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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_1123</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_1122.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_1123</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_1124.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 1123</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">and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">n</font><font face="Times New Roman, Times, Serif" size="3"> - 1 as parameters. Execution of Call 1 pauses until an answer is sent back from this recursive call.</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>Call 2:</i> </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">x</font><font face="Times New Roman, Times, Serif" size="3"> is equal to 2 and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">n</font><font face="Times New Roman, Times, Serif" size="3"> is equal to 2. Because </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">n</font><font face="Times New Roman, Times, Serif" size="3"> is not equal to 1, the function </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Power</font><font face="Times New Roman, Times, Serif" size="3"> is called again, this time with </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">x</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">n</font><font face="Times New Roman, Times, Serif" size="3"> - 1 as parameters. Execution of Call 2 pauses until an answer is sent back from this recursive call.</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>Call 3:</i> </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">x</font><font face="Times New Roman, Times, Serif" size="3"> is equal to 2 and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">n</font><font face="Times New Roman, Times, Serif" size="3"> is equal to 1. Because </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">n</font><font face="Times New Roman, Times, Serif" size="3"> equals 1, the value of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">x</font><font face="Times New Roman, Times, Serif" size="3"> is to be returned. This call to the function has finished executing, and the function return value (which is 2) is passed back to the place in the statement from which the call was made.</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>Call 2:</i> This call to the function can now complete the statement that contained the recursive call because the recursive call has returned. Call 3's return value (which is 2) is multiplied by </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">x</font><font face="Times New Roman, Times, Serif" size="3">. This call to the function has finished executing, and the function return value (which is 4) is passed back to the place in the statement from which the call was made.</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>Call 1:</i> This call to the function can now complete the statement that contained the recursive call because the recursive call has returned. Call 2's return value (which is 4) is multiplied by </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">x</font><font face="Times New Roman, Times, Serif" size="3">. This call to the function has finished executing, and the function return value (which is 8) is passed back to the place in the statement from which the call was made. Because the first call (the nonrecursive call in </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">main</font><font face="Times New Roman, Times, Serif" size="3">) has now completed, this is the final value of the function </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Power</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">This trace is summarized in Figure 19-2. Each box represents a call to the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Power</font><font face="Times New Roman, Times, Serif" size="3"> function. The values for the parameters for that call are shown in each box.</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">What happens if there is no base case? We have infinite recursion, the recursive equivalent of an infinite loop. For example, if the statement</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(n聽==聽l)</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">were omitted, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Power</font><font face="Times New Roman, Times, Serif" size="3"> would be called over and over again. Infinite recursion also occurs if </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Power</font><font face="Times New Roman, Times, Serif" size="3"> is called with </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">n</font><font face="Times New Roman, Times, Serif" size="3"> less than or equal to zero.</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" width="100%"><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 align="center"><font face="Times New Roman, Times, Serif" size="3"><img src="0a6d75a91ea41d1b7d9809454d8bed4a.gif" border="0" alt="1123-01.gif" width="308" height="219" /></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" width="100%"><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 align="center"><font face="Times New Roman, Times, Serif" size="2">Figure聽19-2<br />Execution聽of聽</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Power(2,聽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_1122.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_1123</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_1124.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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