📄 page_231.html
字号:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>page_231</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_230.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_231</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_232.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 231</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>The Problem-Solving Phase: The Algorithm Walk-Through</i></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 test at the problem-solving phase, we do a <i>walk-through</i> of the algorithm. For each module in the top-down design, we establish an assertion called a precondition and another called a postcondition. A precondition is an assertion that must be true before a module is executed in order for the module to execute correctly. A postcondition is an assertion that should be true after the module has executed, if it has done its job correctly. To test a module, we walk through the algorithmic steps to confirm that they produce the required postcondition given the stated precondition.</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" width="100%"><tr><td rowspan="5"><img src="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" border="0" width="96" height="1" alt="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" /></td> <td colspan="3" height="12"></td> <td rowspan="5"><img src="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" border="0" width="96" height="1" alt="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" /></td></tr><tr><td colspan="3" height="2"><table cellpadding="0" cellspacing="0" border="0"><tr><td></td></tr></table></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="2">Precondition An assertion that must be true before a module begins executing.</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" width="100%"><tr><td rowspan="5"><img src="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" border="0" width="96" height="1" alt="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" /></td> <td colspan="3" height="12"></td> <td rowspan="5"><img src="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" border="0" width="96" height="1" alt="3e26ecb1b6ac508ae10a0e39d2fb98b2.gif" /></td></tr><tr><td colspan="3"></td></tr><tr><td></td> <td><font face="Times New Roman, Times, Serif" size="2">Postcondition An assertion that should be true after a module has executed.</font></td><td></td></tr><tr><td colspan="3" height="2"><table cellpadding="0" cellspacing="0" border="0"><tr><td></td></tr></table></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">Our lumberyard algorithm has four modules: the main module, Get Data, Calculate Lumber Amount, and Calculate Plywood Amount. Usually there is no precondition for a main module. Our main module's postcondition is that it outputs the correct result given the correct input. More specifically, the postcondition for the main module is</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 computer has read five input values (a letter and four integers).</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"> either the number of board feet ordered or the number of full sheets of plywood ordered has been calculated and displayed.</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">Because Get Data is the first module executed in the algorithm and because it does not assume anything about the contents of the variables it is about to manipulate, it has no precondition. Its postcondition is that it has input a letter into </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">code</font><font face="Times New Roman, Times, Serif" size="3">, and integer values into </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">size1</font><font face="Times New Roman, Times, Serif" size="3">, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">size2</font><font face="Times New Roman, Times, Serif" size="3">, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">size3</font><font face="Times New Roman, Times, Serif" size="3">, and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">numOrdered</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">The precondition for module Calculate Lumber Amount is that </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">code</font><font face="Times New Roman, Times, Serif" size="3"> equals L, and that </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">size1</font><font face="Times New Roman, Times, Serif" size="3">, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">size2</font><font face="Times New Roman, Times, Serif" size="3">, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">size3</font><font face="Times New Roman, Times, Serif" size="3">, and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">numOrdered</font><font face="Times New Roman, Times, Serif" size="3"> contain integer values. Its postcondition is that the number of board feet ordered has been computed from those values and printed out.</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 precondition for module Calculate Plywood Amount is that </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">code</font><font face="Times New Roman, Times, Serif" size="3"> equals P, and that </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">size1</font><font face="Times New Roman, Times, Serif" size="3">, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">size2</font><font face="Times New Roman, Times, Serif" size="3">, </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">size3</font><font face="Times New Roman, Times, Serif" size="3">, and </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">numOrdered</font><font face="Times New Roman, Times, Serif" size="3"> contain integer values. Its postcondition is that the number of full sheets ordered has been computed and printed out.</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">Below we summarize the module preconditions and postconditions in tabular form. In the table, we use <i>AND</i> with its usual meaning in an assertionthe logical AND operation. Also, a phrase like </font><font face="Courier New, Courier, Mono New, Courier, Mono" size="3">someVariable</font><font face="Times New Roman, Times, Serif" size="3"> is as-</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_230.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_231</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_232.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 + -