Symbolic Constants

<BODY LANG="EN"> <HR><center> <table border=1> <td><A NAME="tex2html26" HREF="c15.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html32" HREF="c15.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html34" HREF="node2.shtml"><img height=36 width=36 alt="NEXT" src="http://www.cs.utah.edu/icons/latex2html/next.gif"></A></td> <td><a href="contents-node1.shtml"><img height=36 width=36 alt="CONTENTS" src="http://www.cs.utah.edu/icons/latex2html/toc.gif"></a></td> <td><a href="reset.hamlet"><img height=36 width=36 alt="RESET" src="http://www.cs.utah.edu/icons/latex2html/stop.gif"></a></td> <td><a href="copyright.html"><img height=36 width=36 alt="COPYRIGHT" src="http://www.cs.utah.edu/icons/latex2html/copyright.gif"></a></td> <td><a href="mailto:hamlet@cs.utah.edu"><img height=36 width=36 alt="FEEDBACK" src="http://www.cs.utah.edu/icons/latex2html/feedback.gif"></a></td> <td>     <a href="http://www.cs.utah.edu/%7Ehamlet/icons.html"><img height=36 width=36 alt=ABOUT src=http://www.cs.utah.edu/icons/latex2html/about.gif></a>  </td> </table> </center> <HR><H1><A NAME="SECTION00010000000000000000">Symbolic Constants</A></H1> <P> Take a look at the program ``circle.c'' in your ``examples'' directory (or <A HREF="../examples/circle.c#circlec">view it directly</A>). Can you think of a way of making it more readable? <P> <P><A NAME="tex2html4" HREF="answer22.html">Click here for the answer</A><P> <P> One approach to solving this problem would be to declare a variable called PI and assign it an initial value of 3.14159. We could then replace both occurrences of 3.14159 with PI. What is misleading about doing this? <P> <P><A NAME="tex2html5" HREF="answer25.html">Click here for the answer</A><P> <P> Try the following modification instead. On a line before the ``main'' procedure, put the line <P> <PRE>#define PI 3.14159</PRE> <P> and then replace both occurrences of 3.14159 with PI. The program should compile and run exactly as before. <P> The <B>#define</B> line is a directive to the preprocessor to replace every occurrence of PI in the file with 3.14159. That is, before the compiler even sees the program, every occurrence of PI has been replaced with 3.14159. (The change is not made to the permanent copy of the file, but is made temporarily for the benefit of the compiler.) <P> Now, what do you suppose would happen if you tried to change the value of PI in the program? Right after the declaration of ``r'' in ``main'', try putting in an assignment statement that changes the value of PI to 3.14. What happens when you try to compile it? <P> <P><A NAME="tex2html6" HREF="answer29.html">Click here for the answer</A><P> <P> This method of defining symbolic constants is actually kind of old fashioned. Try replacing the definition of PI with the executable C statement <P> <PRE>const float PI = 3.14159;</PRE> <P> The keyword ``const'' tells the compiler that the value of PI must never be modified. Now when you compile the program, what error message do you get concerning the assignment that you added? <P> <P><A NAME="tex2html7" HREF="answer32.html">Click here for the answer</A><P> <P> The moral of the story is that if you want to define a symbolic constant, use the ``new-fashioned'' way instead of using <B>#define</B>. If you'd like to know a little bit more about how the ``const'' keyword works, take a look at section 5.8 of the book. <P> <HR><center> <table border=1> <td><A NAME="tex2html26" HREF="c15.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html32" HREF="c15.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html34" HREF="node2.shtml"><img height=36 width=36 alt="NEXT" src="http://www.cs.utah.edu/icons/latex2html/next.gif"></A></td> <td><a href="contents-node1.shtml"><img height=36 width=36 alt="CONTENTS" src="http://www.cs.utah.edu/icons/latex2html/toc.gif"></a></td> <td><a href="reset.hamlet"><img height=36 width=36 alt="RESET" src="http://www.cs.utah.edu/icons/latex2html/stop.gif"></a></td> <td><a href="copyright.html"><img height=36 width=36 alt="COPYRIGHT" src="http://www.cs.utah.edu/icons/latex2html/copyright.gif"></a></td> <td><a href="mailto:hamlet@cs.utah.edu"><img height=36 width=36 alt="FEEDBACK" src="http://www.cs.utah.edu/icons/latex2html/feedback.gif"></a></td> <td>     <a href="http://www.cs.utah.edu/%7Ehamlet/icons.html"><img height=36 width=36 alt=ABOUT src=http://www.cs.utah.edu/icons/latex2html/about.gif></a>  </td> </table> </center> <HR><P><ADDRESS> <I><A NAME="tex2html1" HREF="http://www.cs.utah.edu/~hamlet">Hamlet Project</A> <BR> <A NAME="tex2html2" HREF="mailto:hamlet@cs.utah.edu">hamlet@cs.utah.edu</A></I> </ADDRESS> </BODY>