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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 V1.73.2" /><meta name="keywords" content=" ISO C++ , library " /><link rel="start" 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> |
