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<a name="Vector-views"></a>
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Next: <a href="Copying-vectors.html#Copying-vectors" accesskey="n" rel="next">Copying vectors</a>, Previous: <a href="Reading-and-writing-vectors.html#Reading-and-writing-vectors" accesskey="p" rel="previous">Reading and writing vectors</a>, Up: <a href="Vectors.html#Vectors" accesskey="u" rel="up">Vectors</a> &nbsp; [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Vector-views-1"></a>
<h4 class="subsection">8.3.5 Vector views</h4>

<p>In addition to creating vectors from slices of blocks it is also
possible to slice vectors and create vector views.  For example, a
subvector of another vector can be described with a view, or two views
can be made which provide access to the even and odd elements of a
vector.
</p>
<a name="index-gsl_005fvector_005fview"></a>
<a name="index-gsl_005fvector_005fconst_005fview"></a>
<p>A vector view is a temporary object, stored on the stack, which can be
used to operate on a subset of vector elements.  Vector views can be
defined for both constant and non-constant vectors, using separate types
that preserve constness.  A vector view has the type
<code>gsl_vector_view</code> and a constant vector view has the type
<code>gsl_vector_const_view</code>.  In both cases the elements of the view
can be accessed as a <code>gsl_vector</code> using the <code>vector</code> component
of the view object.  A pointer to a vector of type <code>gsl_vector *</code>
or <code>const gsl_vector *</code> can be obtained by taking the address of
this component with the <code>&amp;</code> operator.  
</p>
<p>When using this pointer it is important to ensure that the view itself
remains in scope&mdash;the simplest way to do so is by always writing the
pointer as <code>&amp;</code><var>view</var><code>.vector</code>, and never storing this value
in another variable.  
</p>
<dl>
<dt><a name="index-gsl_005fvector_005fsubvector"></a>Function: <em>gsl_vector_view</em> <strong>gsl_vector_subvector</strong> <em>(gsl_vector * <var>v</var>, size_t <var>offset</var>, size_t <var>n</var>)</em></dt>
<dt><a name="index-gsl_005fvector_005fconst_005fsubvector"></a>Function: <em>gsl_vector_const_view</em> <strong>gsl_vector_const_subvector</strong> <em>(const gsl_vector * <var>v</var>, size_t <var>offset</var>, size_t <var>n</var>)</em></dt>
<dd><p>These functions return a vector view of a subvector of another vector
<var>v</var>.  The start of the new vector is offset by <var>offset</var> elements
from the start of the original vector.  The new vector has <var>n</var>
elements.  Mathematically, the <var>i</var>-th element of the new vector
<var>v&rsquo;</var> is given by,
</p>
<div class="example">
<pre class="example">v'(i) = v-&gt;data[(offset + i)*v-&gt;stride]
</pre></div>

<p>where the index <var>i</var> runs from 0 to <code>n-1</code>.
</p>
<p>The <code>data</code> pointer of the returned vector struct is set to null if
the combined parameters (<var>offset</var>,<var>n</var>) overrun the end of the
original vector.
</p>
<p>The new vector is only a view of the block underlying the original
vector, <var>v</var>.  The block containing the elements of <var>v</var> is not
owned by the new vector.  When the view goes out of scope the original
vector <var>v</var> and its block will continue to exist.  The original
memory can only be deallocated by freeing the original vector.  Of
course, the original vector should not be deallocated while the view is
still in use.
</p>
<p>The function <code>gsl_vector_const_subvector</code> is equivalent to
<code>gsl_vector_subvector</code> but can be used for vectors which are
declared <code>const</code>.
</p></dd></dl>


<dl>
<dt><a name="index-gsl_005fvector_005fsubvector_005fwith_005fstride"></a>Function: <em>gsl_vector_view</em> <strong>gsl_vector_subvector_with_stride</strong> <em>(gsl_vector * <var>v</var>, size_t <var>offset</var>, size_t <var>stride</var>, size_t <var>n</var>)</em></dt>
<dt><a name="index-gsl_005fvector_005fconst_005fsubvector_005fwith_005fstride"></a>Function: <em>gsl_vector_const_view</em> <strong>gsl_vector_const_subvector_with_stride</strong> <em>(const gsl_vector * <var>v</var>, size_t <var>offset</var>, size_t <var>stride</var>, size_t <var>n</var>)</em></dt>
<dd><p>These functions return a vector view of a subvector of another vector
<var>v</var> with an additional stride argument. The subvector is formed in
the same way as for <code>gsl_vector_subvector</code> but the new vector has
<var>n</var> elements with a step-size of <var>stride</var> from one element to
the next in the original vector.  Mathematically, the <var>i</var>-th element
of the new vector <var>v&rsquo;</var> is given by,
</p>
<div class="example">
<pre class="example">v'(i) = v-&gt;data[(offset + i*stride)*v-&gt;stride]
</pre></div>

<p>where the index <var>i</var> runs from 0 to <code>n-1</code>.
</p>
<p>Note that subvector views give direct access to the underlying elements
of the original vector. For example, the following code will zero the
even elements of the vector <code>v</code> of length <code>n</code>, while leaving the
odd elements untouched,
</p>
<div class="example">
<pre class="example">gsl_vector_view v_even 
  = gsl_vector_subvector_with_stride (v, 0, 2, n/2);
gsl_vector_set_zero (&amp;v_even.vector);
</pre></div>

