Next: Copying vectors, Previous: Reading and writing vectors, Up: Vectors [Index]
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.
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
gsl_vector_view and a constant vector view has the type
gsl_vector_const_view. In both cases the elements of the view
can be accessed as a gsl_vector using the vector component
of the view object. A pointer to a vector of type gsl_vector *
or const gsl_vector * can be obtained by taking the address of
this component with the & 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 &view.vector, and never storing this value
in another variable.
These functions return a vector view of a subvector of another vector v. The start of the new vector is offset by offset elements from the start of the original vector. The new vector has n elements. Mathematically, the i-th element of the new vector v’ is given by,
v'(i) = v->data[(offset + i)*v->stride]
where the index i runs from 0 to n-1.
The data pointer of the returned vector struct is set to null if
the combined parameters (offset,n) overrun the end of the
original vector.
The new vector is only a view of the block underlying the original vector, v. The block containing the elements of v is not owned by the new vector. When the view goes out of scope the original vector 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 gsl_vector_const_subvector is equivalent to
gsl_vector_subvector but can be used for vectors which are
declared const.
These functions return a vector view of a subvector of another vector
v with an additional stride argument. The subvector is formed in
the same way as for gsl_vector_subvector but the new vector has
n elements with a step-size of stride from one element to
the next in the original vector. Mathematically, the i-th element
of the new vector v’ is given by,
v'(i) = v->data[(offset + i*stride)*v->stride]
where the index i runs from 0 to 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 v of length 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
&view.vector. For example, the following code
computes the norm of the odd elements of v using the BLAS
routine 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 gsl_vector_const_subvector_with_stride is equivalent
to gsl_vector_subvector_with_stride but can be used for vectors
which are declared const.
These functions return a vector view of the real parts of the complex vector v.
The function gsl_vector_complex_const_real is equivalent to
gsl_vector_complex_real but can be used for vectors which are
declared const.
These functions return a vector view of the imaginary parts of the complex vector v.
The function gsl_vector_complex_const_imag is equivalent to
gsl_vector_complex_imag but can be used for vectors which are
declared const.
These functions return a vector view of an array. The start of the new vector is given by base and has n elements. Mathematically, the i-th element of the new vector v’ is given by,
v'(i) = base[i]
where the index i runs from 0 to n-1.
The array containing the elements of 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 base. Of course, the original array should not be deallocated while the view is still in use.
The function gsl_vector_const_view_array is equivalent to
gsl_vector_view_array but can be used for arrays which are
declared const.
These functions return a vector view of an array base with an
additional stride argument. The subvector is formed in the same way as
for gsl_vector_view_array but the new vector has n elements
with a step-size of stride from one element to the next in the
original array. Mathematically, the i-th element of the new
vector v’ is given by,
v'(i) = base[i*stride]
where the index i runs from 0 to 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 &view.vector.
The function gsl_vector_const_view_array_with_stride is
equivalent to gsl_vector_view_array_with_stride but can be used
for arrays which are declared const.
Next: Copying vectors, Previous: Reading and writing vectors, Up: Vectors [Index]