </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">loopCount</font><font face="Times New Roman, Times, Serif" size="3"> starts out at 1 and runs up to 10, the loop body is executed 10 times. If we want the loop to execute 11 times, we have to either initialize </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">loopCount</font><font face="Times New Roman, Times, Serif" size="3"> to 0 or change the test to</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">loopCount聽<=聽11</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"><i>Event-Controlled Loops</i></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">There are several kinds of event-controlled loops: sentinel-controlled, endof-file-controlled, and flag-controlled. In all of these loops, the termination condition depends on some event occurring while the loop body is executing.</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">Sentinel-Controlled Loops</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">Loops often are used to read in and process long lists of data. Each time the loop body is executed, a new piece of data is read and processed. Often a special data value, called a <i>sentinel or trailer</i></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_258.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_259</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_260.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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