std::gslice
Defined in header <valarray>
|
||
class gslice; |
||
std::gslice is the selector class that identifies a subset of std::valarray indices defined by a multi-level set of strides and sizes. Objects of type std::gslice can be used as indices with valarray's operator[] to select, for example, columns of a multidimensional array represented as a valarray
Given the starting value s, a list of strides ij and a list of sizes dj, a std::gslice constructed from these values selects the set of indices kj=s+Σj(ijdj)
For example, a gslice with starting index 3, strides {19,4,1} and lengths {2,4,3} generates the following set of 24=2*4*3 indices:
3
+
0
*
19
+
0
*
4
+
0
*
1
=
3,
3
+
0
*
19
+
0
*
4
+
1
*
1
=
4,
3
+
0
*
19
+
0
*
4
+
2
*
1
=
5,
3
+
0
*
19
+
1
*
4
+
0
*
1
=
7,
3
+
0
*
19
+
1
*
4
+
1
*
1
=
8,
3
+
0
*
19
+
1
*
4
+
2
*
1
=
9,
3
+
0
*
19
+
2
*
4
+
0
*
1
=
11,
...
3
+
1
*
19
+
3
*
4
+
1
*
1
=
35,
3
+
1
*
19
+
3
*
4
+
2
*
1
=
36
It is possible to construct std::gslice objects that select some indices more than once: if the above example used the strides {1,1,1}, the indices would have been {3, 4, 5, 4, 5, 6, ...}. Such gslices may only be used as arguments to the const version of std::valarray::operator[]
Member functions
(constructor) |
constructs a generic slice (public member function) |
startsizestride |
returns the parameters of the slice (public member function) |
std::gslice::gslice
gslice() |
(1) | |
|
gslice(
std::size_t start, const
std::valarray
<
std::size_t
>
& sizes,
const std::valarray < std::size_t > & strides ) ; |
(2) | |
gslice( const gslice& other ); |
(3) | |
Constructs a new generic slice.
Parameters
| start | - | the position of the first element |
| sizes | - | an array that defines the number of elements in each dimension |
| strides | - | an array that defines the number of positions between successive elements in each dimension |
| other | - | another slice to copy |
std::slice::start, size, stride
|
std::size_t start(
)
const
;
|
(1) | |
|
std::valarray
<
std::size_t
> size(
)
const
;
|
(2) | |
|
std::valarray
<
std::size_t
> stride(
)
const
;
|
(3) | |
Returns the parameters passed to the slice on construction - start, sizes and strides respectively.
Parameters
(none)
Return value
The parameters of the slice -- start, sizes and strides respectively.
Complexity
Constant.
Example
Demonstrates the use of gslices to address columns of a 3D array:
#include <iostream> #include <valarray> void test_print(std::valarray<int>& v, int planes, int rows, int cols) { for (int r = 0; r < rows; ++r) { for (int z = 0; z < planes; ++z) { for (int c = 0; c < cols; ++c) std::cout << v[z * rows * cols + r * cols + c] << ' '; std::cout << " "; } std::cout << '\n'; } } int main() { std::valarray<int> v = // 3d array: 2 x 4 x 3 elements {111,112,113 , 121,122,123 , 131,132,133 , 141,142,143, 211,212,213 , 221,222,223 , 231,232,233 , 241,242,243}; // int ar3d[2][4][3] std::cout << "Initial 2x4x3 array:\n"; test_print(v, 2, 4, 3); // update every value in the first columns of both planes v[std::gslice(0, {2, 4}, {4 * 3, 3})] = 1; // two level one strides of 12 elements // then four level two strides of 3 elements // subtract the third column from the second column in the 1st plane v[std::gslice(1, {1, 4}, {4 * 3, 3})] -= v[std::gslice(2, {1, 4}, {4 * 3, 3})]; std::cout << "\n" "After column operations:\n"; test_print(v, 2, 4, 3); }
Output:
Initial 2x4x3 array: 111 112 113 211 212 213 121 122 123 221 222 223 131 132 133 231 232 233 141 142 143 241 242 243 After column operations: 1 -1 113 1 212 213 1 -1 123 1 222 223 1 -1 133 1 232 233 1 -1 143 1 242 243
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 543 | C++98 | it was unclear whether a default constructed generic slice is usable | it is usable (as an empty subset) |
See also
| get/set valarray element, slice, or mask (public member function) |
|
| BLAS-like slice of a valarray: starting index, length, stride (class) |
|
| proxy to a subset of a valarray after applying a gslice (class template) |