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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_260</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_259.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_260</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_261.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 260</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"><i>value,</i> is used to signal the program that there is no more data to be processed. Looping continues as long as the data value read is <i>not</i> the sentinel; the loop stops when the program recognizes the sentinel. In other words, reading the sentinel value is the event that controls the looping process.</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 sentinel value must be something that never shows up in the normal input to a program. For example, if a program reads calendar dates, we could use February 31 as a sentinel value:</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This聽code聽is聽incorrect:<br /><br />while聽(聽!(month聽==聽2聽&&聽day聽==聽31)聽)<br />{<br />聽聽聽聽cin聽>>聽month聽>>聽day;聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Get聽a聽date<br />聽聽聽聽.<br />聽聽聽聽.聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Process聽it<br />聽聽聽聽.<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">There is a problem in the loop in the example above. The values </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">of聽month</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">day</font><font face="Times New Roman, Times, Serif" size="3"> are not defined before the first pass through the loop. Somehow we have to initialize these variables. We could assign them arbitrary values, but then we would run the risk of those values being processed as data. Also, it's inefficient to initialize variables with values that are never used.</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 can solve the problem by reading the first set of data values <i>before</i> entering the loop. This is called a <i>priming read.</i> (The idea is similar to priming a pump by pouring a bucket of water into the mechanism before starting it.) Let's add the priming read to the loop:</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This聽is聽still聽incorrect:<br /><br />cin聽>>聽month聽>>聽day;聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Get聽a聽date--priming聽read<br />while聽(聽!(month聽==聽2聽&&聽day聽==聽31)聽)<br />{<br />聽聽聽聽cin聽>>聽month聽>>聽day;聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Get聽a聽date<br />聽聽聽聽.<br />聽聽聽聽.<br />聽聽聽聽.聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Process聽it<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">There is still a problem here. Notice that the first thing the program does inside the loop is to get a date, destroying the values obtained by the priming read. Thus, the first date in the data list is never processed. Given the priming read, the <i>first</i> thing that the loop body should do is process the data that have already been read. But then at what point do we read the next data set? We do this <i>last.</i> In this way, the While condition is applied to the next data set before it gets processed. Here's how it looks:</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_259.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_260</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_261.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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