********************
Vectors and Matrices
********************

.. include:: include.rst

The functions described in this chapter provide a simple vector and
matrix interface to ordinary C arrays. The memory management of these
arrays is implemented using a single underlying type, known as a
block. By writing your functions in terms of vectors and matrices you
can pass a single structure containing both data and dimensions as an
argument without needing additional function parameters.  The structures
are compatible with the vector and matrix formats used by BLAS
routines.

Data types
==========

All the functions are available for each of the standard data-types.
The versions for :code:`double` have the prefix :code:`gsl_block`,
:code:`gsl_vector` and :code:`gsl_matrix`.  Similarly the versions for
single-precision :code:`float` arrays have the prefix
:code:`gsl_block_float`, :code:`gsl_vector_float` and
:code:`gsl_matrix_float`.  The full list of available types is given
below,

=============================== ====================
Prefix                          Type
=============================== ====================
gsl_block                       double         
gsl_block_float                 float         
gsl_block_long_double           long double   
gsl_block_int                   int           
gsl_block_uint                  unsigned int  
gsl_block_long                  long          
gsl_block_ulong                 unsigned long 
gsl_block_short                 short         
gsl_block_ushort                unsigned short
gsl_block_char                  char          
gsl_block_uchar                 unsigned char 
gsl_block_complex               complex double        
gsl_block_complex_float         complex float         
gsl_block_complex_long_double   complex long double   
=============================== ====================

Corresponding types exist for the :code:`gsl_vector` and
:code:`gsl_matrix` functions.

.. index:: blocks

Blocks
======

For consistency all memory is allocated through a :code:`gsl_block`
structure.  The structure contains two components, the size of an area of
memory and a pointer to the memory.  The :code:`gsl_block` structure looks
like this,

.. type:: gsl_block

   ::
   
      typedef struct
      {
        size_t size;
        double * data;
      } gsl_block;

Vectors and matrices are made by *slicing* an underlying block. A
slice is a set of elements formed from an initial offset and a
combination of indices and step-sizes. In the case of a matrix the
step-size for the column index represents the row-length.  The step-size
for a vector is known as the *stride*.

The functions for allocating and deallocating blocks are defined in
:file:`gsl_block.h`.

Block allocation
----------------

The functions for allocating memory to a block follow the style of
:code:`malloc` and :code:`free`.  In addition they also perform their own
error checking.  If there is insufficient memory available to allocate a
block then the functions call the GSL error handler (with an error
number of :macro:`GSL_ENOMEM`) in addition to returning a null
pointer.  Thus if you use the library error handler to abort your program
then it isn't necessary to check every :code:`alloc`.  

.. function:: gsl_block * gsl_block_alloc (size_t n)

   This function allocates memory for a block of :data:`n` double-precision
   elements, returning a pointer to the block struct.  The block is not
   initialized and so the values of its elements are undefined.  Use the
   function :func:`gsl_block_calloc` if you want to ensure that all the
   elements are initialized to zero.

   Zero-sized requests are valid and return a non-null result.  A null pointer
   is returned if insufficient memory is available to create the block.

.. function:: gsl_block * gsl_block_calloc (size_t n)

   This function allocates memory for a block and initializes all the
   elements of the block to zero.

.. function:: void gsl_block_free (gsl_block * b)

   This function frees the memory used by a block :data:`b` previously
   allocated with :func:`gsl_block_alloc` or :func:`gsl_block_calloc`.

Reading and writing blocks
--------------------------

The library provides functions for reading and writing blocks to a file
as binary data or formatted text.

.. function:: int gsl_block_fwrite (FILE * stream, const gsl_block * b)

   This function writes the elements of the block :data:`b` to the stream
   :data:`stream` in binary format.  The return value is 0 for success and
   :macro:`GSL_EFAILED` if there was a problem writing to the file.  Since the
   data is written in the native binary format it may not be portable
   between different architectures.

.. function:: int gsl_block_fread (FILE * stream, gsl_block * b)

   This function reads into the block :data:`b` from the open stream
   :data:`stream` in binary format.  The block :data:`b` must be preallocated
   with the correct length since the function uses the size of :data:`b` to
   determine how many bytes to read.  The return value is 0 for success and
   :macro:`GSL_EFAILED` if there was a problem reading from the file.  The
   data is assumed to have been written in the native binary format on the
   same architecture.

.. function:: int gsl_block_fprintf (FILE * stream, const gsl_block * b, const char * format)

   This function writes the elements of the block :data:`b` line-by-line to
   the stream :data:`stream` using the format specifier :data:`format`, which
   should be one of the :code:`%g`, :code:`%e` or :code:`%f` formats for
   floating point numbers and :code:`%d` for integers.  The function returns
   0 for success and :macro:`GSL_EFAILED` if there was a problem writing to
   the file.

.. function:: int gsl_block_fscanf (FILE * stream, gsl_block * b)

   This function reads formatted data from the stream :data:`stream` into the
   block :data:`b`.  The block :data:`b` must be preallocated with the correct
   length since the function uses the size of :data:`b` to determine how many
   numbers to read.  The function returns 0 for success and
   :macro:`GSL_EFAILED` if there was a problem reading from the file.

Example programs for blocks
---------------------------

The following program shows how to allocate a block,

.. include:: examples/block.c
   :code:

Here is the output from the program,

.. include:: examples/block.txt
   :code:

.. index::
   single: vectors
   single: stride, of vector index

Vectors
=======

Vectors are defined by a :type:`gsl_vector` structure which describes a
slice of a block.  Different vectors can be created which point to the
same block.  A vector slice is a set of equally-spaced elements of an
area of memory.

The :type:`gsl_vector` structure contains five components, the
*size*, the *stride*, a pointer to the memory where the elements
are stored, :data:`data`, a pointer to the block owned by the vector,
:data:`block`, if any, and an ownership flag, :data:`owner`.  The structure
is very simple and looks like this,

.. type:: gsl_vector

   ::

      typedef struct
      {
        size_t size;
        size_t stride;
        double * data;
        gsl_block * block;
        int owner;
      } gsl_vector;

The :data:`size` is simply the number of vector elements.  The range of
valid indices runs from 0 to :code:`size-1`.  The :data:`stride` is the
step-size from one element to the next in physical memory, measured in
units of the appropriate datatype.  The pointer :data:`data` gives the
location of the first element of the vector in memory.  The pointer
:data:`block` stores the location of the memory block in which the vector
elements are located (if any).  If the vector owns this block then the
:data:`owner` field is set to one and the block will be deallocated when the
vector is freed.  If the vector points to a block owned by another
object then the :data:`owner` field is zero and any underlying block will not be
deallocated with the vector.

The functions for allocating and accessing vectors are defined in
:file:`gsl_vector.h`.

Vector allocation
-----------------

The functions for allocating memory to a vector follow the style of
:code:`malloc` and :code:`free`.  In addition they also perform their own
error checking.  If there is insufficient memory available to allocate a
vector then the functions call the GSL error handler (with an error
number of :macro:`GSL_ENOMEM`) in addition to returning a null
pointer.  Thus if you use the library error handler to abort your program
then it isn't necessary to check every :code:`alloc`.

.. function:: gsl_vector * gsl_vector_alloc (size_t n)

   This function creates a vector of length n, returning a pointer to
   a newly initialized vector struct. A new block is allocated for the
   elements of the vector, and stored in the :data:`block` component of the
   vector struct.  The block is "owned" by the vector, and will be
   deallocated when the vector is deallocated.
   Zero-sized requests are valid and return a non-null result.

.. function:: gsl_vector * gsl_vector_calloc (size_t n)

   This function allocates memory for a vector of length :data:`n` and
   initializes all the elements of the vector to zero.

.. function:: void gsl_vector_free (gsl_vector * v)

   This function frees a previously allocated vector :data:`v`.  If the
   vector was created using :func:`gsl_vector_alloc` then the block
   underlying the vector will also be deallocated.  If the vector has
   been created from another object then the memory is still owned by
   that object and will not be deallocated.

.. index::
   single: vectors, range-checking
   single: range-checking for vectors
   single: bounds checking, extension to GCC
   single: gcc extensions, range-checking
   single: Fortran range checking, equivalent in gcc

Accessing vector elements
-------------------------

Unlike Fortran compilers, C compilers do not usually provide
support for range checking of vectors and matrices. [#f1]_
The functions :func:`gsl_vector_get` and
:func:`gsl_vector_set` can perform portable range checking for you and
report an error if you attempt to access elements outside the allowed
range.

