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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_147</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_146.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_147</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_148.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 147</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">Declaring File Streams</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 a program, you declare stream variables the same way that you declare any variableyou specify the data type and then the variable name:</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">int聽聽聽聽聽聽someInt;<br />float聽聽聽聽someFloat;<br />ifstream聽inFile;<br />ofstream聽outFile;</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">(You don't have to declare the stream variables </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">cin</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">cout</font><font face="Times New Roman, Times, Serif" size="3">. The header file </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">iostream.h</font><font face="Times New Roman, Times, Serif" size="3"> already does this for you.)</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">For our Mileage program, let's name the input and output file streams </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inMPG</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">outMPG</font><font face="Times New Roman, Times, Serif" size="3">. We declare them like this:</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">ifstream聽inMPG;聽聽聽聽聽聽聽聽//聽Holds聽gallon聽amounts聽and聽mileages<br />ofstream聽outMPG;聽聽聽聽聽聽聽//聽Holds聽miles聽per聽gallon聽output</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">Note that the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ifstream</font><font face="Times New Roman, Times, Serif" size="3"> type is for input files only, and the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ofstream</font><font face="Times New Roman, Times, Serif" size="3"> type is for output files only. With these data types, you cannot read from and write to the same file. If you wanted to do so, you would use a third data type named </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">fstream</font><font face="Times New Roman, Times, Serif" size="3">, the details of which we don't explore in this book.</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">Opening Files</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">The third thing we have to do is prepare each file for reading or writing, an act called <i>opening a file.</i> Opening a file causes the computer's operating system to perform certain actions that allow us to proceed with file I/O.</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 our example, we want to read from the file stream </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inMPG</font><font face="Times New Roman, Times, Serif" size="3"> and write to the file stream </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">outMPG</font><font face="Times New Roman, Times, Serif" size="3">. We open the relevant files by using these 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="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">inMPG.open(inmpg.dat);<br />outMPG.open聽(outmpg.dat);</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">These statements are both function calls (notice the telltale parametersthe mark of a function). In each function call, the parameter is a string enclosed by quotes. The first statement is a call to a function named open, which is associated with the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ifstream</font><font face="Times New Roman, Times, Serif" size="3"> data type. The second is a call to another function (also named open) associated with the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ofstream</font><font face="Times New Roman, Times, Serif" size="3"> data type. As we discussed earlier, we use dot notation (as in </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inMPG.open</font><font face="Times New Roman, Times, Serif" size="3">) to call certain library functions that are tightly associated with data types.</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">Exactly what does an open function do? First, it associates a stream variable used in your program with a physical file on disk. Our first function call creates a connection between the stream variable </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inMPG</font><font face="Times New Roman, Times, Serif" size="3"> and the actual disk file, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inmpg.dat.</font><font face="Times New Roman, Times, Serif" size="3"> (Names of file streams must be identifiers; they are vari-</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_146.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_147</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_148.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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