<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<!-- Copyright David Abrahams 2006. Distributed under the Boost -->
<!-- Software License, Version 1.0. (See accompanying -->
<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
<html>
<head>
  <meta name="generator"
 content="HTML Tidy for Windows (vers 1st August 2002), see www.w3.org">
  <meta http-equiv="Content-Type"
 content="text/html; charset=iso-8859-1">
  <link rel="stylesheet" type="text/css" href="../boost.css">
  <title>Boost.Python - &lt;boost/python/slice.hpp&gt;</title>
</head>
<body>
<table border="0" cellpadding="7" cellspacing="0" width="100%"
 summary="header">
  <tbody>
    <tr>
      <td valign="top" width="300">
      <h3><a href="../../../../index.htm"><img height="86" width="277"
 alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
      </td>
      <td valign="top">
      <h1 align="center"><a href="../index.html">Boost.Python</a></h1>
      <h2 align="center">Header &lt;boost/python/slice.hpp&gt;</h2>
      </td>
    </tr>
  </tbody>
</table>
<hr>
<h2>Contents</h2>
<dl class="page-index">
  <dt><a href="#introduction">Introduction</a></dt>
  <dt><a href="#classes">Classes</a></dt>
  <dd>
    <dl class="page-index">
      <dt><a href="#slice-spec">Class <code>slice</code></a></dt>
      <dd>
        <dl class="page-index">
          <dt><a href="#slice-spec-synopsis">Class <code>slice</code>
synopsis</a></dt>
          <dt><a href="#slice-spec-ctors">Class <code>slice</code>
constructors</a></dt>
          <dt><a href="#slice-spec-observers">Class <code>slice</code>
observer functions</a></dt>
        </dl>
      </dd>
    </dl>
  </dd>
  <dt><a href="#examples">Example(s)</a></dt>
</dl>
<hr>
<h2><a name="introduction"></a>Introduction</h2>
<p>Exposes a <a href="ObjectWrapper.html#TypeWrapper-concept">TypeWrapper</a>
for the Python <a
 href="http://www.python.org/doc/2.3.3/api/slice-objects.html">slice</a>
type.</p>
<h2><a name="classes"></a>Classes</h2>
<h3><a name="slice-spec"></a>Class <code>slice</code></h3>
<p>Exposes the extended slicing protocol by wrapping the built-in slice
type. The semantics of the constructors and member functions defined
below can be fully understood by reading the <a
 href="ObjectWrapper.html#TypeWrapper-concept">TypeWrapper</a> concept
definition. Since <code>slice</code> is publicly derived from <code><a
 href="object.html#object-spec">object</a></code>, the public object
interface applies to <code>slice</code> instances as well.<br>
</p>
<h4><a name="slice-spec-synopsis"></a>Class <code>slice</code> synopsis</h4>
<pre>
namespace boost { namespace python
{
  class slice : public object
  {
   public:
      slice(); // create an empty slice, equivalent to [::]

      template &lt;typename Int1, typename Int2&gt;
      slice(Int1 start, Int2 stop);

      template &lt;typename Int1, typename Int2, typename Int3&gt;
      slice(Int1 start, Int2 stop, Int3 step);

      // Access the parameters this slice was created with.
      object start();
      object stop();
      object step();

      // The return type of slice::get_indicies()
      template &lt;typename RandomAccessIterator&gt;
      struct range
      {
          RandomAccessIterator start;
          RandomAccessIterator stop;
          int step;
      };