The functions for accessing the elements of a vector or matrix are
defined in :file:`gsl_vector.h` and declared :code:`extern inline` to
eliminate function-call overhead.  You must compile your program with
the preprocessor macro :macro:`HAVE_INLINE` defined to use these
functions.

.. macro:: GSL_RANGE_CHECK_OFF

   If necessary you can turn off range checking completely without
   modifying any source files by recompiling your program with the
   preprocessor definition :macro:`GSL_RANGE_CHECK_OFF`.  Provided your
   compiler supports inline functions the effect of turning off range
   checking is to replace calls to :code:`gsl_vector_get(v,i)` by
   :code:`v->data[i*v->stride]` and calls to :code:`gsl_vector_set(v,i,x)` by
   :code:`v->data[i*v->stride]=x`.  Thus there should be no performance
   penalty for using the range checking functions when range checking is
   turned off.

.. macro:: GSL_C99_INLINE

   If you use a C99 compiler which requires inline functions in header
   files to be declared :code:`inline` instead of :code:`extern inline`,
   define the macro :macro:`GSL_C99_INLINE` (see :ref:`sec_inline-functions`).
   With GCC this is selected automatically when compiling in C99 mode
   (:code:`-std=c99`).

.. var:: int gsl_check_range

   If inline functions are not used, calls to the functions
   :func:`gsl_vector_get` and :func:`gsl_vector_set` will link to the
   compiled versions of these functions in the library itself.  The range
   checking in these functions is controlled by the global integer
   variable :code:`gsl_check_range`.  It is enabled by default---to
   disable it, set :code:`gsl_check_range` to zero.  Due to function-call
   overhead, there is less benefit in disabling range checking here than
   for inline functions.

.. function:: double gsl_vector_get (const gsl_vector * v, const size_t i)

   This function returns the :data:`i`-th element of a vector :data:`v`.  If
   :data:`i` lies outside the allowed range of 0 to :code:`size - 1` then the error
   handler is invoked and 0 is returned. |inlinefn|

.. function:: void gsl_vector_set (gsl_vector * v, const size_t i, double x)

   This function sets the value of the :data:`i`-th element of a vector
   :data:`v` to :data:`x`.  If :data:`i` lies outside the allowed range of 0 to
   :code:`size - 1` then the error handler is invoked. |inlinefn|

.. function:: double * gsl_vector_ptr (gsl_vector * v, size_t i)
              const double * gsl_vector_const_ptr (const gsl_vector * v, size_t i)

   These functions return a pointer to the :data:`i`-th element of a vector
   :data:`v`.  If :data:`i` lies outside the allowed range of 0 to :code:`size - 1`
   then the error handler is invoked and a null pointer is returned. |inlinefns|

.. index::
   single: vectors, initializing
   single: initializing vectors

Initializing vector elements
----------------------------

.. function:: void gsl_vector_set_all (gsl_vector * v, double x)

   This function sets all the elements of the vector :data:`v` to the value
   :data:`x`.

.. function:: void gsl_vector_set_zero (gsl_vector * v)

   This function sets all the elements of the vector :data:`v` to zero.

.. function:: int gsl_vector_set_basis (gsl_vector * v, size_t i)

   This function makes a basis vector by setting all the elements of the
   vector :data:`v` to zero except for the :data:`i`-th element which is set to
   one.

Reading and writing vectors
---------------------------

The library provides functions for reading and writing vectors to a file
as binary data or formatted text.

.. function:: int gsl_vector_fwrite (FILE * stream, const gsl_vector * v)

   This function writes the elements of the vector :data:`v` to the stream
   :data:`stream` in binary format.  The return value is 0 for success and
   :macro:`GSL_EFAILED` if there was a problem writing to the file.  Since the
   data is written in the native binary format it may not be portable
   between different architectures.

.. function:: int gsl_vector_fread (FILE * stream, gsl_vector * v)

   This function reads into the vector :data:`v` from the open stream
   :data:`stream` in binary format.  The vector :data:`v` must be preallocated
   with the correct length since the function uses the size of :data:`v` to
   determine how many bytes to read.  The return value is 0 for success and
   :macro:`GSL_EFAILED` if there was a problem reading from the file.  The
   data is assumed to have been written in the native binary format on the
   same architecture.

.. function:: int gsl_vector_fprintf (FILE * stream, const gsl_vector * v, const char * format)

   This function writes the elements of the vector :data:`v` line-by-line to
   the stream :data:`stream` using the format specifier :data:`format`, which
   should be one of the :code:`%g`, :code:`%e` or :code:`%f` formats for
   floating point numbers and :code:`%d` for integers.  The function returns
   0 for success and :macro:`GSL_EFAILED` if there was a problem writing to
   the file.

.. function:: int gsl_vector_fscanf (FILE * stream, gsl_vector * v)

   This function reads formatted data from the stream :data:`stream` into the
   vector :data:`v`.  The vector :data:`v` must be preallocated with the correct
   length since the function uses the size of :data:`v` to determine how many
   numbers to read.  The function returns 0 for success and
   :macro:`GSL_EFAILED` if there was a problem reading from the file.

Vector views
------------

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.

.. type:: gsl_vector_view
          gsl_vector_const_view

   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
   :type:`gsl_vector_view` and a constant vector view has the type
   :type:`gsl_vector_const_view`.  In both cases the elements of the view
   can be accessed as a :type:`gsl_vector` using the :code:`vector` component
   of the view object.  A pointer to a vector of type :code:`gsl_vector *`
   or :code:`const gsl_vector *` can be obtained by taking the address of
   this component with the :code:`&` operator.  

   When using this pointer it is important to ensure that the view itself
   remains in scope---the simplest way to do so is by always writing the
   pointer as :code:`&view.vector`, and never storing this value
   in another variable.  

.. function:: gsl_vector_view gsl_vector_subvector (gsl_vector * v, size_t offset, size_t n)
              gsl_vector_const_view gsl_vector_const_subvector (const gsl_vector * v, size_t offset, size_t n)

   These functions return a vector view of a subvector of another vector
   :data:`v`.  The start of the new vector is offset by :data:`offset` elements
   from the start of the original vector.  The new vector has :data:`n`
   elements.  Mathematically, the :data:`i`-th element of the new vector
   :code:`v'` is given by::

      v'(i) = v->data[(offset + i)*v->stride]

   where the index :data:`i` runs from 0 to :code:`n - 1`.

   The :code:`data` pointer of the returned vector struct is set to null if
   the combined parameters (:data:`offset`, :data:`n`) overrun the end of the
   original vector.

   The new vector is only a view of the block underlying the original
   vector, :data:`v`.  The block containing the elements of :data:`v` is not
   owned by the new vector.  When the view goes out of scope the original
   vector :data:`v` 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.

   The function :func:`gsl_vector_const_subvector` is equivalent to
   :func:`gsl_vector_subvector` but can be used for vectors which are
   declared :code:`const`.

.. function:: gsl_vector_view gsl_vector_subvector_with_stride (gsl_vector * v, size_t offset, size_t stride, size_t n)
              gsl_vector_const_view gsl_vector_const_subvector_with_stride (const gsl_vector * v, size_t offset, size_t stride, size_t n)

   These functions return a vector view of a subvector of another vector
   :data:`v` with an additional stride argument. The subvector is formed in
   the same way as for :func:`gsl_vector_subvector` but the new vector has
   :data:`n` elements with a step-size of :data:`stride` from one element to
   the next in the original vector.  Mathematically, the :data:`i`-th element
   of the new vector :code:`v'` is given by::

      v'(i) = v->data[(offset + i*stride)*v->stride]

   where the index :data:`i` runs from 0 to :code:`n - 1`.

