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+<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Tokenizing</title><meta name="generator" content="DocBook XSL Stylesheets V1.74.0" /><meta name="keywords" content=" ISO C++ , library " /><link rel="home" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="bk01pt05ch13.html" title="Chapter 13. String Classes" /><link rel="prev" href="bk01pt05ch13s03.html" title="Arbitrary Character Types" /><link rel="next" href="bk01pt05ch13s05.html" title="Shrink to Fit" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Tokenizing</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="bk01pt05ch13s03.html">Prev</a> </td><th width="60%" align="center">Chapter 13. String Classes</th><td width="20%" align="right"> <a accesskey="n" href="bk01pt05ch13s05.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="strings.string.token"></a>Tokenizing</h2></div></div></div><p>
+ </p><p>The Standard C (and C++) function <code class="code">strtok()</code> leaves a lot to
+ be desired in terms of user-friendliness. It's unintuitive, it
+ destroys the character string on which it operates, and it requires
+ you to handle all the memory problems. But it does let the client
+ code decide what to use to break the string into pieces; it allows
+ you to choose the "whitespace," so to speak.
+ </p><p>A C++ implementation lets us keep the good things and fix those
+ annoyances. The implementation here is more intuitive (you only
+ call it once, not in a loop with varying argument), it does not
+ affect the original string at all, and all the memory allocation
+ is handled for you.
+ </p><p>It's called stringtok, and it's a template function. Sources are
+ as below, in a less-portable form than it could be, to keep this
+ example simple (for example, see the comments on what kind of
+ string it will accept).
+ </p><pre class="programlisting">
+#include <string>
+template <typename Container>
+void
+stringtok(Container &container, string const &in,
+ const char * const delimiters = " \t\n")
+{
+ const string::size_type len = in.length();
+ string::size_type i = 0;
+
+ while (i < len)
+ {
+ // Eat leading whitespace
+ i = in.find_first_not_of(delimiters, i);
+ if (i == string::npos)
+ return; // Nothing left but white space
+
+ // Find the end of the token
+ string::size_type j = in.find_first_of(delimiters, i);
+
+ // Push token
+ if (j == string::npos)
+ {
+ container.push_back(in.substr(i));
+ return;
+ }
+ else
+ container.push_back(in.substr(i, j-i));
+
+ // Set up for next loop
+ i = j + 1;
+ }
+}
+</pre><p>
+ The author uses a more general (but less readable) form of it for
+ parsing command strings and the like. If you compiled and ran this
+ code using it:
+ </p><pre class="programlisting">
+ std::list<string> ls;
+ stringtok (ls, " this \t is\t\n a test ");
+ for (std::list<string>const_iterator i = ls.begin();
+ i != ls.end(); ++i)
+ {
+ std::cerr << ':' << (*i) << ":\n";
+ } </pre><p>You would see this as output:
+ </p><pre class="programlisting">
+ :this:
+ :is:
+ :a:
+ :test: </pre><p>with all the whitespace removed. The original <code class="code">s</code> is still
+ available for use, <code class="code">ls</code> will clean up after itself, and
+ <code class="code">ls.size()</code> will return how many tokens there were.
+ </p><p>As always, there is a price paid here, in that stringtok is not
+ as fast as strtok. The other benefits usually outweigh that, however.
+ <a class="ulink" href="stringtok_std_h.txt" target="_top">Another version of stringtok is given
+ here</a>, suggested by Chris King and tweaked by Petr Prikryl,
+ and this one uses the
+ transformation functions mentioned below. If you are comfortable
+ with reading the new function names, this version is recommended
+ as an example.
+ </p><p><span class="emphasis"><em>Added February 2001:</em></span> Mark Wilden pointed out that the
+ standard <code class="code">std::getline()</code> function can be used with standard
+ <a class="ulink" href="../27_io/howto.html" target="_top">istringstreams</a> to perform
+ tokenizing as well. Build an istringstream from the input text,
+ and then use std::getline with varying delimiters (the three-argument
+ signature) to extract tokens into a string.
+ </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="bk01pt05ch13s03.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="bk01pt05ch13.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="bk01pt05ch13s05.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Arbitrary Character Types </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Shrink to Fit</td></tr></table></div></body></html>
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