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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_267</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_266.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_267</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_268.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 267</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 initialize sum to 0 before the loop starts so that the first time the loop body executes, 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">sum聽=聽sum聽+聽number;</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">adds the current value of sum (0) to </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">number</font><font face="Times New Roman, Times, Serif" size="3"> to form the new value of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">sum</font><font face="Times New Roman, Times, Serif" size="3">. After the entire code fragment has executed, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">sum</font><font face="Times New Roman, Times, Serif" size="3"> contains the total of the 10 values read, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">count</font><font face="Times New Roman, Times, Serif" size="3"> contains 11, and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">number</font><font face="Times New Roman, Times, Serif" size="3"> contains the last value read.</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 </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">count</font><font face="Times New Roman, Times, Serif" size="3"> is being incremented in each iteration. For each new value of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">count</font><font face="Times New Roman, Times, Serif" size="3">, there is a new value for </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">number</font><font face="Times New Roman, Times, Serif" size="3">. Does this mean we could decrement </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">count</font><font face="Times New Roman, Times, Serif" size="3"> by 1 and inspect the previous value of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">number</font><font face="Times New Roman, Times, Serif" size="3">? No. Once a new value has been read into </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">number</font><font face="Times New Roman, Times, Serif" size="3">, the previous value is gone forever unless we've saved it in another variable. You'll see how to do that in the next section.</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">Let's look at another example. We want to count and sum the first 10 odd numbers in a data set. We need to test each number to see if it is even or odd. (We can use the modulus operator to find out. If </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">number聽%聽2</font><font face="Times New Roman, Times, Serif" size="3"> equals 1, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">number</font><font face="Times New Roman, Times, Serif" size="3"> is odd; otherwise, it's even.) If the input value is even, we do nothing. If it is odd, we increment the counter and add the value to our sum. We use a flag to control the loop because this is not a normal count-controlled loop. In the following code segment, all of the variables are of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">int</font><font face="Times New Roman, Times, Serif" size="3"> except the Boolean flag, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">lessThanTen</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="Courier New, Courier, Mono New, Courier, Mono" size="2">count聽=聽0;聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Initialize聽event聽counter<br />sum聽=聽0;聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Initialize聽sum<br />lessThanTen聽=聽TRUE;聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Initialize聽loop聽control聽flag<br />while聽(lessThanTen)<br />{<br />聽聽聽聽cin聽>>聽number;聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Get聽the聽next聽value<br />聽聽聽聽if聽(number聽%聽2聽==聽1)聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Is聽the聽value聽odd?<br />聽聽聽聽{<br />聽聽聽聽聽聽聽聽count++;聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Yes--Increment聽counter<br />聽聽聽聽聽聽聽聽sum聽=聽sum聽+聽number;聽聽聽聽聽聽聽聽聽聽//聽Add聽value聽to聽sum<br />聽聽聽聽聽聽聽聽lessThanTen聽=聽(count聽<聽10);聽//聽Update聽loop聽control聽flag<br />聽聽聽聽聽{<br />}</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 this example, there is no relationship between the value of the counter variable and the number of times the loop is executed. We could have written the While expression this way:</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">while聽(count聽<聽10)</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 this might mislead a reader into thinking that the loop is countcontrolled in the normal way. So, instead, we choose to control the loop with the flag </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">lessThanTen</font><font face="Times New Roman, Times, Serif" size="3">, to emphasize that </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">count</font><font face="Times New Roman, Times, Serif" size="3"> is incremented only</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_266.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_267</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_268.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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