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
i
j
and a list of sizes
d
j
, a
std::gslice
constructed from these values selects the set of indices
k
j
=s+Σ
j
(i
j
d
j
)
.
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[]
, otherwise the behavior is undefined.
Member functions
|
(constructor)
|
constructs a generic slice
(public member function) |
|
start
size
stride
|
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) |