   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 :data:`v` of length :code:`n`, while leaving the
   odd elements untouched::

      gsl_vector_view v_even = gsl_vector_subvector_with_stride (v, 0, 2, n/2);
      gsl_vector_set_zero (&v_even.vector);

   A vector view can be passed to any subroutine which takes a vector
   argument just as a directly allocated vector would be, using
   :code:`&view.vector`.  For example, the following code
   computes the norm of the odd elements of :data:`v` using the BLAS
   routine :code:`dnrm2`::

      gsl_vector_view v_odd = gsl_vector_subvector_with_stride (v, 1, 2, n/2);
      double r = gsl_blas_dnrm2 (&v_odd.vector);

   The function :func:`gsl_vector_const_subvector_with_stride` is equivalent
   to :func:`gsl_vector_subvector_with_stride` but can be used for vectors
   which are declared :code:`const`.

.. function:: gsl_vector_view gsl_vector_complex_real (gsl_vector_complex * v)
              gsl_vector_const_view gsl_vector_complex_const_real (const gsl_vector_complex * v)

   These functions return a vector view of the real parts of the complex
   vector :data:`v`.

   The function :func:`gsl_vector_complex_const_real` is equivalent to
   :func:`gsl_vector_complex_real` but can be used for vectors which are
   declared :code:`const`.

.. function:: gsl_vector_view gsl_vector_complex_imag (gsl_vector_complex * v)
              gsl_vector_const_view gsl_vector_complex_const_imag (const gsl_vector_complex * v)

   These functions return a vector view of the imaginary parts of the
   complex vector :data:`v`.

   The function :func:`gsl_vector_complex_const_imag` is equivalent to
   :func:`gsl_vector_complex_imag` but can be used for vectors which are
   declared :code:`const`.

.. function:: gsl_vector_view gsl_vector_view_array (double * base, size_t n) 
              gsl_vector_const_view gsl_vector_const_view_array (const double * base, size_t n)

   These functions return a vector view of an array.  The start of the new
   vector is given by :data:`base` and has :data:`n` elements.  Mathematically,
   the :data:`i`-th element of the new vector :code:`v'` is given by::

      v'(i) = base[i]

   where the index :data:`i` runs from 0 to :code:`n - 1`.

   The array containing the elements of :data:`v` 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 :data:`base`.  Of course, the original array
   should not be deallocated while the view is still in use.

   The function :func:`gsl_vector_const_view_array` is equivalent to
   :func:`gsl_vector_view_array` but can be used for arrays which are
   declared :code:`const`.

.. function:: gsl_vector_view gsl_vector_view_array_with_stride (double * base, size_t stride, size_t n)
              gsl_vector_const_view gsl_vector_const_view_array_with_stride (const double * base, size_t stride, size_t n)

   These functions return a vector view of an array :data:`base` with an
   additional stride argument. The subvector is formed in the same way as
   for :func:`gsl_vector_view_array` but the new vector has :data:`n` elements
   with a step-size of :data:`stride` from one element to the next in the
   original array.  Mathematically, the :data:`i`-th element of the new
   vector :code:`v'` is given by::

      v'(i) = base[i*stride]

   where the index :data:`i` runs from 0 to :code:`n - 1`.

   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:`&view.vector`.

   The function :func:`gsl_vector_const_view_array_with_stride` is
   equivalent to :func:`gsl_vector_view_array_with_stride` but can be used
   for arrays which are declared :code:`const`.

.. @node Modifying subvector views
.. @subsection Modifying subvector views
.. 
.. @deftypefun int gsl_vector_view_from_vector (gsl_vector * v, gsl_vector * base, size_t offset, size_t n, size_t stride)
.. This function modifies and existing vector view :data:`v` to form a new
.. view of a vector :data:`base`, starting from element :data:`offset`.  The
.. vector has :data:`n` elements separated by stride :data:`stride`.  Any
.. existing view in :data:`v` will be lost as a result of this function.
.. @end deftypefun
.. 
.. @deftypefun int gsl_vector_view_from_array (gsl_vector * v, double * base, size_t offset, size_t n, size_t stride)
.. This function modifies and existing vector view :data:`v` to form a new
.. view of an array :data:`base`, starting from element :data:`offset`.  The
.. vector has :data:`n` elements separated by stride :data:`stride`.  Any
.. existing view in :data:`v` will be lost as a result of this function.
.. @end deftypefun

.. @deftypefun {gsl_vector *} gsl_vector_alloc_from_block (gsl_block * b, size_t offset, size_t n, size_t stride)
.. This function creates a vector as a slice of an existing block :data:`b`,
.. returning a pointer to a newly initialized vector struct.  The start of
.. the vector is offset by :data:`offset` elements from the start of the
.. block.  The vector has :data:`n` elements, with a step-size of :data:`stride`
.. from one element to the next.  Mathematically, the :data:`i`-th element of
.. the vector is given by,
.. 
.. @example
.. v(i) = b->data[offset + i*stride]
.. @end example
.. @noindent
.. where the index :data:`i` runs from 0 to :code:`n-1`.
.. 
.. A null pointer is returned if the combined parameters
.. (:data:`offset`, :data:`n`, :data:`stride`) overrun the end of the block or if
.. insufficient memory is available to store the vector.
.. 
.. The vector is only a view of the block :data:`b`, and the block is not
.. owned by the vector.  When the vector is deallocated the block :data:`b`
.. will continue to exist.  This memory can only be deallocated by freeing
.. the block itself.  Of course, this block should not be deallocated while
.. the vector is still in use.
.. @end deftypefun
.. 
.. @deftypefun {gsl_vector *} gsl_vector_alloc_from_vector (gsl_vector * :data:`v`, size_t offset, size_t n, size_t stride)
.. This function creates a vector as a slice of another vector :data:`v`,
.. returning a pointer to a newly initialized vector struct.  The start of
.. the new vector is offset by :data:`offset` elements from the start of the
.. original vector.  The new vector has :data:`n` elements, with a step-size
.. of :data:`stride` from one element to the next in the original vector.
.. Mathematically, the :data:`i`-th element of the new vector :data:`v'` is
.. given by,
.. 
.. @example
.. v'(i) = v->data[(offset + i*stride)*v->stride]
.. @end example
.. @noindent
.. where the index :data:`i` runs from 0 to :code:`n-1`.
.. 
.. A null pointer is returned if the combined parameters
.. (:data:`offset`, :data:`n`, :data:`stride`) overrun the end of the original
.. vector or if insufficient memory is available store the new vector.
.. 
.. The new vector is only a view of the block underlying the original
.. vector, :data:`v`.  The block is not owned by the new vector.  When the new
.. vector is deallocated the original vector :data:`v` 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 new vector is still in use.
.. @end deftypefun

Copying vectors
---------------

Common operations on vectors such as addition and multiplication are
available in the BLAS part of the library (see :ref:`chap_blas-support`).
However, it is useful to have a small number of utility
functions which do not require the full BLAS code.  The following
functions fall into this category.

.. function:: int gsl_vector_memcpy (gsl_vector * dest, const gsl_vector * src)

   This function copies the elements of the vector :data:`src` into the
   vector :data:`dest`.  The two vectors must have the same length.

.. function:: int gsl_vector_swap (gsl_vector * v, gsl_vector * w)

   This function exchanges the elements of the vectors :data:`v` and :data:`w`
   by copying.  The two vectors must have the same length.

Exchanging elements
-------------------

The following functions can be used to exchange, or permute, the elements
of a vector.

.. function:: int gsl_vector_swap_elements (gsl_vector * v, size_t i, size_t j)

   This function exchanges the :data:`i`-th and :data:`j`-th elements of the
   vector :data:`v` in-place.

.. function:: int gsl_vector_reverse (gsl_vector * v)

   This function reverses the order of the elements of the vector :data:`v`.

Vector operations
-----------------

.. function:: int gsl_vector_add (gsl_vector * a, const gsl_vector * b)

   This function adds the elements of vector :data:`b` to the elements of
   vector :data:`a`.  The result :math:`a_i \leftarrow a_i + b_i` is stored
   in :data:`a` and :data:`b` remains unchanged.  The two vectors must have
   the same length.

.. function:: int gsl_vector_sub (gsl_vector * a, const gsl_vector * b)

   This function subtracts the elements of vector :data:`b` from the elements of
   vector :data:`a`.  The result :math:`a_i \leftarrow a_i - b_i` is stored
   in :data:`a` and :data:`b` remains unchanged.  The two vectors must have the
   same length.

.. function:: int gsl_vector_mul (gsl_vector * a, const gsl_vector * b)

   This function multiplies the elements of vector :data:`a` by the
   elements of vector :data:`b`.  The result :math:`a_i \leftarrow a_i * b_i`
   is stored in :data:`a` and :data:`b` remains unchanged. The two
   vectors must have the same length.

.. function:: int gsl_vector_div (gsl_vector * a, const gsl_vector * b)

   This function divides the elements of vector :data:`a` by the elements
   of vector :data:`b`.  The result :math:`a_i \leftarrow a_i / b_i` is
   stored in :data:`a` and :data:`b` remains unchanged. The two vectors must
   have the same length.

.. function:: int gsl_vector_scale (gsl_vector * a, const double x)

   This function multiplies the elements of vector :data:`a` by the
   constant factor :data:`x`.  The result :math:`a_i \leftarrow x a_i` is
   stored in :data:`a`.

.. function:: int gsl_vector_add_constant (gsl_vector * a, const double x)

   This function adds the constant value :data:`x` to the elements of the
   vector :data:`a`.  The result :math:`a_i \leftarrow a_i + x` is stored in
   :data:`a`.

.. function:: double gsl_vector_sum (const gsl_vector * a)

   This function returns the sum of the elements of :data:`a`, defined as :math:`\sum_{i=1}^n a_i`

.. function:: int gsl_vector_axpby (const double alpha, const gsl_vector * x, const double beta, gsl_vector * y)

   This function performs the operation :math:`y \leftarrow \alpha x + \beta y`. The
   vectors :data:`x` and :data:`y` must have the same length.

Finding maximum and minimum elements of vectors
-----------------------------------------------

The following operations are only defined for real vectors.

.. function:: double gsl_vector_max (const gsl_vector * v)

   This function returns the maximum value in the vector :data:`v`.

.. function:: double gsl_vector_min (const gsl_vector * v)

   This function returns the minimum value in the vector :data:`v`.

.. function:: void gsl_vector_minmax (const gsl_vector * v, double * min_out, double * max_out)

   This function returns the minimum and maximum values in the vector
   :data:`v`, storing them in :data:`min_out` and :data:`max_out`.

.. function:: size_t gsl_vector_max_index (const gsl_vector * v)

   This function returns the index of the maximum value in the vector :data:`v`.
   When there are several equal maximum elements then the lowest index is
   returned.

.. function:: size_t gsl_vector_min_index (const gsl_vector * v)

   This function returns the index of the minimum value in the vector :data:`v`.
   When there are several equal minimum elements then the lowest index is
   returned.

.. function:: void gsl_vector_minmax_index (const gsl_vector * v, size_t * imin, size_t * imax)

   This function returns the indices of the minimum and maximum values in
   the vector :data:`v`, storing them in :data:`imin` and :data:`imax`. When
   there are several equal minimum or maximum elements then the lowest
   indices are returned.

Vector properties
-----------------

The following functions are defined for real and complex vectors.  For
complex vectors both the real and imaginary parts must satisfy the
conditions.

.. function:: int gsl_vector_isnull (const gsl_vector * v)
              int gsl_vector_ispos (const gsl_vector * v)
              int gsl_vector_isneg (const gsl_vector * v)
              int gsl_vector_isnonneg (const gsl_vector * v)

   These functions return 1 if all the elements of the vector :data:`v` are
   zero, strictly positive, strictly negative, or non-negative
   respectively, and 0 otherwise.

.. function:: int gsl_vector_equal (const gsl_vector * u, const gsl_vector * v)

   This function returns 1 if the vectors :data:`u` and :data:`v` are equal
   (by comparison of element values) and 0 otherwise.

Example programs for vectors
----------------------------

This program shows how to allocate, initialize and read from a vector
using the functions :func:`gsl_vector_alloc`, :func:`gsl_vector_set` and
:func:`gsl_vector_get`.

.. include:: examples/vector.c
   :code:

Here is the output from the program.  The final loop attempts to read
outside the range of the vector :code:`v`, and the error is trapped by
the range-checking code in :func:`gsl_vector_get`.

::

  $ ./a.out
  v_0 = 1.23
  v_1 = 2.23
  v_2 = 3.23
  gsl: vector_source.c:12: ERROR: index out of range
  Default GSL error handler invoked.
  Aborted (core dumped)

The next program shows how to write a vector to a file.

.. include:: examples/vectorw.c
   :code:

After running this program the file :file:`test.dat` should contain the
elements of :code:`v`, written using the format specifier
:code:`%.5g`.  The vector could then be read back in using the function
:code:`gsl_vector_fscanf (f, v)` as follows:

.. include:: examples/vectorr.c
   :code:

.. index::
   single: matrices
   single: physical dimension, matrices
   single: trailing dimension, matrices
   single: leading dimension, matrices
   single: ordering, matrix elements

Matrices
========

Matrices are defined by a :type:`gsl_matrix` structure which describes a
generalized slice of a block.  Like a vector it represents a set of
elements in an area of memory, but uses two indices instead of one.

.. type:: gsl_matrix

   The :type:`gsl_matrix` structure contains six components, the two
   dimensions of the matrix, a physical dimension, a pointer to the memory
   where the elements of the matrix are stored, :data:`data`, a pointer to
   the block owned by the matrix :data:`block`, if any, and an ownership
   flag, :data:`owner`.  The physical dimension determines the memory layout
   and can differ from the matrix dimension to allow the use of
   submatrices.  The :type:`gsl_matrix` structure is very simple and looks
   like this::

      typedef struct
      {
        size_t size1;
        size_t size2;
        size_t tda;
        double * data;
        gsl_block * block;
        int owner;
      } gsl_matrix;

Matrices are stored in row-major order, meaning that each row of
elements forms a contiguous block in memory.  This is the standard
"C-language ordering" of two-dimensional arrays. Note that Fortran
stores arrays in column-major order. The number of rows is :data:`size1`.
The range of valid row indices runs from 0 to :code:`size1 - 1`.  Similarly
:data:`size2` is the number of columns.  The range of valid column indices
runs from 0 to :code:`size2 - 1`.  The physical row dimension :data:`tda`, or
*trailing dimension*, specifies the size of a row of the matrix as
laid out in memory.

For example, in the following matrix :data:`size1` is 3, :data:`size2` is 4,
and :data:`tda` is 8.  The physical memory layout of the matrix begins in
the top left hand-corner and proceeds from left to right along each row
in turn.

::

   00 01 02 03 XX XX XX XX
   10 11 12 13 XX XX XX XX
   20 21 22 23 XX XX XX XX

Each unused memory location is represented by ":code:`XX`".  The
pointer :data:`data` gives the location of the first element of the matrix
in memory.  The pointer :data:`block` stores the location of the memory
block in which the elements of the matrix are located (if any).  If the
matrix owns this block then the :data:`owner` field is set to one and the
block will be deallocated when the matrix is freed.  If the matrix is
only a slice of a block owned by another object then the :data:`owner` field is
zero and any underlying block will not be freed.

The functions for allocating and accessing matrices are defined in
:file:`gsl_matrix.h`.

Matrix allocation
-----------------

The functions for allocating memory to a matrix follow the style of
:code:`malloc` and :code:`free`.  They also perform their own error
checking.  If there is insufficient memory available to allocate a matrix
then the functions call the GSL error handler (with an error number of
:macro:`GSL_ENOMEM`) in addition to returning a null pointer.  Thus if you
use the library error handler to abort your program then it isn't
necessary to check every :code:`alloc`.

.. function:: gsl_matrix * gsl_matrix_alloc (size_t n1, size_t n2)

   This function creates a matrix of size :data:`n1` rows by :data:`n2` columns,
   returning a pointer to a newly initialized matrix struct. A new block is
   allocated for the elements of the matrix, and stored in the :data:`block`
   component of the matrix struct.  The block is "owned" by the matrix,
   and will be deallocated when the matrix is deallocated.  Requesting zero
   for :data:`n1` or :data:`n2` is valid and returns a non-null result.

.. function:: gsl_matrix * gsl_matrix_calloc (size_t n1, size_t n2)

   This function allocates memory for a matrix of size :data:`n1` rows by
   :data:`n2` columns and initializes all the elements of the matrix to zero.

