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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_479</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_478.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_479</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_480.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 479</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">聽聽聽聽cin聽>>聽age;<br /><br />聽聽聽聽聽聽聽聽//聽Invariant:<br />聽聽聽聽聽聽聽聽//聽聽聽聽聽age聽has聽been聽input<br />聽聽聽聽聽聽聽聽//聽聽&&聽For聽all聽values聽of聽age聽prior聽to<br />聽聽聽聽聽聽聽聽//聽聽聽聽聽current聽value,聽age聽<=聽0<br /><br />}聽while聽(age聽<=聽0);</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">After the loop terminates, the conjunction of the termination condition and the invariant yields the postcondition</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">(</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">age聽>聽0</font><font face="Times New Roman, Times, Serif" size="3">) AND (For all values of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">age</font><font face="Times New Roman, Times, Serif" size="3"> prior to the current value, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">age聽<u><</u>聽0</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">Writing loop invariants as program comments is not always easy to do. Sometimes an invariant consists of 15 or 20 separate assertions. Although these assertions may all be important when you design and verify the loop, it's not practical to include them all as comments. For documentation purposes, what is important is to summarize concisely the task accomplished by the loop, mentioning values of key variables such as counters, summing variables, and input or output variables. Using mathematical notation wherever possible helps keep the comments brief and precise. And remember to express an invariant as an assertion, not as a general informational comment. Assertions don't look like This loop will do such-and-such or This loop does so-and-so. An assertion is a truth-valued statementone that is either true or false (true, one hopes).</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">One final remark about loop invariants: If you write an infinite loop with </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">break</font><font face="Times New Roman, Times, Serif" size="3"> statements to allow multiple exits from the loop, you need a different loop invariant for each of the exit points. The loop invariants are all bound to be different because the state of the program changes between any two exit points. The prospect of creating four or five different invariants for a given loop may keep you from becoming too casual about using </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">break</font><font face="Times New Roman, Times, Serif" size="3"> statements!</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="17"></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">Problem-Solving Case Study The Rich Uncle</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"><img src="0d8942efec72cc84520824ecd0b1b12a.gif" border="0" alt="0479-01.gif" width="119" height="156" /></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">Problem: Your rich uncle has just died, and in his desk you find two wills. One of them, dated several months ago, leaves you and your relatives a substantial part of his fortune; the other, dated last week, gives it all to his next-door neighbor. Being suspicious that the second will is a forgery, you decide to write a program to analyze writing style in order to compare the wills. The program reads and prints alphanumeric characters from a file</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_478.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_479</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_480.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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