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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 V1.75.2" /><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" title="Tokenizing"><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.
</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
<code class="code">istringstreams</code> 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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