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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_210</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_209.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_210</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_211.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 210</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">In general, any problem that involves a <i>multi-way branch</i> (more than two alternative courses of action) can be coded using nested If statements. For example, to print out the name of a month given its number, we could use a sequence of If statements (unnested):</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(month聽==聽1)<br />聽聽聽聽cout聽<<聽January;<br />if聽(month聽==聽2)<br />聽聽聽聽cout聽<<聽February;<br />if聽(month聽==聽3)<br />聽聽聽聽cout聽<<聽March;<br />聽聽.<br />聽聽.<br />聽聽.<br />if聽(month聽==聽12)<br />聽聽聽聽cout聽<<聽December;</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">But the equivalent nested If structure,</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(month聽==聽1)<br />聽聽聽聽cout聽<<聽January;<br />else<br />聽聽聽聽if聽(month聽==2)聽聽聽聽聽聽聽聽聽聽//聽Nested聽If<br />聽聽聽聽聽聽聽聽cout聽<<聽February;<br />聽聽聽聽else<br />聽聽聽聽聽聽聽聽if聽(month聽==聽3)聽聽聽聽聽聽聽聽聽聽//聽Nested聽If<br />聽聽聽聽聽聽聽聽聽聽聽聽cout聽<<聽March;<br />聽聽聽聽聽聽聽聽else<br />聽聽聽聽聽聽聽聽聽聽聽聽if聽(month聽==聽4)聽聽聽聽聽聽聽聽聽聽//聽Nested聽If<br />聽聽聽聽聽聽聽聽聽聽聽聽聽聽.<br />聽聽聽聽聽聽聽聽聽聽聽聽聽聽.<br />聽聽聽聽聽聽聽聽聽聽聽聽聽聽.</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">is more efficient because it makes fewer comparisons. The first versionthe sequence of independent If statementsalways tests every condition (all 12 of them), even if the first one is satisfied. In contrast, the nested If solution skips all remaining comparisons after one alternative has been selected.</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">In the last example, notice how the indentation of the thenand elseclauses causes the statements to move continually to the right. We use a special indentation style with deeply nested If-Then-Else statements to indicate that the complex structure is just choosing one of a set of alternatives. This general multi-way branch is known as an <i>If-Then-Else-If</i>control structure:</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(month聽==聽1)<br />聽聽聽聽cout聽<<聽January;<br /></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_209.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_210</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_211.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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