      template &lt;typename RandomAccessIterator&gt;
      range&lt;RandomAccessIterator&gt;
      get_indicies(
          RandomAccessIterator const&amp; begin, 
          RandomAccessIterator const&amp; end);
  };
}}
</pre>
<h4><a name="slice-spec-ctors"></a>Class <code>slice</code>
constructors<br>
</h4>
<pre>slice();<br></pre>
<dl class="function-semantics">
  <dt><b>Effects:</b> constructs a <code>slice</code> with default stop, start, and
step values.&nbsp; Equivalent to the slice object created as part of the Python
expression <code>base[::].</code></dt>
  <dt><b>Throws:</b> nothing.</dt>
</dl>
<pre>
template &lt;typename Int1, typename Int2&gt;
slice(Int1 start, Int2 stop);
</pre>
<dl class="function-semantics">
  <dt><b>Requires:</b> <code>start</code>, <code>stop</code>, and <code>step</code>
    are of type <code><a href="object.html#slice_nil-spec">slice_nil</a></code> 
    or convertible to type <code>object</code>.</dt>
  <dt><b>Effects:</b> constructs a new slice with default step value
and the provided start and stop values.&nbsp; Equivalent to the slice
object
created by the built-in Python function <code><a
 href="http://www.python.org/doc/current/lib/built-in-funcs.html#12h-62">slice(start,stop)</a></code>,
or as part of the Python expression <code>base[start:stop]</code>.</dt>
  <dt><b>Throws:</b> <code>error_already_set</code> and sets a Python <code>TypeError</code>
exception if no conversion is possible from the arguments to type <code>object</code>.</dt>
</dl>
<pre>
template &lt;typename Int1, typename Int2, typename Int3&gt;
slice(Int1 start, Int2 stop, Int3 step);
</pre>
  <dt><b>Requires:</b> <code>start</code>, <code>stop</code>, and <code>step</code> are <code>slice_nil</code> or convertible to type <code>object</code>.</dt>
  <dt><b>Effects:</b> constructs a new slice with start stop and step
values.&nbsp; Equivalent to the slice object created
by the built-in Python function <code><a
 href="http://www.python.org/doc/current/lib/built-in-funcs.html">slice(start,stop,step)</a></code>,
or as part of the Python expression <code>base[start:stop:step]</code>.</dt>
  <dt><b>Throws:</b> <code>error_already_set</code> and sets a Python <code>TypeError</code>
exception if no conversion is possible from the arguments to type
object.</dt>
<h4><a name="slice-spec-observers"></a>Class <code>slice</code>
observer functions<br>
</h4>
<pre>
object slice::start() const;
object slice::stop() const;
object slice::step() const;
</pre>
<dl class="function-semantics">
  <dt><b>Effects:</b> None.</dt>
  <dt><b>Throws:</b> nothing.</dt>
  <dt><b>Returns:</b>the parameter that
the slice was created with.&nbsp;If the parameter was omitted or
slice_nil was used when the slice was created, than that parameter will
be a reference to PyNone and compare equal to a default-constructed
object.&nbsp; In principal, any object may be used when creating a
slice object, but in practice they are usually integers.</dt>
</dl>
<br>
<pre>
template &lt;typename RandomAccessIterator&gt;
slice::range&lt;RandomAccessIterator&gt;
slice::get_indicies( 
    RandomAccessIterator const&amp; begin, 
    RandomAccessIterator const&amp; end) const;
</pre>
<dl class="function-semantics">
  <dt><b>Arguments:</b> A pair of STL-conforming Random Access
Iterators that form a half-open range.</dt>
  <dt><b>Effects:</b> Create a RandomAccessIterator pair that defines a
fully-closed range within the [begin,end) range of its arguments.&nbsp;
This function translates this slice's indicies while accounting for the
effects of any PyNone or negative indicies, and non-singular step sizes.</dt>
  <dt><b>Returns:</b> a slice::range
that has been initialized with a non-zero value of step and a pair of
RandomAccessIterators that point within the range of this functions
arguments and define a closed interval.</dt>
  <dt><b>Throws:</b> <a href="definitions.html#raise">Raises</a> a Python <code>TypeError</code> exception if any of this slice's arguments
are neither references to <code>PyNone</code> nor convertible to <code>int</code>.&nbsp; Throws
<code>std::invalid_argument</code> if the resulting range would be empty.&nbsp; You
should always wrap calls to <code>slice::get_indicies()</code>
within <code>try { ...; } catch (std::invalid_argument) {}</code> to
handle this case and take appropriate action.</dt>
  <dt><b>Rationale</b>: closed-interval: If
an open interval were used, then for step
size other than 1, the required state for the end iterator would point
beyond the one-past-the-end position or before the beginning of the
specified range.<br>
exceptions on empty slice: It is impossible to define a closed interval
over an empty range, so some other form of error checking would have to
be used to prevent undefined behavior.&nbsp;In the case where the
exception is not caught, it will simply be translated to Python by the
default exception handling mechanisms. </dt>
</dl>
<h2><a name="examples"></a><b>Examples</b></h2>
<pre>
using namespace boost::python;

// Perform an extended slice of a Python list.
// Warning: extended slicing was not supported for built-in types prior 
// to Python 2.3
list odd_elements(list l)
{
    return l[slice(_,_,2)];
}

// Perform a multidimensional extended slice of a Numeric.array
numeric::array even_columns(numeric::array arr)
{
    // select every other column, starting with the second, of a 2-D array.
    // Equivalent to "return arr[:, 1::2]" in Python.
    return arr[make_tuple( slice(), slice(1,_,2))];
}

// Perform a summation over a slice of a std::vector.
double partial_sum(std::vector&lt;double&gt; const&amp; Foo, const slice index)
{
    slice::range&lt;std::vector&lt;double&gt;::const_iterator&gt; bounds;
    try {
        bounds = index.get_indicies&lt;&gt;(Foo.begin(), Foo.end());
    }
    catch (std::invalid_argument) {
        return 0.0;
    }
    double sum = 0.0;
    while (bounds.start != bounds.stop) {
        sum += *bounds.start;
        std::advance( bounds.start, bounds.step);
    }
    sum += *bounds.start;
    return sum;
}
</pre>
<p>Revised 07 Febuary, 2004</p>
<p><i>&copy; Copyright <a
 href="mailto:jbrandmeyer@users.sourceforge.net">Jonathan Brandmeyer</a>,
2004.&nbsp; Modification, copying and redistribution of this document
is permitted under the terms and conditions of the Boost Software
License, version 1.0.<br>
</i></p>
</body>
</html>