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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_212</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_211.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_212</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_213.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 212</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">We code this information with an If-Then-Else nested within an If-Then:</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">if聽(average聽<聽70.0)<br />聽聽聽聽if聽(average聽<聽60.0)<br />聽聽聽聽聽聽聽聽cout聽<<聽Failing;<br />聽聽聽聽else<br />聽聽聽聽聽聽聽聽cout聽<<聽Passing聽but聽marginal;</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 do we know to which </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">if</font><font face="Times New Roman, Times, Serif" size="3"> the else belongs? Here is the rule that the C++ compiler follows: In the absence of braces, an </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">else</font><font face="Times New Roman, Times, Serif" size="3"> is always paired with the closest preceding </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">if</font><font face="Times New Roman, Times, Serif" size="3"> that doesn't already have an </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">else</font><font face="Times New Roman, Times, Serif" size="3"> paired with it. We indented the code to reflect this pairing.</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">Suppose we write the fragment like this:</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">if聽(average聽>=聽60.0)聽聽聽聽聽聽//聽Incorrect聽version<br />聽聽聽聽if聽(average聽<聽70.0)<br />聽聽聽聽聽聽聽聽cout聽<<聽Passing聽but聽marginal;<br />else<br />聽聽聽聽cout聽<<聽Failing;</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">Here we want the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">else</font><font face="Times New Roman, Times, Serif" size="3"> branch attached to the outer If statement, not the inner, so we indent the code as you see it. But indentation does not affect the execution of the code. Even though the else aligns with the first </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">if</font><font face="Times New Roman, Times, Serif" size="3">, the compiler pairs it with the second </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">if</font><font face="Times New Roman, Times, Serif" size="3">. An </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">else</font><font face="Times New Roman, Times, Serif" size="3"> that follows a nested If-Then is called a <i>dangling </i></font><i><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">else</font><font face="Times New Roman, Times, Serif" size="3">.</font></i><font face="Times New Roman, Times, Serif" size="3"> It doesn't logically belong with the nested If but is attached to it by the compiler.</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 attach the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">else</font><font face="Times New Roman, Times, Serif" size="3"> to the first </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">if</font><font face="Times New Roman, Times, Serif" size="3">, not the second, you can turn the outer then-clause into a block:</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">if聽(average聽>=聽60.0)聽聽聽聽聽聽//聽Correct聽version<br />{<br />聽聽聽聽if聽(average聽<聽70.0)<br />聽聽聽聽聽聽聽聽cout聽<<聽Passing聽but聽marginal;<br />}<br />else<br />聽聽聽聽cout聽<<聽Failing;</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">{聽}</font><font face="Times New Roman, Times, Serif" size="3"> pair indicates that the inner If statement is complete, so the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">else</font><font face="Times New Roman, Times, Serif" size="3"> must belong to the outer </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">if</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></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_211.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_212</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_213.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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