247 lines
9.5 KiB
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247 lines
9.5 KiB
HTML
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<title>Boost.Python - <boost/python/slice.hpp></title>
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<table border="0" cellpadding="7" cellspacing="0" width="100%"
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summary="header">
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<h3><a href="../../../../index.htm"><img height="86" width="277"
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alt="C++ Boost" src="../../../../boost.png" border="0"></a></h3>
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</td>
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<td valign="top">
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<h1 align="center"><a href="../index.html">Boost.Python</a></h1>
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<h2 align="center">Header <boost/python/slice.hpp></h2>
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</td>
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</tr>
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</tbody>
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</table>
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<hr>
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<h2>Contents</h2>
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<dl class="page-index">
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<dt><a href="#introduction">Introduction</a></dt>
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<dt><a href="#classes">Classes</a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#slice-spec">Class <code>slice</code></a></dt>
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<dd>
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<dl class="page-index">
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<dt><a href="#slice-spec-synopsis">Class <code>slice</code>
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synopsis</a></dt>
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<dt><a href="#slice-spec-ctors">Class <code>slice</code>
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constructors</a></dt>
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<dt><a href="#slice-spec-observers">Class <code>slice</code>
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observer functions</a></dt>
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</dl>
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</dd>
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</dl>
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</dd>
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<dt><a href="#examples">Example(s)</a></dt>
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</dl>
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<hr>
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<h2><a name="introduction"></a>Introduction</h2>
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<p>Exposes a <a href="ObjectWrapper.html#TypeWrapper-concept">TypeWrapper</a>
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for the Python <a
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href="http://www.python.org/doc/2.3.3/api/slice-objects.html">slice</a>
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type.</p>
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<h2><a name="classes"></a>Classes</h2>
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<h3><a name="slice-spec"></a>Class <code>slice</code></h3>
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<p>Exposes the extended slicing protocol by wrapping the built-in slice
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type. The semantics of the constructors and member functions defined
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below can be fully understood by reading the <a
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href="ObjectWrapper.html#TypeWrapper-concept">TypeWrapper</a> concept
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definition. Since <code>slice</code> is publicly derived from <code><a
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href="object.html#object-spec">object</a></code>, the public object
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interface applies to <code>slice</code> instances as well.<br>
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</p>
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<h4><a name="slice-spec-synopsis"></a>Class <code>slice</code> synopsis</h4>
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<pre>
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namespace boost { namespace python
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{
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class slice : public object
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{
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public:
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slice(); // create an empty slice, equivalent to [::]
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template <typename Int1, typename Int2>
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slice(Int1 start, Int2 stop);
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template <typename Int1, typename Int2, typename Int3>
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slice(Int1 start, Int2 stop, Int3 step);
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// Access the parameters this slice was created with.
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object start();
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object stop();
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object step();
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// The return type of slice::get_indicies()
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template <typename RandomAccessIterator>
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struct range
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{
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RandomAccessIterator start;
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RandomAccessIterator stop;
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int step;
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};
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template <typename RandomAccessIterator>
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range<RandomAccessIterator>
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get_indicies(
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RandomAccessIterator const& begin,
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RandomAccessIterator const& end);
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};
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}}
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</pre>
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<h4><a name="slice-spec-ctors"></a>Class <code>slice</code>
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constructors<br>
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</h4>
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<pre>slice();<br></pre>
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<dl class="function-semantics">
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<dt><b>Effects:</b> constructs a <code>slice</code> with default stop, start, and
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step values. Equivalent to the slice object created as part of the Python
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expression <code>base[::].</code></dt>
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<dt><b>Throws:</b> nothing.</dt>
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</dl>
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<pre>
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template <typename Int1, typename Int2>
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slice(Int1 start, Int2 stop);
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</pre>
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<dl class="function-semantics">
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<dt><b>Requires:</b> <code>start</code>, <code>stop</code>, and <code>step</code>
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are of type <code><a href="object.html#slice_nil-spec">slice_nil</a></code>
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or convertible to type <code>object</code>.</dt>
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<dt><b>Effects:</b> constructs a new slice with default step value
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and the provided start and stop values. Equivalent to the slice
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object
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created by the built-in Python function <code><a
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href="http://www.python.org/doc/current/lib/built-in-funcs.html#12h-62">slice(start,stop)</a></code>,
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or as part of the Python expression <code>base[start:stop]</code>.</dt>
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<dt><b>Throws:</b> <code>error_already_set</code> and sets a Python <code>TypeError</code>
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exception if no conversion is possible from the arguments to type <code>object</code>.</dt>
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</dl>
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<pre>
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template <typename Int1, typename Int2, typename Int3>
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slice(Int1 start, Int2 stop, Int3 step);
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</pre>
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<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>
