📄 page_211.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>page_211</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_210.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_211</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_212.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 211</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">else聽if聽(month聽==聽2)聽聽聽聽聽聽聽聽聽聽//聽Nested聽If<br />聽聽聽聽cout聽<<聽February;<br />else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cout聽<<聽March;<br />else聽if聽(month聽==聽4)聽聽聽聽聽聽聽聽聽聽//聽Nested聽If<br />聽聽.<br />聽聽.<br />聽聽.<br />else<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">This style prevents the indentation from marching continuously to the right. But, more importantly, it visually conveys the idea that we are using a 12way branch based on the variable </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">month</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="Times New Roman, Times, Serif" size="3">It's important to note one difference between the sequence of If statements and the nested If: More than one alternative can be taken by the sequence of Ifs, but the nested If can select only one. To see why this is important, consider the analogy of filling out a questionnaire. Some questions are like a sequence of If statements, asking you to circle all the items in a list that apply to you (such as all your hobbies). Other questions ask you to circle only one item in a list (your age group, for example) and are thus like a nested If structure. Both kinds of questions occur in programming problems. Being able to recognize which type of question is being asked permits you to immediately select the appropriate 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="Times New Roman, Times, Serif" size="3">Another particularly helpful use of the nested If is when you want to compare a series of consecutive ranges of values. For example, the first Problem-Solving Case Study in this chapter involves printing different messages for different ranges of temperatures. We present two solutions for one of the modules, one using a sequence of If statements, the other using a nested If structure. As you'll see, the nested If version uses fewer comparisons, so it's more efficient.</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">As fast as modern computers are, many applications require so much computation that inefficient algorithms can waste hours of computer time. Always be on the lookout for ways to make your programs more efficient, as long as doing so doesn't make them difficult for other programmers to understand. It's usually better to sacrifice a little efficiency for the sake of readability.</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>The Dangling Else</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">When If statements are nested, you may find yourself confused about the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">if-else</font><font face="Times New Roman, Times, Serif" size="3"> pairings: 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"> does an </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">else</font><font face="Times New Roman, Times, Serif" size="3"> belong? For example, suppose that if a student's average is below 60, we want to print Failing; if it is at least 60 but less than 70, we want to print Passing but marginal; and if it is 70 or greater, we don't want to print anything.</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_210.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_211</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_212.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -