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<title>GNU Scientific Library &ndash; Reference Manual: Matrix views</title>

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<a name="Matrix-views"></a>
<div class="header">
<p>
Next: <a href="Creating-row-and-column-views.html#Creating-row-and-column-views" accesskey="n" rel="next">Creating row and column views</a>, Previous: <a href="Reading-and-writing-matrices.html#Reading-and-writing-matrices" accesskey="p" rel="previous">Reading and writing matrices</a>, Up: <a href="Matrices.html#Matrices" accesskey="u" rel="up">Matrices</a> &nbsp; [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Matrix-views-1"></a>
<h4 class="subsection">8.4.5 Matrix views</h4>
<a name="index-gsl_005fmatrix_005fview"></a>
<a name="index-gsl_005fmatrix_005fconst_005fview"></a>

<p>A matrix view is a temporary object, stored on the stack, which can be
used to operate on a subset of matrix elements.  Matrix views can be
defined for both constant and non-constant matrices using separate types
that preserve constness.  A matrix view has the type
<code>gsl_matrix_view</code> and a constant matrix view has the type
<code>gsl_matrix_const_view</code>.  In both cases the elements of the view
can by accessed using the <code>matrix</code> component of the view object.  A
pointer <code>gsl_matrix *</code> or <code>const gsl_matrix *</code> can be obtained
by taking the address of the <code>matrix</code> component with the <code>&amp;</code>
operator.  In addition to matrix views it is also possible to create
vector views of a matrix, such as row or column views.
</p>
<dl>
<dt><a name="index-gsl_005fmatrix_005fsubmatrix"></a>Function: <em>gsl_matrix_view</em> <strong>gsl_matrix_submatrix</strong> <em>(gsl_matrix * <var>m</var>, size_t <var>k1</var>, size_t <var>k2</var>, size_t <var>n1</var>, size_t <var>n2</var>)</em></dt>
<dt><a name="index-gsl_005fmatrix_005fconst_005fsubmatrix"></a>Function: <em>gsl_matrix_const_view</em> <strong>gsl_matrix_const_submatrix</strong> <em>(const gsl_matrix * <var>m</var>, size_t <var>k1</var>, size_t <var>k2</var>, size_t <var>n1</var>, size_t <var>n2</var>)</em></dt>
<dd><p>These functions return a matrix view of a submatrix of the matrix
<var>m</var>.  The upper-left element of the submatrix is the element
(<var>k1</var>,<var>k2</var>) of the original matrix.  The submatrix has <var>n1</var>
rows and <var>n2</var> columns.  The physical number of columns in memory
given by <var>tda</var> is unchanged.  Mathematically, the
<em>(i,j)</em>-th element of the new matrix is given by,
</p>
<div class="example">
<pre class="example">m'(i,j) = m-&gt;data[(k1*m-&gt;tda + k2) + i*m-&gt;tda + j]
</pre></div>

<p>where the index <var>i</var> runs from 0 to <code>n1-1</code> and the index <var>j</var>
runs from 0 to <code>n2-1</code>.
</p>
<p>The <code>data</code> pointer of the returned matrix struct is set to null if
the combined parameters (<var>i</var>,<var>j</var>,<var>n1</var>,<var>n2</var>,<var>tda</var>)
overrun the ends of the original matrix.
</p>
<p>The new matrix view is only a view of the block underlying the existing
matrix, <var>m</var>.  The block containing the elements of <var>m</var> is not
owned by the new matrix view.  When the view goes out of scope the
original matrix <var>m</var> and its block will continue to exist.  The
original memory can only be deallocated by freeing the original matrix.
Of course, the original matrix should not be deallocated while the view
is still in use.
</p>
<p>The function <code>gsl_matrix_const_submatrix</code> is equivalent to
<code>gsl_matrix_submatrix</code> but can be used for matrices which are
declared <code>const</code>.
</p></dd></dl>


<dl>
<dt><a name="index-gsl_005fmatrix_005fview_005farray"></a>Function: <em>gsl_matrix_view</em> <strong>gsl_matrix_view_array</strong> <em>(double * <var>base</var>, size_t <var>n1</var>, size_t <var>n2</var>)</em></dt>
<dt><a name="index-gsl_005fmatrix_005fconst_005fview_005farray"></a>Function: <em>gsl_matrix_const_view</em> <strong>gsl_matrix_const_view_array</strong> <em>(const double * <var>base</var>, size_t <var>n1</var>, size_t <var>n2</var>)</em></dt>
<dd><p>These functions return a matrix view of the array <var>base</var>.  The
matrix has <var>n1</var> rows and <var>n2</var> columns.  The physical number of
columns in memory is also given by <var>n2</var>.  Mathematically, the
<em>(i,j)</em>-th element of the new matrix is given by,
</p>
<div class="example">
<pre class="example">m'(i,j) = base[i*n2 + j]
</pre></div>

<p>where the index <var>i</var> runs from 0 to <code>n1-1</code> and the index <var>j</var>
runs from 0 to <code>n2-1</code>.
</p>
<p>The new matrix is only a view of the array <var>base</var>.  When the view
goes out of scope the original array <var>base</var> will continue to exist.
The original memory can only be deallocated by freeing the original
array.  Of course, the original array should not be deallocated while
the view is still in use.
</p>
<p>The function <code>gsl_matrix_const_view_array</code> is equivalent to
<code>gsl_matrix_view_array</code> but can be used for matrices which are
declared <code>const</code>.
</p></dd></dl>


