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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_193</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_192.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_193</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_194.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 193</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"><i>Caution:</i> It's easy to confuse the assignment operator (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">=</font><font face="Times New Roman, Times, Serif" size="3">) and the </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">==</font><font face="Times New Roman, Times, Serif" size="3"> relational operator. These two operators have very different effects in a program. Some people pronounce the relational operator as equals-equals to remind themselves of the difference.</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">Logical Operators</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 mathematics, the <i>logical operators</i> AND, OR, and NOT take logical expressions as operands. C++ uses special symbols for the logical operators: </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">&&</font><font face="Times New Roman, Times, Serif" size="3"> (for AND), </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">||</font><font face="Times New Roman, Times, Serif" size="3"> (for OR), and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">!</font><font face="Times New Roman, Times, Serif" size="3"> (for NOT). By combining relational operators with logical operators, we can make more complex assertions. For example, suppose we want to determine whether a final score is greater than 90 <i>and</i> a midterm score is greater than 70. In C++, we would write the 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">finalScore聽>聽90聽&&聽midtermScore聽>聽70</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 AND operation (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">&&</font><font face="Times New Roman, Times, Serif" size="3">) requires both relationships to be TRUE in order for the overall result to be TRUE. If either or both of the relationships are FALSE, the entire result is FALSE.</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 OR operation (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">||</font><font face="Times New Roman, Times, Serif" size="3">) takes two logical expressions and combines them. If <i>either</i> or <i>both</i> are TRUE, the result is TRUE. Both values must be FALSE for the result to be FALSE. Now we can determine whether the midterm grade is an A <i>or</i> the final grade is an A. If either the midterm grade or the final grade equals A, the assertion is TRUE. In C++, we write the expression 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">midtermGrade聽==聽A聽||聽finalGrade聽==聽A</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 && and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">||</font><font face="Times New Roman, Times, Serif" size="3"> operators always appear between two expressions; they are binary (two-operand) operators. The NOT operator (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">!</font><font face="Times New Roman, Times, Serif" size="3">) is a unary (one-operand) operator. It precedes a single logical expression and gives its opposite as the result. If (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">grade</font><font face="Times New Roman, Times, Serif" size="3"> == A) is FALSE, then </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">!</font><font face="Times New Roman, Times, Serif" size="3"> (</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">grade</font><font face="Times New Roman, Times, Serif" size="3"> == A) is TRUE. NOT gives us a convenient way of reversing the meaning of an assertion. For example,</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">!(hours聽>聽40)</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 the equivalent of</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">hours聽<=聽40</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 some contexts, the first form is clearer; in others, the second makes more sense.</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_192.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_193</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_194.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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