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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_157</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_156.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_157</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_158.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 157</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>Implementing the Design</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">The product of top-down design is a hierarchical solution to a problem with multiple levels of abstraction. Figure 4-5 shows a top-down design for the ConePaint program in Chapter 3. This kind of solution forms the basis for the implementation phase of programming.</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">How do we translate a top-down design into a C++ program? If you look closely at Figure 4-5, you can see that the concrete steps (those that are shaded) can be assembled into a complete algorithm for solving the problem. The order in which they are assembled is determined by their position in the tree. We start at the top of the tree, at level 0, with the first step, Convert dimensions to feet. Because it is abstract, we must go to the next level, level 1. There we find a series of concrete steps that correspond to this step; this series of steps becomes the first part of our algorithm. Because the conversion process is now concrete, we can go back to level 0 and go on to the next step, finding the radius of the cone. This step is concrete; we can copy it directly into the algorithm. The last three steps at level 0 are abstract, so we work with each of them in order at level 1, making them concrete. Here's the resulting algorithm:</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">Set聽heightInFeet聽=聽heightInInches聽/聽12<br />Set聽diameterInFeet聽=聽diameterInInches聽/聽12<br />Set聽radius聽=聽diameterInFeet聽/聽2<br />Set聽surfaceArea聽=聽pi聽</font><font face="Symbol" size="2">*</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">聽radius聽</font><font face="Symbol" size="2">*</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">聽sqrt(radius</font><font face="Symbol" size="2">*</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">radius聽+聽heightInFeet</font><font face="Symbol" size="2">*</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">heightInFeet)<br />Set聽redCost聽=聽surfaceArea聽</font><font face="Symbol" size="2">*</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">聽0.10<br />Set聽blueCost聽=聽surfaceArea聽</font><font face="Symbol" size="2">*</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">聽0.15<br />Set聽greenCost聽=聽surfaceArea聽</font><font face="Symbol" size="2">*</font><font face="Courier New, Courier, Mono New, Courier, Mono" size="2">聽0.18<br />Print聽surfaceArea<br />Print聽redCost<br />Print聽blueCost<br />Print聽greenCost</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">From this algorithm we can construct a table of the constants and variables required, and then write the declarations and executable statements of the program.</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 practice, you write your design not as a tree diagram but as a series of modules grouped by levels of abstraction, as we've done below.</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">Main聽Module聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽聽<i>Level聽0</i></font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table><table cellspacing="0" border="0" width="278" cellpadding="4"><tr><td valign="top"><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">Convert聽dimensions聽to聽feet<br />Set聽radius聽=聽diameterInFeet聽/聽2<br />Compute聽surface聽area<br />Compute聽costs<br />Print聽results</font></td><td></td></tr><tr><td colspan="3"></td></tr><tr><td colspan="3" height="1"></td></tr></table></td></tr></table><br /></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_156.html">< previous page</a></td> <td align="center" width="40%" style="background: #EEF3E2"><strong style="color: #2F4F4F; font-size: 120%;">page_157</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_158.html">next page ></a></td> </tr> </table> </body> </html>⌨️ 快捷键说明
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