C Strings

<BODY LANG="EN"> <HR><center> <table border=1> <td><A NAME="tex2html23" HREF="c18.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html29" HREF="c18.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html31" 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">C Strings</A></H1> <P> C has no special data type for character strings. Instead they are stored as arrays of characters. Nevertheless, C provides special support for character arrays that do not extend to other kinds of arrays. For example, C provides ways to print them (in printf and other output statements) and to read them (in scanf and other input statements). C also provides a number of library routines that compute the lengths of strings, append two strings, and the like. <P> For example, one such C library routine is called <B>strlen</B>. Here is the header for <B>strlen</B>: <P> <PRE>int strlen (char s[]);</PRE> <P> As you can see, this function takes an array of characters as an argument and returns its length as a result. Now think carefully-what is unusual about this header? Think about how you would go about implementing this function. <P> <P><A NAME="tex2html4" HREF="answer17.html">Click here for the answer</A><P> <P> Most of the C string routines do not take the length of the string as an argument. This is possible because, by convention, all C strings are assumed to be terminated by the null character. (The null character is simply the number 0.) That is, if you think of a string as containing the characters ``Hello'', what it actually looks like is: <P> <PRE> _________________________________________________ | | | | | | | | | | 72 | 101 | 108 | 108 | 111 | 0 | | | |_____|_____|_____|_____|_____|_____|_____|_____|</PRE> <P> That is, the ASCII codes for each of the characters 'H', 'e', 'l', 'l', and 'o' are stored, followed by the number 0. <P> So when we ask <B>strlen</B> for the length of a string, we are not asking for the physical length of the array in which the string is stored. Above, the physical array has room for 8 characters, but the actual string consists of only 5 characters. (The null character is not counted as part of the string.) <P> Why is 0 a good choice for marking the end of a string? <P> <P><A NAME="tex2html5" HREF="answer23.html">Click here for the answer</A><P> <P> If this is such a good idea, why don't we use null-termination for arrays of all types? <P> <P><A NAME="tex2html6" HREF="answer26.html">Click here for the answer</A><P><HR><center> <table border=1> <td><A NAME="tex2html23" HREF="c18.shtml"><img height=36 width=36 alt="PREVIOUS" src="http://www.cs.utah.edu/icons/latex2html/previous.gif"></A></td> <td><A NAME="tex2html29" HREF="c18.shtml"><img height=36 width=36 alt="UP" src="http://www.cs.utah.edu/icons/latex2html/up.gif"></A></td> <td><A NAME="tex2html31" 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>