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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_520</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_519.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_520</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_521.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 520</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">If you answered E, you are right. The first statement declares </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ch</font><font face="Times New Roman, Times, Serif" size="3"> and initializes it to the integer value 68 (assuming ASCII). The next statement increments </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ch</font><font face="Times New Roman, Times, Serif" size="3"> to 69, and then its external representation (the letter <i>E)</i> is printed. Extending this idea of incrementing a </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">char</font><font face="Times New Roman, Times, Serif" size="3"> variable, we could print the letters <i>A</i> through <i>G</i> as follows:</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">char聽ch;<br /><br />for聽(ch聽=聽A;聽ch聽<=聽G;聽ch++)<br />聽聽聽聽cout聽<聽ch;</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 code initializes </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ch</font><font face="Times New Roman, Times, Serif" size="3"> to A (65 in ASCII). Each time through the loop, the external representation of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ch</font><font face="Times New Roman, Times, Serif" size="3"> is printed. On the final loop iteration, the <i>G</i> is printed and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">ch</font><font face="Times New Roman, Times, Serif" size="3"> is incremented to H (72 in ASCII). The loop test is then false, so the loop terminates.</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>C++ </i></font><i><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">char</font><font face="Times New Roman, Times, Serif" size="3"> Constants</font></i></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 C++, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">char</font><font face="Times New Roman, Times, Serif" size="3"> constants come in two different forms. The first form, which we have been using regularly, is a single printable character enclosed by apostrophes (single quotes):</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">A聽8聽)聽+</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">Notice that we said <i>printable</i> character. Character sets include both printable characters and <i>control characters</i> (or <i>nonprintable characters).</i> Control characters are not meant to be printed but are used to control the screen, printer, and other hardware devices. If you look at the ASCII character table, you see that the printable characters are those with integer values 32126. The remaining characters (with values 031 and 127) are nonprintable control characters. In the EBCDIC character set, the control characters are those with values 063 and 250255 (and some that are intermingled with the printable characters). One control character you already know about is the newline character, which causes the screen cursor to advance to the next line.</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">To accommodate control characters, C++ provides a second form of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">char</font><font face="Times New Roman, Times, Serif" size="3">constant: the <i>escape sequence.</i> An escape sequence is one or more characters preceded by a backslash (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">\</font><font face="Times New Roman, Times, Serif" size="3">). You are familiar with the escape sequence </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">\n</font><font face="Times New Roman, Times, Serif" size="3">, which represents the newline character. Here is the complete description of the two forms of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">char</font><font face="Times New Roman, Times, Serif" size="3"> constant in C++:</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">1. A single printable characterexcept an apostrophe (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">'</font><font face="Times New Roman, Times, Serif" size="3">) or backslash (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">\</font><font face="Times New Roman, Times, Serif" size="3">) enclosed by apostrophes.</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_519.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_520</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_521.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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