📄 page_264.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>page_264</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_263.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_264</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_265.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 264</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">third time, printing the value 30 and executing the input statement. This time, the input statement fails; we're trying to read beyond the end of the file. The stream </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inData</font><font face="Times New Roman, Times, Serif" size="3"> enters the fail state. Looping back to the loop test, the value of the expression is zero (FALSE) and we exit the loop.</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 we write EOF-controlled loops like the one above, we are expecting that end-of-file is the reason for stream failure. But keep in mind that <i>any</i> input error causes stream failure. The above loop terminates, for example, if input fails because of invalid characters in the input data. This fact emphasizes again the importance of echo printing. It helps us verify that all the data were read correctly before EOF was encountered.</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">EOF-controlled loops are similar to sentinel-controlled loops in that the program doesn't know in advance how many data items are to be input. In the case of sentinel-controlled loops, the program reads until it encounters the sentinel value. With EOF-controlled loops, it reads until it reaches the end of the file.</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 it possible to use an EOF-controlled loop when we read from the standard input device (via the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">cin</font><font face="Times New Roman, Times, Serif" size="3"> stream) instead of a data file? On many systems, yes. With the UNIX operating system, you can type Ctrl/D (that is, you hold down the Ctrl key and tap the D key) to signify end-of-file during interactive input. With the MS-DOS operating system, the end-of-file keystrokes are Ctrl/Z. Other systems use similar keystrokes. Here's a program segment that tests for EOF on the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">cin</font><font face="Times New Roman, Times, Serif" size="3"> stream in UNIX:</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">cout聽<<聽Enter聽an聽integer聽(or聽Ctrl/D聽to聽quit):聽;<br />cin聽>>聽someInt;<br />while聽(cin)<br />{<br />聽聽聽聽cout聽<<聽someInt聽<<聽聽doubled聽is聽聽<<聽2聽*聽someInt聽<<聽endl;<br />聽聽聽聽cout聽<<聽Next聽number聽(or聽Ctrl/D聽to聽quit):聽;<br />聽聽聽聽cin聽>>聽someInt;<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">Flag-Controlled Loops</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">A <i>flag</i> is a Boolean variable that is used to control the logical flow of a program. We can set a Boolean variable to TRUE before a While loop; then, when we want to stop executing the loop, we reset it to FALSE. That is, we can use the Boolean variable to record whether or not the event that controls the process has occurred. For example, the following program segment reads and sums values until the input value is negative. (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">nonNegative</font><font face="Times New Roman, Times, Serif" size="3"> is the Boolean flag; all of the other variables are of type int.)</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0;<br />nonNegative聽=聽TRUE;聽聽聽聽聽聽聽聽聽聽聽聽聽聽//聽Initialize聽flag<br />while聽(nonNegative)<br />{<br />聽聽聽聽cin聽>>聽number;</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_263.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_264</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_265.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 + -