<dl>
<dt><a name="index-gsl_005fmatrix_005fview_005farray_005fwith_005ftda"></a>Function: <em>gsl_matrix_view</em> <strong>gsl_matrix_view_array_with_tda</strong> <em>(double * <var>base</var>, size_t <var>n1</var>, size_t <var>n2</var>, size_t <var>tda</var>)</em></dt>
<dt><a name="index-gsl_005fmatrix_005fconst_005fview_005farray_005fwith_005ftda"></a>Function: <em>gsl_matrix_const_view</em> <strong>gsl_matrix_const_view_array_with_tda</strong> <em>(const double * <var>base</var>, size_t <var>n1</var>, size_t <var>n2</var>, size_t <var>tda</var>)</em></dt>
<dd><p>These functions return a matrix view of the array <var>base</var> with a
physical number of columns <var>tda</var> which may differ from the corresponding
dimension of the matrix.  The matrix has <var>n1</var> rows and <var>n2</var>
columns, and the physical number of columns in memory is given by
<var>tda</var>.  Mathematically, the <em>(i,j)</em>-th element of the new
matrix is given by,
</p>
<div class="example">
<pre class="example">m'(i,j) = base[i*tda + j]
</pre></div>

<p>where the index <var>i</var> runs from 0 to <code>n1-1</code> and the index <var>j</var>
runs from 0 to <code>n2-1</code>.
</p>
<p>The new matrix is only a view of the array <var>base</var>.  When the view
goes out of scope the original array <var>base</var> will continue to exist.
The original memory can only be deallocated by freeing the original
array.  Of course, the original array should not be deallocated while
the view is still in use.
</p>
<p>The function <code>gsl_matrix_const_view_array_with_tda</code> is equivalent
to <code>gsl_matrix_view_array_with_tda</code> but can be used for matrices
which are declared <code>const</code>.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005fmatrix_005fview_005fvector"></a>Function: <em>gsl_matrix_view</em> <strong>gsl_matrix_view_vector</strong> <em>(gsl_vector * <var>v</var>, size_t <var>n1</var>, size_t <var>n2</var>)</em></dt>
<dt><a name="index-gsl_005fmatrix_005fconst_005fview_005fvector"></a>Function: <em>gsl_matrix_const_view</em> <strong>gsl_matrix_const_view_vector</strong> <em>(const gsl_vector * <var>v</var>, size_t <var>n1</var>, size_t <var>n2</var>)</em></dt>
<dd><p>These functions return a matrix view of the vector <var>v</var>.  The matrix
has <var>n1</var> rows and <var>n2</var> columns. The vector must have unit
stride. The physical number of columns in memory is also given by
<var>n2</var>.  Mathematically, the <em>(i,j)</em>-th element of the new
matrix is given by,
</p>
<div class="example">
<pre class="example">m'(i,j) = v-&gt;data[i*n2 + j]
</pre></div>

<p>where the index <var>i</var> runs from 0 to <code>n1-1</code> and the index <var>j</var>
runs from 0 to <code>n2-1</code>.
</p>
<p>The new matrix is only a view of the vector <var>v</var>.  When the view
goes out of scope the original vector <var>v</var> 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_matrix_const_view_vector</code> is equivalent to
<code>gsl_matrix_view_vector</code> but can be used for matrices which are
declared <code>const</code>.
</p></dd></dl>


<dl>
<dt><a name="index-gsl_005fmatrix_005fview_005fvector_005fwith_005ftda"></a>Function: <em>gsl_matrix_view</em> <strong>gsl_matrix_view_vector_with_tda</strong> <em>(gsl_vector * <var>v</var>, size_t <var>n1</var>, size_t <var>n2</var>, size_t <var>tda</var>)</em></dt>
<dt><a name="index-gsl_005fmatrix_005fconst_005fview_005fvector_005fwith_005ftda"></a>Function: <em>gsl_matrix_const_view</em> <strong>gsl_matrix_const_view_vector_with_tda</strong> <em>(const gsl_vector * <var>v</var>, size_t <var>n1</var>, size_t <var>n2</var>, size_t <var>tda</var>)</em></dt>
<dd><p>These functions return a matrix view of the vector <var>v</var> with a
physical number of columns <var>tda</var> which may differ from the
corresponding matrix dimension.  The vector must have unit stride. The
matrix has <var>n1</var> rows and <var>n2</var> columns, and the physical number
of columns in memory is given by <var>tda</var>.  Mathematically, the
<em>(i,j)</em>-th element of the new matrix is given by,
</p>
<div class="example">
<pre class="example">m'(i,j) = v-&gt;data[i*tda + j]
</pre></div>

<p>where the index <var>i</var> runs from 0 to <code>n1-1</code> and the index <var>j</var>
runs from 0 to <code>n2-1</code>.
</p>
<p>The new matrix is only a view of the vector <var>v</var>.  When the view
goes out of scope the original vector <var>v</var> 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_matrix_const_view_vector_with_tda</code> is equivalent
to <code>gsl_matrix_view_vector_with_tda</code> but can be used for matrices
which are declared <code>const</code>.
</p></dd></dl>



<hr>
<div class="header">
<p>
Next: <a href="Creating-row-and-column-views.html#Creating-row-and-column-views" accesskey="n" rel="next">Creating row and column views</a>, Previous: <a href="Reading-and-writing-matrices.html#Reading-and-writing-matrices" accesskey="p" rel="previous">Reading and writing matrices</a>, Up: <a href="Matrices.html#Matrices" accesskey="u" rel="up">Matrices</a> &nbsp; [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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