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<dt><b>Effects:</b> constructs a new slice with start stop and step
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values. Equivalent to the slice object created
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by the built-in Python function <code><a
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href="http://www.python.org/doc/current/lib/built-in-funcs.html">slice(start,stop,step)</a></code>,
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or as part of the Python expression <code>base[start:stop:step]</code>.</dt>
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<dt><b>Throws:</b> <code>error_already_set</code> and sets a Python <code>TypeError</code>
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exception if no conversion is possible from the arguments to type
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object.</dt>
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<h4><a name="slice-spec-observers"></a>Class <code>slice</code>
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observer functions<br>
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</h4>
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<pre>
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object slice::start() const;
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object slice::stop() const;
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object slice::step() const;
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</pre>
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<dl class="function-semantics">
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<dt><b>Effects:</b> None.</dt>
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<dt><b>Throws:</b> nothing.</dt>
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<dt><b>Returns:</b>the parameter that
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the slice was created with. If the parameter was omitted or
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slice_nil was used when the slice was created, than that parameter will
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be a reference to PyNone and compare equal to a default-constructed
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object. In principal, any object may be used when creating a
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slice object, but in practice they are usually integers.</dt>
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</dl>
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<br>
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<pre>
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template <typename RandomAccessIterator>
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slice::range<RandomAccessIterator>
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slice::get_indicies(
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RandomAccessIterator const& begin,
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RandomAccessIterator const& end) const;
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</pre>
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<dl class="function-semantics">
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<dt><b>Arguments:</b> A pair of STL-conforming Random Access
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Iterators that form a half-open range.</dt>
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<dt><b>Effects:</b> Create a RandomAccessIterator pair that defines a
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fully-closed range within the [begin,end) range of its arguments.
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This function translates this slice's indicies while accounting for the
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effects of any PyNone or negative indicies, and non-singular step sizes.</dt>
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<dt><b>Returns:</b> a slice::range
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that has been initialized with a non-zero value of step and a pair of
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RandomAccessIterators that point within the range of this functions
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arguments and define a closed interval.</dt>
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<dt><b>Throws:</b> <a href="definitions.html#raise">Raises</a> a Python <code>TypeError</code> exception if any of this slice's arguments
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are neither references to <code>PyNone</code> nor convertible to <code>int</code>. Throws
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<code>std::invalid_argument</code> if the resulting range would be empty. You
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should always wrap calls to <code>slice::get_indicies()</code>
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within <code>try { ...; } catch (std::invalid_argument) {}</code> to
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handle this case and take appropriate action.</dt>
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<dt><b>Rationale</b>: closed-interval: If
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an open interval were used, then for step
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size other than 1, the required state for the end iterator would point
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beyond the one-past-the-end position or before the beginning of the
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specified range.<br>
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exceptions on empty slice: It is impossible to define a closed interval
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over an empty range, so some other form of error checking would have to
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be used to prevent undefined behavior. In the case where the
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exception is not caught, it will simply be translated to Python by the
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default exception handling mechanisms. </dt>
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</dl>
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<h2><a name="examples"></a><b>Examples</b></h2>
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<pre>
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using namespace boost::python;
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// Perform an extended slice of a Python list.
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// Warning: extended slicing was not supported for built-in types prior
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// to Python 2.3
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list odd_elements(list l)
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{
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return l[slice(_,_,2)];
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}
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// Perform a multidimensional extended slice of a Numeric.array
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numeric::array even_columns(numeric::array arr)
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{
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// select every other column, starting with the second, of a 2-D array.
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// Equivalent to "return arr[:, 1::2]" in Python.
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return arr[make_tuple( slice(), slice(1,_,2))];
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}
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// Perform a summation over a slice of a std::vector.
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double partial_sum(std::vector<double> const& Foo, const slice index)
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{
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slice::range<std::vector<double>::const_iterator> bounds;
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try {
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bounds = index.get_indicies<>(Foo.begin(), Foo.end());
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}
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catch (std::invalid_argument) {
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return 0.0;
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}
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double sum = 0.0;
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while (bounds.start != bounds.stop) {
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sum += *bounds.start;
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std::advance( bounds.start, bounds.step);
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}
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sum += *bounds.start;
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return sum;
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}
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</pre>
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<p>Revised 07 Febuary, 2004</p>
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<p><i>© Copyright <a
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href="mailto:jbrandmeyer@users.sourceforge.net">Jonathan Brandmeyer</a>,
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2004. Modification, copying and redistribution of this document
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is permitted under the terms and conditions of the Boost Software
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License, version 1.0.<br>
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</i></p>
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</body>
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