Arrays as pointers?

<BODY LANG="EN"> <HR><center> <table border=1> <td><A NAME="tex2html19" HREF="c22.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html25" HREF="c22.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html27" 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">Arrays as pointers?</A></H1> <P> Think back to when we first discussed what happens when you pass arrays as arguments to functions. How were arrays different from every other data type that we had studied to that point? <P> <P><A NAME="tex2html4" HREF="answer15.html">Click here for the answer</A><P> <P> When we introduced arrays, we told you to treat them as an exception to the way that arguments are passed by value in C. Now that you have a little more perspective, how would you characterize the way that arrays are passed as arguments in C? <P> <P><A NAME="tex2html5" HREF="answer19.html">Click here for the answer</A><P> <P> Now let's look at the first of a sequence of programs. ``aptr1.c'' in your ``examples'' directory (or <A HREF="../examples/aptr1.c#aptr1c">view it directly</A>) passes an integer variable by reference (through the use of pointers) to a function which initializes the variable. There's nothing new about this program, but be sure that you understand it. <P> Now take a look at ``aptr2.c'' (or <A HREF="../examples/aptr2.c#aptr2c">view it directly</A>). What's the difference between this program and the previous one? <P> <P><A NAME="tex2html6" HREF="answer36.html">Click here for the answer</A><P> <P> In the first program we passed a pointer to an integer through which ``init'' communicated the value 17, and in the second program we passed an array of one element through which ``init'' communicated the value 17. <P> Now, examine the code of the two programs very carefully and detail exactly what the differences are. <P> <P><A NAME="tex2html7" HREF="answer39.html">Click here for the answer</A><P> <P> Thus there are four differences between how ``n'' and ``x'' are declared and used. In ``aptr1.c'' they are declared and used as pointers (x actually points to ``y''), and in ``aptr2.c'' they are declared and used as arrays of one element. Yet both programs do more or less the same thing. <P> Now try out the following exercise. Change the header of ``init'' in ``aptr1.c'' so that instead of ``int *n'' it contains ``int n[]''. Compile and run the program. Does it work? Keep making modifications to both programs in this way, replacing a piece of one program with the corresponding piece of the other program. Are you surprised at what happens? <P> <P><A NAME="tex2html8" HREF="answer45.html">Click here for an observation</A><P> <P> Let this sink in before moving along. You may not believe it yet, but the evidence is all there. <P> <HR><center> <table border=1> <td><A NAME="tex2html19" HREF="c22.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html25" HREF="c22.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html27" 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>