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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_538</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_537.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_538</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_539.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 538</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="Courier New, Courier, Mono New, Courier, Mono" size="2">enum聽Starch聽{CORN,聽RICE,聽POTATO,聽BEAN};<br />enum聽Grain聽{WHEAT,聽CORN,聽RYE,聽BARLEY,聽SORGHUM};聽聽聽聽//聽No</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">type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Starch</font><font face="Times New Roman, Times, Serif" size="3"> and type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Grain</font><font face="Times New Roman, Times, Serif" size="3"> are legal by themselves, but together they are not. Identifiers in the same scope must be unique. </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">CORN</font><font face="Times New Roman, Times, Serif" size="3"> cannot be defined twice.</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">Suppose you are writing a program for a veterinary office. The program must keep track of different kinds of animals. The following enumeration type might be used for this purpose.</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"><img src="9bcec1dba141fd31c5858d72a1d908ba.gif" border="0" alt="0538-01.gif" width="477" height="99" /></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="3">RODENT</font><font face="Times New Roman, Times, Serif" size="3"> is a literal, one of the values in the data type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Animals</font><font face="Times New Roman, Times, Serif" size="3">. Be sure you understand that </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">RODENT</font><font face="Times New Roman, Times, Serif" size="3"> is not a variable name. Instead, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">RODENT</font><font face="Times New Roman, Times, Serif" size="3"> is one of the values that can be stored into the variables </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inPatient</font><font face="Times New Roman, Times, Serif" size="3"> and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">outPatient</font><font face="Times New Roman, Times, Serif" size="3">. Let's look at the kinds of operations we might want to perform on variables of enumeration 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">Assignment</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 assignment 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">inPatient聽=聽DOG;</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">does not assign to </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inPatient</font><font face="Times New Roman, Times, Serif" size="3"> the character string DOG, nor the contents of a variable named </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">DOG</font><font face="Times New Roman, Times, Serif" size="3">. It assigns the <i>value</i> </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">DOG</font><font face="Times New Roman, Times, Serif" size="3">, which is one of the values in the domain of the data type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Animals</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="Times New Roman, Times, Serif" size="3">Assignment is a valid operation, as long as the value being stored is of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Animals</font><font face="Times New Roman, Times, Serif" size="3">. Both of the 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">inPatient聽=聽DOG;<br />outPatient聽=聽inPatient;</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">are acceptable. Each expression on the right-hand side is of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Animals</font><font face="Times New Roman, Times, Serif" size="3"></font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">DOG</font><font face="Times New Roman, Times, Serif" size="3"> is a literal of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Animals</font><font face="Times New Roman, Times, Serif" size="3">, and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">inPatient</font><font face="Times New Roman, Times, Serif" size="3"> is a variable of type </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">Animals</font><font face="Times New Roman, Times, Serif" size="3">. Although we know that the underlying representation of </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">DOG</font><font face="Times New Roman, Times, Serif" size="3"> is the integer 2, the compiler prevents us from using this assignment:</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">inPatient聽=聽2;</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_537.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_538</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_539.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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