.. function:: void gsl_matrix_free (gsl_matrix * m)

   This function frees a previously allocated matrix :data:`m`.  If the
   matrix was created using :func:`gsl_matrix_alloc` then the block
   underlying the matrix will also be deallocated.  If the matrix has
   been created from another object then the memory is still owned by
   that object and will not be deallocated.

.. index::
   single: matrices, range-checking
   single: range-checking for matrices

Accessing matrix elements
-------------------------

The functions for accessing the elements of a matrix use the same range
checking system as vectors.  You can turn off range checking by recompiling
your program with the preprocessor definition
:macro:`GSL_RANGE_CHECK_OFF`.

The elements of the matrix are stored in "C-order", where the second
index moves continuously through memory.  More precisely, the element
accessed by the function :code:`gsl_matrix_get(m,i,j)` and
:code:`gsl_matrix_set(m,i,j,x)` is::

   m->data[i * m->tda + j]

where :data:`tda` is the physical row-length of the matrix.

.. function:: double gsl_matrix_get (const gsl_matrix * m, const size_t i, const size_t j)

   This function returns the :math:`(i,j)`-th element of a matrix
   :data:`m`.  If :data:`i` or :data:`j` lie outside the allowed range of 0 to
   :code:`n1 - 1` and 0 to :code:`n2 - 1` then the error handler is invoked and 0
   is returned. |inlinefn|

.. function:: void gsl_matrix_set (gsl_matrix * m, const size_t i, const size_t j, double x)

   This function sets the value of the :math:`(i,j)`-th element of a
   matrix :data:`m` to :data:`x`.  If :data:`i` or :data:`j` lies outside the
   allowed range of 0 to :code:`n1 - 1` and 0 to :code:`n2 - 1` then the error
   handler is invoked. |inlinefn|

.. function:: double * gsl_matrix_ptr (gsl_matrix * m, size_t i, size_t j)
              const double * gsl_matrix_const_ptr (const gsl_matrix * m, size_t i, size_t j)

   These functions return a pointer to the :math:`(i,j)`-th element of a
   matrix :data:`m`.  If :data:`i` or :data:`j` lie outside the allowed range of
   0 to :code:`n1 - 1` and 0 to :code:`n2 - 1` then the error handler is invoked
   and a null pointer is returned. |inlinefns|

.. index::
   single: matrices, initializing
   single: initializing matrices
   single: identity matrix
   single: matrix, identity
   single: zero matrix
   single: matrix, zero
   single: constant matrix
   single: matrix, constant

Initializing matrix elements
----------------------------

.. function:: void gsl_matrix_set_all (gsl_matrix * m, double x)

   This function sets all the elements of the matrix :data:`m` to the value
   :data:`x`.

.. function:: void gsl_matrix_set_zero (gsl_matrix * m)

   This function sets all the elements of the matrix :data:`m` to zero.

.. function:: void gsl_matrix_set_identity (gsl_matrix * m)

   This function sets the elements of the matrix :data:`m` to the
   corresponding elements of the identity matrix, :math:`m(i,j) = \delta(i,j)`,
   i.e. a unit diagonal with all off-diagonal elements zero.
   This applies to both square and rectangular matrices.

Reading and writing matrices
----------------------------

The library provides functions for reading and writing matrices to a file
as binary data or formatted text.

.. function:: int gsl_matrix_fwrite (FILE * stream, const gsl_matrix * m)

   This function writes the elements of the matrix :data:`m` to the stream
   :data:`stream` in binary format.  The return value is 0 for success and
   :macro:`GSL_EFAILED` if there was a problem writing to the file.  Since the
   data is written in the native binary format it may not be portable
   between different architectures.

.. function:: int gsl_matrix_fread (FILE * stream, gsl_matrix * m)

   This function reads into the matrix :data:`m` from the open stream
   :data:`stream` in binary format.  The matrix :data:`m` must be preallocated
   with the correct dimensions since the function uses the size of :data:`m` to
   determine how many bytes to read.  The return value is 0 for success and
   :macro:`GSL_EFAILED` if there was a problem reading from the file.  The
   data is assumed to have been written in the native binary format on the
   same architecture.

.. function:: int gsl_matrix_fprintf (FILE * stream, const gsl_matrix * m, const char * format)

   This function writes the elements of the matrix :data:`m` line-by-line to
   the stream :data:`stream` using the format specifier :data:`format`, which
   should be one of the :code:`%g`, :code:`%e` or :code:`%f` formats for
   floating point numbers and :code:`%d` for integers.  The function returns
   0 for success and :macro:`GSL_EFAILED` if there was a problem writing to
   the file.

.. function:: int gsl_matrix_fscanf (FILE * stream, gsl_matrix * m)

   This function reads formatted data from the stream :data:`stream` into the
   matrix :data:`m`.  The matrix :data:`m` must be preallocated with the correct
   dimensions since the function uses the size of :data:`m` to determine how many
   numbers to read.  The function returns 0 for success and
   :macro:`GSL_EFAILED` if there was a problem reading from the file.

Matrix views
------------

.. type:: gsl_matrix_view
          gsl_matrix_const_view

   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
   :type:`gsl_matrix_view` and a constant matrix view has the type
   :type:`gsl_matrix_const_view`.  In both cases the elements of the view
   can by accessed using the :code:`matrix` component of the view object.  A
   pointer :code:`gsl_matrix *` or :code:`const gsl_matrix *` can be obtained
   by taking the address of the :code:`matrix` component with the :code:`&`
   operator.  In addition to matrix views it is also possible to create
   vector views of a matrix, such as row or column views.

.. function:: gsl_matrix_view gsl_matrix_submatrix (gsl_matrix * m, size_t k1, size_t k2, size_t n1, size_t n2)
              gsl_matrix_const_view gsl_matrix_const_submatrix (const gsl_matrix * m, size_t k1, size_t k2, size_t n1, size_t n2)

   These functions return a matrix view of a submatrix of the matrix
   :data:`m`.  The upper-left element of the submatrix is the element
   (:data:`k1`, :data:`k2`) of the original matrix.  The submatrix has :data:`n1`
   rows and :data:`n2` columns.  The physical number of columns in memory
   given by :data:`tda` is unchanged.  Mathematically, the
   :math:`(i,j)`-th element of the new matrix is given by::

      m'(i,j) = m->data[(k1*m->tda + k2) + i*m->tda + j]

   where the index :data:`i` runs from 0 to :code:`n1 - 1` and the index :data:`j`
   runs from 0 to :code:`n2 - 1`.

   The :code:`data` pointer of the returned matrix struct is set to null if
   the combined parameters (:data:`i`, :data:`j`, :data:`n1`, :data:`n2`, :data:`tda`)
   overrun the ends of the original matrix.

   The new matrix view is only a view of the block underlying the existing
   matrix, :data:`m`.  The block containing the elements of :data:`m` is not
   owned by the new matrix view.  When the view goes out of scope the
   original matrix :data:`m` 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.

   The function :func:`gsl_matrix_const_submatrix` is equivalent to
   :func:`gsl_matrix_submatrix` but can be used for matrices which are
   declared :code:`const`.

.. function:: gsl_matrix_view gsl_matrix_view_array (double * base, size_t n1, size_t n2)
              gsl_matrix_const_view gsl_matrix_const_view_array (const double * base, size_t n1, size_t n2)

   These functions return a matrix view of the array :data:`base`.  The
   matrix has :data:`n1` rows and :data:`n2` columns.  The physical number of
   columns in memory is also given by :data:`n2`.  Mathematically, the
   :math:`(i,j)`-th element of the new matrix is given by::

      m'(i,j) = base[i*n2 + j]

   where the index :data:`i` runs from 0 to :code:`n1 - 1` and the index :data:`j`
   runs from 0 to :code:`n2 - 1`.

   The new matrix is only a view of the array :data:`base`.  When the view
   goes out of scope the original array :data:`base` 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.

   The function :func:`gsl_matrix_const_view_array` is equivalent to
   :func:`gsl_matrix_view_array` but can be used for matrices which are
   declared :code:`const`.

