class slice_nil;

static const _ = slice_nil();

>>> x[::-1]

x[slice(_,_,-1)]

<dt><a href="slice.html">slice.hpp</a></dt>

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<h1 align="center"><a href="../index.html">Boost.Python</a></h1>

<h2 align="center">Header &lt;boost/python/slice.hpp&gt;</h2>

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<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>

<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="class-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>

<h4><a name="slice-spec-synopsis"></a>Class <code>slice</code> synopsis</h4>

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);

};

}}

<h4><a name="slice-spec-ctors"></a>Class <code>slice</code>

constructors<br>

<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 by the Python

expression <code>base[::].</code></dt>

<dt><b>Throws:</b> nothing.</dt>

template &lt;typename Int1, typename Int2&gt;

slice(Int1 start, Int2 stop);

<dl class="function-semantics">

<dt><b>Requires:</b> <code>start</code>, <code>stop</code>, and <code>step</code>

are of type <code>slice_nil</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 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>

template &lt;typename Int1, typename Int2, typename Int3&gt;

slice(Int1 start, Int2 stop, Int3 step);

<dt><b>Requires:</b> <code>start</code>, <code>stop</code>, and <code>step</code> are integers, <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-functions.html#12h-62">slice(start,stop,step)</a></code>,

or 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>

object slice::start();

object slice::stop();

object slice::step();

<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>

template &lt;typename RandomAccessIterator&gt;

slice::range&lt;RandomAccessIterator&gt;

slice::get_indicies(

RandomAccessIterator const&amp; begin, RandomAccessIterator const&amp; end);

<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>

<h2><a name="examples"></a><b>Examples</b></h2>

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 rich 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, 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.end) {

sum += *bounds.start;

std::advance( bounds.start, bounds.step);

}

sum += *bounds.start;

return sum;

}

<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>

class slice_nil;

# include <boost/python/slice_nil.hpp>