<p>A vector view can be passed to any subroutine which takes a vector
argument just as a directly allocated vector would be, using
<code>&amp;</code><var>view</var><code>.vector</code>.  For example, the following code
computes the norm of the odd elements of <code>v</code> using the <small>BLAS</small>
routine <small>DNRM2</small>,
</p>
<div class="example">
<pre class="example">gsl_vector_view v_odd 
  = gsl_vector_subvector_with_stride (v, 1, 2, n/2);
double r = gsl_blas_dnrm2 (&amp;v_odd.vector);
</pre></div>

<p>The function <code>gsl_vector_const_subvector_with_stride</code> is equivalent
to <code>gsl_vector_subvector_with_stride</code> but can be used for vectors
which are declared <code>const</code>.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005fvector_005fcomplex_005freal"></a>Function: <em>gsl_vector_view</em> <strong>gsl_vector_complex_real</strong> <em>(gsl_vector_complex * <var>v</var>)</em></dt>
<dt><a name="index-gsl_005fvector_005fcomplex_005fconst_005freal"></a>Function: <em>gsl_vector_const_view</em> <strong>gsl_vector_complex_const_real</strong> <em>(const gsl_vector_complex * <var>v</var>)</em></dt>
<dd><p>These functions return a vector view of the real parts of the complex
vector <var>v</var>.
</p>
<p>The function <code>gsl_vector_complex_const_real</code> is equivalent to
<code>gsl_vector_complex_real</code> but can be used for vectors which are
declared <code>const</code>.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005fvector_005fcomplex_005fimag"></a>Function: <em>gsl_vector_view</em> <strong>gsl_vector_complex_imag</strong> <em>(gsl_vector_complex * <var>v</var>)</em></dt>
<dt><a name="index-gsl_005fvector_005fcomplex_005fconst_005fimag"></a>Function: <em>gsl_vector_const_view</em> <strong>gsl_vector_complex_const_imag</strong> <em>(const gsl_vector_complex * <var>v</var>)</em></dt>
<dd><p>These functions return a vector view of the imaginary parts of the
complex vector <var>v</var>.
</p>
<p>The function <code>gsl_vector_complex_const_imag</code> is equivalent to
<code>gsl_vector_complex_imag</code> but can be used for vectors which are
declared <code>const</code>.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005fvector_005fview_005farray"></a>Function: <em>gsl_vector_view</em> <strong>gsl_vector_view_array</strong> <em>(double * <var>base</var>, size_t <var>n</var>) </em></dt>
<dt><a name="index-gsl_005fvector_005fconst_005fview_005farray"></a>Function: <em>gsl_vector_const_view</em> <strong>gsl_vector_const_view_array</strong> <em>(const double * <var>base</var>, size_t <var>n</var>)</em></dt>
<dd><p>These functions return a vector view of an array.  The start of the new
vector is given by <var>base</var> and has <var>n</var> elements.  Mathematically,
the <var>i</var>-th element of the new vector <var>v&rsquo;</var> is given by,
</p>
<div class="example">
<pre class="example">v'(i) = base[i]
</pre></div>

<p>where the index <var>i</var> runs from 0 to <code>n-1</code>.
</p>
<p>The array containing the elements of <var>v</var> is not owned by the new
vector view.  When the view goes out of scope the original array will
continue to exist.  The original memory can only be deallocated by
freeing the original pointer <var>base</var>.  Of course, the original array
should not be deallocated while the view is still in use.
</p>
<p>The function <code>gsl_vector_const_view_array</code> is equivalent to
<code>gsl_vector_view_array</code> but can be used for arrays which are
declared <code>const</code>.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005fvector_005fview_005farray_005fwith_005fstride"></a>Function: <em>gsl_vector_view</em> <strong>gsl_vector_view_array_with_stride</strong> <em>(double * <var>base</var>, size_t <var>stride</var>, size_t <var>n</var>)</em></dt>
<dt><a name="index-gsl_005fvector_005fconst_005fview_005farray_005fwith_005fstride"></a>Function: <em>gsl_vector_const_view</em> <strong>gsl_vector_const_view_array_with_stride</strong> <em>(const double * <var>base</var>, size_t <var>stride</var>, size_t <var>n</var>)</em></dt>
<dd><p>These functions return a vector view of an array <var>base</var> with an
additional stride argument. The subvector is formed in the same way as
for <code>gsl_vector_view_array</code> but the new vector has <var>n</var> elements
with a step-size of <var>stride</var> from one element to the next in the
original array.  Mathematically, the <var>i</var>-th element of the new
vector <var>v&rsquo;</var> is given by,
</p>
<div class="example">
<pre class="example">v'(i) = base[i*stride]
</pre></div>

<p>where the index <var>i</var> runs from 0 to <code>n-1</code>.
</p>
<p>Note that the view gives direct access to the underlying elements of the
original array.  A vector view can be passed to any subroutine which
takes a vector argument just as a directly allocated vector would be,
using <code>&amp;</code><var>view</var><code>.vector</code>.
</p>
<p>The function <code>gsl_vector_const_view_array_with_stride</code> is
equivalent to <code>gsl_vector_view_array_with_stride</code> but can be used
for arrays which are declared <code>const</code>.
</p></dd></dl>




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