About Ragged Arrays

<BODY LANG="EN"> <HR><center> <table border=1> <td><A NAME="tex2html21" HREF="c24.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html27" HREF="c24.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html29" 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">About Ragged Arrays</A></H1> <P> We saw in the last lesson that the two declarations <P> <PRE> int A[10]; int *A;</PRE> <P> both declare A to be a pointer to an integer (or, equivalently, an array of integers). Nevertheless, there are two crucial differences between the two declarations. What are they? <P> <P><A NAME="tex2html4" HREF="answer15.html">Click here for the answer</A><P> <P> Given this, how do you think the following three declarations compare? <P> <PRE> char A[4][4]; char *B[4]; char **C;</PRE> <P> <P><A NAME="tex2html5" HREF="answer20.html">Click here for the answer</A><P> <P> Here's a way to visualize what's going on. In the first case, <P> <PRE> A: [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ]</PRE> <P> A is a name for the constant address of a 4x4 block of storage locations. In the second case, <P> <PRE> B: [ ] [ ] [ ] [ ]</PRE> <P> B is a name for the constant address of a block of of 4 storage locations, each of which can someday contain a pointer to a character (or characters). Once we initialize these four storage locations, we might have a picture like <P> <PRE> B: [ ] [ ] [ ] [ ] | | | | \|/ \|/ \|/ \|/ [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ]</PRE> <P> Notice that each of the four pointers can point to a different size block of characters. (Thus the name ``ragged array''.) <P> Finally, in the third case <P> <PRE>C:[ ]</PRE> <P> C is the name of a variable that can someday be assigned a pointer to a pointer to characters. For example, we might initialize C as <P> <PRE>C:[--]-->[ ] [ ] [ ] [ ]</PRE> <P> and could then initialize each of the four new storage locations as we did in the example for B to obtain <P> <PRE>C:[--]-->[ ] [ ] [ ] [ ] | | | | \|/ \|/ \|/ \|/ [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ]</PRE> <P> The important thing is to realize that whereas the value of C can be changed (via assignment), the values of A and B cannot. Further, whereas the values of each of the B[i] and C[i] can be changed, the values of the A[i] cannot. On the other hand, A, B, and the A[i] are actually preallocated, whereas C, the B[i], and the C[i] are not. <P> However, once they are all fully initialized, A[i] (or B[i] or C[i]) will be a pointer to a character (i.e., a string), and A[i][j] (or B[i][j] or C[i][j]) will be a character. <P> <HR><center> <table border=1> <td><A NAME="tex2html21" HREF="c24.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html27" HREF="c24.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html29" 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>