.. function:: gsl_matrix_view gsl_matrix_view_array_with_tda (double * base, size_t n1, size_t n2, size_t tda)
              gsl_matrix_const_view gsl_matrix_const_view_array_with_tda (const double * base, size_t n1, size_t n2, size_t tda)

   These functions return a matrix view of the array :data:`base` with a
   physical number of columns :data:`tda` which may differ from the corresponding
   dimension of the matrix.  The matrix has :data:`n1` rows and :data:`n2`
   columns, and the physical number of columns in memory is given by
   :data:`tda`.  Mathematically, the :math:`(i,j)`-th element of the new
   matrix is given by::

      m'(i,j) = base[i*tda + j]

   where the index :data:`i` runs from 0 to :code:`n1 - 1` and the index :data:`j`
   runs from 0 to :code:`n2 - 1`.

   The new matrix is only a view of the array :data:`base`.  When the view
   goes out of scope the original array :data:`base` 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.

   The function :func:`gsl_matrix_const_view_array_with_tda` is equivalent
   to :func:`gsl_matrix_view_array_with_tda` but can be used for matrices
   which are declared :code:`const`.

.. function:: gsl_matrix_view gsl_matrix_view_vector (gsl_vector * v, size_t n1, size_t n2)
              gsl_matrix_const_view gsl_matrix_const_view_vector (const gsl_vector * v, size_t n1, size_t n2)

   These functions return a matrix view of the vector :data:`v`.  The matrix
   has :data:`n1` rows and :data:`n2` columns. The vector must have unit
   stride. The physical number of columns in memory is also given by
   :data:`n2`.  Mathematically, the :math:`(i,j)`-th element of the new
   matrix is given by::

      m'(i,j) = v->data[i*n2 + j]

   where the index :data:`i` runs from 0 to :code:`n1 - 1` and the index :data:`j`
   runs from 0 to :code:`n2 - 1`.

   The new matrix is only a view of the vector :data:`v`.  When the view
   goes out of scope the original vector :data:`v` 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.

   The function :func:`gsl_matrix_const_view_vector` is equivalent to
   :func:`gsl_matrix_view_vector` but can be used for matrices which are
   declared :code:`const`.

.. function:: gsl_matrix_view gsl_matrix_view_vector_with_tda (gsl_vector * v, size_t n1, size_t n2, size_t tda)
              gsl_matrix_const_view gsl_matrix_const_view_vector_with_tda (const gsl_vector * v, size_t n1, size_t n2, size_t tda)

   These functions return a matrix view of the vector :data:`v` with a
   physical number of columns :data:`tda` which may differ from the
   corresponding matrix dimension.  The vector must have unit stride. The
   matrix has :data:`n1` rows and :data:`n2` columns, and the physical number
   of columns in memory is given by :data:`tda`.  Mathematically, the
   :math:`(i,j)`-th element of the new matrix is given by::

      m'(i,j) = v->data[i*tda + j]

   where the index :data:`i` runs from 0 to :code:`n1 - 1` and the index :data:`j`
   runs from 0 to :code:`n2 - 1`.

   The new matrix is only a view of the vector :data:`v`.  When the view
   goes out of scope the original vector :data:`v` 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.

   The function :func:`gsl_matrix_const_view_vector_with_tda` is equivalent
   to :func:`gsl_matrix_view_vector_with_tda` but can be used for matrices
   which are declared :code:`const`.

.. @node Modifying matrix views
.. @subsection Modifying matrix views
.. 
.. @deftypefun int gsl_matrix_view_from_matrix (gsl_matrix * m, gsl_matrix * mm, const size_t k1, const size_t k2, const size_t n1, const size_t n2)
.. This function modifies and existing matrix view :data:`m` to form a new
.. view of a matrix :data:`mm`, starting from element (:data:`k1`, :data:`k2`).
.. The matrix view has :data:`n1` rows and :data:`n2` columns.  Any existing
.. view in :data:`m` will be lost as a result of this function.
.. @end deftypefun
.. 
.. @deftypefun int gsl_matrix_view_from_vector (gsl_matrix * m, gsl_vector * v, const size_t offset, const size_t n1, const size_t n2)
.. This function modifies and existing matrix view :data:`m` to form a new
.. view of a vector :data:`v`, starting from element :data:`offset`.  The
.. vector has :data:`n1` rows and :data:`n2` columns.  Any
.. existing view in :data:`m` will be lost as a result of this function.
.. @end deftypefun
.. 
.. @deftypefun int gsl_matrix_view_from_array (gsl_matrix * m, double * base, const size_t offset, const size_t n1, const size_t n2)
.. This function modifies and existing matrix view :data:`m` to form a new
.. view of an array :data:`base`, starting from element :data:`offset`.  The
.. matrix has :data:`n1` rows and :data:`n2` columns.  Any
.. existing view in :data:`m` will be lost as a result of this function.
.. @end deftypefun
.. 
.. @deftypefun {gsl_matrix *} gsl_matrix_alloc_from_block (gsl_block * b, size_t offset, size_t n1, size_t n2, size_t tda)
.. This function creates a matrix as a slice of the block :data:`b`,
.. returning a pointer to a newly initialized matrix struct.  The start of
.. the matrix is offset by :data:`offset` elements from the start of the
.. block.  The matrix has :data:`n1` rows and :data:`n2` columns, with the
.. physical number of columns in memory given by :data:`tda`.
.. Mathematically, the (:data:`i`, :data:`j`)-th element of the matrix is given by,
.. 
.. @example
.. m(i,j) = b->data[offset + i*tda + j]
.. @end example
.. @noindent
.. where the index :data:`i` runs from 0 to :code:`n1-1` and the index :data:`j`
.. runs from 0 to :code:`n2-1`.
.. 
.. A null pointer is returned if the combined parameters
.. (:data:`offset`, :data:`n1`, :data:`n2`, :data:`tda`) overrun the end of the block
.. or if insufficient memory is available to store the matrix.
.. 
.. The matrix is only a view of the block :data:`b`, and the block is not
.. owned by the matrix.  When the matrix is deallocated the block :data:`b`
.. will continue to exist.  This memory can only be deallocated by freeing
.. the block itself.  Of course, this block should not be deallocated while
.. the matrix is still in use.
.. @end deftypefun
.. 
.. @deftypefun {gsl_matrix *} gsl_matrix_alloc_from_matrix (gsl_matrix * m, size_t k1, size_t k2, size_t n1, size_t n2)
.. 
.. This function creates a matrix as a submatrix of the matrix :data:`m`,
.. returning a pointer to a newly initialized matrix struct.  The upper-left
.. element of the submatrix is the element (:data:`k1`, :data:`k2`) of the
.. original matrix.  The submatrix has :data:`n1` rows and :data:`n2` columns.
.. The physical number of columns in memory given by :data:`tda` is
.. unchanged.  Mathematically, the (:data:`i`, :data:`j`)-th element of the
.. new matrix is given by,
.. 
.. @example
.. m'(i,j) = m->data[(k1*m->tda + k2) + i*m->tda + j]
.. @end example
.. @noindent
.. where the index :data:`i` runs from 0 to :code:`n1-1` and the index :data:`j`
.. runs from 0 to :code:`n2-1`.
.. 
.. A null pointer is returned if the combined parameters
.. (:data:`k1`, :data:`k2`, :data:`n1`, :data:`n2`, :data:`tda`) overrun the end of the
.. original matrix or if insufficient memory is available to store the matrix.
.. 
.. The new matrix is only a view of the block underlying the existing
.. matrix, :data:`m`.  The block is not owned by the new matrix.  When the new
.. matrix is deallocated the original matrix :data:`m` 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 new matrix is still in use.
.. @end deftypefun

Creating row and column views
-----------------------------

In general there are two ways to access an object, by reference or by
copying.  The functions described in this section create vector views
which allow access to a row or column of a matrix by reference.
Modifying elements of the view is equivalent to modifying the matrix,
since both the vector view and the matrix point to the same memory
block.

.. function:: gsl_vector_view gsl_matrix_row (gsl_matrix * m, size_t i)
              gsl_vector_const_view gsl_matrix_const_row (const gsl_matrix * m, size_t i)

   These functions return a vector view of the :data:`i`-th row of the matrix
   :data:`m`.  The :code:`data` pointer of the new vector is set to null if
   :data:`i` is out of range.

   The function :func:`gsl_matrix_const_row` is equivalent to
   :func:`gsl_matrix_row` but can be used for matrices which are declared
   :code:`const`.

