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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_261</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_260.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_261</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_262.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 261</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">//聽This聽version聽is聽correct:<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.<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the聽next聽date<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">This segment works fine. The first data set is read in; if it is not the sentinel, it gets processed. At the end of the loop, the next data set is read in, and we go back to the beginning of the loop. If the new data set is not the sentinel, it gets processed just like the first. When the sentinel value is read, the While expression becomes FALSE, and the loop exits (<i>without</i> processing the sentinel).</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">Many times the problem dictates the value of the sentinel. For example, if the problem does not allow data values of 0, then the sentinel value should be 0. Sometimes a combination of values is invalid. The combination of February and 31 as a date is such a case. Sometimes a range of values (negative numbers, for example) is the sentinel. And when you process char data, one line of input at a time, the newline character (\n] often serves as the sentinel. Here's a code segment that reads and prints all of the characters on an input line (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inChar</font><font face="Times New Roman, Times, Serif" size="3"> is of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">char</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="Courier New, Courier, Mono New, Courier, Mono" size="2">cin.get(inChar);聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Get聽first聽character<br />while聽(inChar聽!=聽\n)<br />{<br />聽聽聽聽聽cout聽<<聽inChar;聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Echo聽it<br />聽聽聽聽聽cin.get(inChar);聽聽聽聽聽聽聽聽聽聽聽聽//聽Get聽next聽character<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">(Notice that for this particular task we use the get function, not the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">>></font><font face="Times New Roman, Times, Serif" size="3"> operator, to input a character. Remember that the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">>></font><font face="Times New Roman, Times, Serif" size="3"> operator skips whitespace charactersincluding blanks and newlinesto find the next data value in the input stream. In this example, we want to input <i>every</i> character, even a blank and especially the newline character.)</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 you are choosing a value to use as a sentinel, what happens if there aren't any invalid data values? Then you may have to input an extra value in each iteration, a value whose only purpose is to signal the end of the data. For example, look at this code segment:</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">cin聽>>聽dataValue聽>>聽sentinel;聽聽聽聽聽聽聽//聽Get聽first聽data聽value<br />while聽(sentinel聽==聽1)</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_260.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_261</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_262.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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