Array Arguments

<BODY LANG="EN"> <HR><center> <table border=1> <td><A NAME="tex2html20" HREF="c17.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html26" HREF="c17.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html28" 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">Array Arguments</A></H1> <P> An array, like any other kind of value in C, can be passed as an argument to a function. In a better world that is all we would have to say about array arguments, since you already know about integer arguments and real arguments and char arguments. Unfortunately, array arguments in C do not behave the way that beginning C programmers would expect. In the end, when you understand more of C, array arguments will make perfect sense. For now, however, they require careful and detailed study. <P> So that you can understand what I am talking about, take a look at ``arraydemo.c'' in your examples directory (or <A HREF="../examples/arraydemo.c#arraydemoc">view it directly</A>). <P> Read the program carefully, and notice that there are three function calls (two to ``modifyInt'' and one to ``modifyArray'') in the main function. Very carefully try to predict what will be printed out when you run the program. Then compile and run the program and see if your intuition is correct. <P> Here's an explanation for what went on. <P> <OL> <LI> In the first call to ``modifyInt'', we are passing the value of an integer variable as the argument. Recall that C uses <B> call-by-value</B>, which means that the dummy argument in ``modifyInt'' (n) contains the value of the actual argument (x), but has nothing more in common. So when the value of ``n'' is changed, there is no effect on ``x''. (There's nothing new going on here--we've talked about this before.) <LI> In the second call to ``modifyInt'', we are passing the value of an integer array element as the argument. Since an element of an integer array behaves identically to a simple integer variable, we would expect this case to behave just like the previous one. And, in fact, it does. <LI> In the call to ``modifyArray'', we are passing the entire integer array as an argument. You might reasonably assume that the entire array would be copied during the function call, so that any modifications to the ``n'' made by ``modifyArray'' would not be visible in the original version ``y''. (After all, this is what happened in the first two cases.) But in this case, as you can see, the assignment to ``n[0]'' in ``modifyArray'' results in a change to ``y[0]'' in the main function. This seems like an inconsistency. What is going on? <P> </OL> <P> If this seems like an inconsistency, it is because it <I>is</I> an inconsistency! When you pass an entire array as an argument in C, nothing is copied. Instead, the <I>address</I> in memory of the actual array argument is passed up to the function. In effect, the dummy argument becomes another name for the actual argument, and any modifications made to the dummy argument are actually made in the original argument. <P> In our example, when we call ``modifyArray'' we are passing the address of ``y''. So the assignment to ``n[0]'' in ``modifyArray'' is actually an assignment to ``y[0]''. This is known as ``call-by-reference,'' which is how <I>all</I> argument passing is done in Fortran, and how <I>var</I> parameters are passed in Pascal. For the time being you should treat this as a special case of argument passing. (It will make a lot more sense, and seem a lot more intuitive, once we study pointers.) <P> This difference between array arguments and other kinds of arguments may be confusing. Can you think of a good reason for passing array arguments in this way? <P> <P><A NAME="tex2html4" HREF="answer29.html">Click here for the answer</A><P><HR><center> <table border=1> <td><A NAME="tex2html20" HREF="c17.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html26" HREF="c17.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html28" 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>