.. function:: gsl_vector_view gsl_matrix_column (gsl_matrix * m, size_t j)
              gsl_vector_const_view gsl_matrix_const_column (const gsl_matrix * m, size_t j)

   These functions return a vector view of the :data:`j`-th column of the
   matrix :data:`m`.  The :code:`data` pointer of the new vector is set to
   null if :data:`j` is out of range.

   The function :func:`gsl_matrix_const_column` is equivalent to
   :func:`gsl_matrix_column` but can be used for matrices which are declared
   :code:`const`.

.. function:: gsl_vector_view gsl_matrix_subrow (gsl_matrix * m, size_t i, size_t offset, size_t n)
              gsl_vector_const_view gsl_matrix_const_subrow (const gsl_matrix * m, size_t i, size_t offset, size_t n)

   These functions return a vector view of the :data:`i`-th row of the matrix
   :data:`m` beginning at :data:`offset` elements past the first column and
   containing :data:`n` elements. The :code:`data` pointer of the new vector
   is set to null if :data:`i`, :data:`offset`, or :data:`n` are out of range.

   The function :func:`gsl_matrix_const_subrow` is equivalent to
   :func:`gsl_matrix_subrow` but can be used for matrices which are declared
   :code:`const`.

.. function:: gsl_vector_view gsl_matrix_subcolumn (gsl_matrix * m, size_t j, size_t offset, size_t n)
              gsl_vector_const_view gsl_matrix_const_subcolumn (const gsl_matrix * m, size_t j, size_t offset, size_t n)

   These functions return a vector view of the :data:`j`-th column of the matrix
   :data:`m` beginning at :data:`offset` elements past the first row and
   containing :data:`n` elements. The :code:`data` pointer of the new vector
   is set to null if :data:`j`, :data:`offset`, or :data:`n` are out of range.

   The function :func:`gsl_matrix_const_subcolumn` is equivalent to
   :func:`gsl_matrix_subcolumn` but can be used for matrices which are declared
   :code:`const`.

.. index::
   single: matrix diagonal
   single: diagonal, of a matrix

.. function:: gsl_vector_view gsl_matrix_diagonal (gsl_matrix * m)
              gsl_vector_const_view gsl_matrix_const_diagonal (const gsl_matrix * m)

   These functions return a vector view of the diagonal of the matrix
   :data:`m`. The matrix :data:`m` is not required to be square. For a
   rectangular matrix the length of the diagonal is the same as the smaller
   dimension of the matrix.

   The function :func:`gsl_matrix_const_diagonal` is equivalent to
   :func:`gsl_matrix_diagonal` but can be used for matrices which are
   declared :code:`const`.

.. index::
   single: matrix subdiagonal
   single: subdiagonal, of a matrix

.. function:: gsl_vector_view gsl_matrix_subdiagonal (gsl_matrix * m, size_t k) 
              gsl_vector_const_view gsl_matrix_const_subdiagonal (const gsl_matrix * m, size_t k)

   These functions return a vector view of the :data:`k`-th subdiagonal of
   the matrix :data:`m`. The matrix :data:`m` is not required to be square.
   The diagonal of the matrix corresponds to :math:`k = 0`.

   The function :func:`gsl_matrix_const_subdiagonal` is equivalent to
   :func:`gsl_matrix_subdiagonal` but can be used for matrices which are
   declared :code:`const`.

.. index::
   single: matrix superdiagonal
   single: superdiagonal, matrix

.. function:: gsl_vector_view gsl_matrix_superdiagonal (gsl_matrix * m, size_t k) 
              gsl_vector_const_view gsl_matrix_const_superdiagonal (const gsl_matrix * m, size_t k)

   These functions return a vector view of the :data:`k`-th superdiagonal of
   the matrix :data:`m`. The matrix :data:`m` is not required to be square. The
   diagonal of the matrix corresponds to :math:`k = 0`.

   The function :func:`gsl_matrix_const_superdiagonal` is equivalent to
   :func:`gsl_matrix_superdiagonal` but can be used for matrices which are
   declared :code:`const`.

.. @deftypefun {gsl_vector *} gsl_vector_alloc_row_from_matrix (gsl_matrix * m, size_t i)
.. This function allocates a new :type:`gsl_vector` struct which points to
.. the :data:`i`-th row of the matrix :data:`m`.
.. @end deftypefun
.. 
.. @deftypefun {gsl_vector *} gsl_vector_alloc_col_from_matrix (gsl_matrix * m, size_t j)
.. This function allocates a new :type:`gsl_vector` struct which points to
.. the :data:`j`-th column of the matrix :data:`m`.
.. @end deftypefun

Copying matrices
----------------

.. function:: int gsl_matrix_memcpy (gsl_matrix * dest, const gsl_matrix * src)

   This function copies the elements of the matrix :data:`src` into the
   matrix :data:`dest`.  The two matrices must have the same size.

.. function:: int gsl_matrix_swap (gsl_matrix * m1, gsl_matrix * m2)

   This function exchanges the elements of the matrices :data:`m1` and
   :data:`m2` by copying.  The two matrices must have the same size.

Copying rows and columns
------------------------

The functions described in this section copy a row or column of a matrix
into a vector.  This allows the elements of the vector and the matrix to
be modified independently.  Note that if the matrix and the vector point
to overlapping regions of memory then the result will be undefined.  The
same effect can be achieved with more generality using
:func:`gsl_vector_memcpy` with vector views of rows and columns.

.. function:: int gsl_matrix_get_row (gsl_vector * v, const gsl_matrix * m, size_t i)

   This function copies the elements of the :data:`i`-th row of the matrix
   :data:`m` into the vector :data:`v`.  The length of the vector must be the
   same as the length of the row.

.. function:: int gsl_matrix_get_col (gsl_vector * v, const gsl_matrix * m, size_t j)

   This function copies the elements of the :data:`j`-th column of the matrix
   :data:`m` into the vector :data:`v`.  The length of the vector must be the
   same as the length of the column.

.. function:: int gsl_matrix_set_row (gsl_matrix * m, size_t i, const gsl_vector * v)

   This function copies the elements of the vector :data:`v` into the
   :data:`i`-th row of the matrix :data:`m`.  The length of the vector must be
   the same as the length of the row.

.. function:: int gsl_matrix_set_col (gsl_matrix * m, size_t j, const gsl_vector * v)

   This function copies the elements of the vector :data:`v` into the
   :data:`j`-th column of the matrix :data:`m`.  The length of the vector must be
   the same as the length of the column.

Exchanging rows and columns
---------------------------

The following functions can be used to exchange the rows and columns of
a matrix.

.. function:: int gsl_matrix_swap_rows (gsl_matrix * m, size_t i, size_t j)

   This function exchanges the :data:`i`-th and :data:`j`-th rows of the matrix
   :data:`m` in-place.

.. function:: int gsl_matrix_swap_columns (gsl_matrix * m, size_t i, size_t j)

   This function exchanges the :data:`i`-th and :data:`j`-th columns of the
   matrix :data:`m` in-place.

.. function:: int gsl_matrix_swap_rowcol (gsl_matrix * m, size_t i, size_t j)

   This function exchanges the :data:`i`-th row and :data:`j`-th column of the
   matrix :data:`m` in-place.  The matrix must be square for this operation to
   be possible.

.. function:: int gsl_matrix_transpose_memcpy (gsl_matrix * dest, const gsl_matrix * src)

   This function makes the matrix :data:`dest` the transpose of the matrix
   :data:`src` by copying the elements of :data:`src` into :data:`dest`.  This
   function works for all matrices provided that the dimensions of the matrix
   :data:`dest` match the transposed dimensions of the matrix :data:`src`.

.. function:: int gsl_matrix_transpose (gsl_matrix * m)

   This function replaces the matrix :data:`m` by its transpose by copying
   the elements of the matrix in-place.  The matrix must be square for this
   operation to be possible.

.. function:: int gsl_matrix_complex_conjtrans_memcpy (gsl_matrix * dest, const gsl_matrix * src)

   This function makes the matrix :data:`dest` the conjugate transpose of the matrix
   :data:`src` by copying the complex conjugate elements of :data:`src` into :data:`dest`.  This
   function works for all complex matrices provided that the dimensions of the matrix
   :data:`dest` match the transposed dimensions of the matrix :data:`src`.

