1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
|
.. Copyright (c) 2017, Wolf Vollprecht, Johan Mabille and Sylvain Corlay
Distributed under the terms of the BSD 3-Clause License.
The full license is in the file LICENSE, distributed with this software.
.. raw:: html
<style>
.rst-content .section>img {
width: 30px;
margin-bottom: 0;
margin-top: 0;
margin-right: 15px;
margin-left: 15px;
float: left;
}
</style>
Usage
=====
To use xtensor-blas functions, the ``xlinalg.hpp`` header has to be included.
In the ``xt::linalg`` namespace, many of NumPy's ``np.linalg`` functions are implemented.
We make an effort to keep the interfaces very similar.
For example, calculating a determinant:
.. highlight cpp
.. code-block:: cpp
#include "xtensor-blas/xlinalg.hpp"
int main()
{
xt::xarray<double> a = {{3, 2, 1}, {0, 4, 2}, {1, 3, 5}};
auto d = xt::linalg::det(a);
std::cout << d << std::endl; // 42.0
}
We can also try to compute the same determinant using the ``slogdet`` function, which
is more robust against under- or overflows by summing up the logarithm. The slogdet
function in NumPy returns a tuple of (sign, val). In C++, we emulate the behaviour by
returning a ``std::tuple``, which can be unpacked using ``std::get<N>(tuple)``.
.. code-block:: cpp
xt::xarray<double> a = {{3, 2, 1}, {0, 4, 2}, {1, 3, 5}};
auto d = xt::linalg::slogdet(a);
std::cout << std::get<0>(d) << ", " << std::get<1>(d) << std::endl; // +1, 3.7376696
Returning tuples is used throughout the xlinalg package.
Using xblas and xlapack directly
--------------------------------
It is not necessarily recommended to use ``xblas`` or ``xlapack`` directly, but it's possible and can improve performance in certain cases. Some things have to be taken into consideration:
For one thing, the result container needs to be allocated and passed into the
function beforehand. And for the LAPACK functions, all arguments have to be in column_major order.
Furthermore it's required that the xexpressions are evaluated and are stored in contiguous memory.
All of this is taken care of when using ``xlinalg``.
|