Matrix operations
-----------------

The following operations are defined for real and complex matrices.

.. function:: int gsl_matrix_add (gsl_matrix * a, const gsl_matrix * b)

   This function adds the elements of matrix :data:`b` to the elements of
   matrix :data:`a`.  The result :math:`a(i,j) \leftarrow a(i,j) + b(i,j)`
   is stored in :data:`a` and :data:`b` remains unchanged. The two matrices
   must have the same dimensions.

.. function:: int gsl_matrix_sub (gsl_matrix * a, const gsl_matrix * b)

   This function subtracts the elements of matrix :data:`b` from the
   elements of matrix :data:`a`.  The result :math:`a(i,j) \leftarrow a(i,j) - b(i,j)`
   is stored in :data:`a` and :data:`b` remains unchanged. The two
   matrices must have the same dimensions.

.. function:: int gsl_matrix_mul_elements (gsl_matrix * a, const gsl_matrix * b)

   This function multiplies the elements of matrix :data:`a` by the
   elements of matrix :data:`b`.  The result :math:`a(i,j) \leftarrow a(i,j) * b(i,j)`
   is stored in :data:`a` and :data:`b` remains unchanged.  The two
   matrices must have the same dimensions.

.. function:: int gsl_matrix_div_elements (gsl_matrix * a, const gsl_matrix * b)

   This function divides the elements of matrix :data:`a` by the elements
   of matrix :data:`b`.  The result :math:`a(i,j) \leftarrow a(i,j) / b(i,j)`
   is stored in :data:`a` and :data:`b` remains unchanged. The two
   matrices must have the same dimensions.

.. function:: int gsl_matrix_scale (gsl_matrix * a, const double x)

   This function multiplies the elements of matrix :data:`a` by the
   constant factor :data:`x`.  The result :math:`a(i,j) \leftarrow x a(i,j)`
   is stored in :data:`a`.

.. function:: int gsl_matrix_scale_columns (gsl_matrix * A, const gsl_vector * x)

   This function scales the columns of the :math:`M`-by-:math:`N` matrix
   :data:`A` by the elements of the vector :data:`x`, of length :math:`N`. The
   :math:`j`-th column of :data:`A` is multiplied by :math:`x_j`. This is equivalent to
   forming

   .. math:: A \rightarrow A X

   where :math:`X = \textrm{diag}(x)`.

.. function:: int gsl_matrix_scale_rows (gsl_matrix * A, const gsl_vector * x)

   This function scales the rows of the :math:`M`-by-:math:`N` matrix
   :data:`A` by the elements of the vector :data:`x`, of length :math:`M`. The
   :math:`i`-th row of :data:`A` is multiplied by :math:`x_i`. This is equivalent to
   forming

   .. math:: A \rightarrow X A

   where :math:`X = \textrm{diag}(x)`.

.. function:: int gsl_matrix_add_constant (gsl_matrix * a, const double x)

   This function adds the constant value :data:`x` to the elements of the
   matrix :data:`a`.  The result :math:`a(i,j) \leftarrow a(i,j) + x` is
   stored in :data:`a`.

Finding maximum and minimum elements of matrices
------------------------------------------------

The following operations are only defined for real matrices.

.. function:: double gsl_matrix_max (const gsl_matrix * m)

   This function returns the maximum value in the matrix :data:`m`.

.. function:: double gsl_matrix_min (const gsl_matrix * m)

   This function returns the minimum value in the matrix :data:`m`.

.. function:: void gsl_matrix_minmax (const gsl_matrix * m, double * min_out, double * max_out)

   This function returns the minimum and maximum values in the matrix
   :data:`m`, storing them in :data:`min_out` and :data:`max_out`.

.. function:: void gsl_matrix_max_index (const gsl_matrix * m, size_t * imax, size_t * jmax)

   This function returns the indices of the maximum value in the matrix
   :data:`m`, storing them in :data:`imax` and :data:`jmax`.  When there are
   several equal maximum elements then the first element found is returned,
   searching in row-major order.

.. function:: void gsl_matrix_min_index (const gsl_matrix * m, size_t * imin, size_t * jmin)

   This function returns the indices of the minimum value in the matrix
   :data:`m`, storing them in :data:`imin` and :data:`jmin`.  When there are
   several equal minimum elements then the first element found is returned,
   searching in row-major order.

.. function:: void gsl_matrix_minmax_index (const gsl_matrix * m, size_t * imin, size_t * jmin, size_t * imax, size_t * jmax)

   This function returns the indices of the minimum and maximum values in
   the matrix :data:`m`, storing them in (:data:`imin`, :data:`jmin`) and
   (:data:`imax`, :data:`jmax`). When there are several equal minimum or maximum
   elements then the first elements found are returned, searching in
   row-major order.

Matrix properties
-----------------

The following functions are defined for real and complex matrices.
For complex matrices both the real and imaginary parts must satisfy
the conditions.

.. function:: int gsl_matrix_isnull (const gsl_matrix * m)
              int gsl_matrix_ispos (const gsl_matrix * m)
              int gsl_matrix_isneg (const gsl_matrix * m)
              int gsl_matrix_isnonneg (const gsl_matrix * m)

   These functions return 1 if all the elements of the matrix :data:`m` are
   zero, strictly positive, strictly negative, or non-negative
   respectively, and 0 otherwise. To test whether a matrix is
   positive-definite, use the :ref:`Cholesky decomposition <sec_cholesky-decomposition>`.

.. function:: int gsl_matrix_equal (const gsl_matrix * a, const gsl_matrix * b)

   This function returns 1 if the matrices :data:`a` and :data:`b` are equal
   (by comparison of element values) and 0 otherwise.

.. function:: double gsl_matrix_norm1 (const gsl_matrix * A)

   This function returns the 1-norm of the :math:`m`-by-:math:`n` matrix :data:`A`, defined as
   the maximum column sum,

   .. math:: ||A||_1 = \textrm{max}_{1 \le j \le n} \sum_{i=1}^m |A_{ij}|

Example programs for matrices
-----------------------------

The program below shows how to allocate, initialize and read from a matrix
using the functions :func:`gsl_matrix_alloc`, :func:`gsl_matrix_set` and
:func:`gsl_matrix_get`.

.. include:: examples/matrix.c
   :code:

Here is the output from the program.  The final loop attempts to read
outside the range of the matrix :code:`m`, and the error is trapped by
the range-checking code in :func:`gsl_matrix_get`.

::

   $ ./a.out
   m(0,0) = 0.23
   m(0,1) = 1.23
   m(0,2) = 2.23
   m(1,0) = 100.23
   m(1,1) = 101.23
   m(1,2) = 102.23
   ...
   m(9,2) = 902.23
   gsl: matrix_source.c:13: ERROR: first index out of range
   Default GSL error handler invoked.
   Aborted (core dumped)

The next program shows how to write a matrix to a file.

.. include:: examples/matrixw.c
   :code:

After running this program the file :file:`test.dat` should contain the
elements of :code:`m`, written in binary format.  The matrix which is read
back in using the function :func:`gsl_matrix_fread` should be exactly
equal to the original matrix.

The following program demonstrates the use of vector views.  The program
computes the column norms of a matrix.

.. include:: examples/vectorview.c
   :code:

Here is the output of the program, 

.. include:: examples/vectorview.txt
   :code:

The results can be confirmed using |octave|::

   $ octave
   GNU Octave, version 2.0.16.92
   octave> m = sin(0:9)' * ones(1,10) 
                  + ones(10,1) * cos(0:9); 
   octave> sqrt(sum(m.^2))
   ans =
     4.3146  3.1205  2.1932  3.2611  2.5342  2.5728
     4.2047  3.6520  2.0852  3.0731

References and Further Reading
------------------------------

The block, vector and matrix objects in GSL follow the :code:`valarray`
model of C++.  A description of this model can be found in the following
reference,

* B. Stroustrup, The C++ Programming Language (3rd Ed), 
  Section 22.4 Vector Arithmetic. Addison-Wesley 1997, ISBN 0-201-88954-4.

.. rubric:: Footnotes

.. [#f1] Range checking is available in the GNU C Compiler bounds-checking extension,
         but it is not part of the default installation of GCC. Memory accesses
         can also be checked with Valgrind or the :code:`gcc -fmudflap`
         memory protection option.