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[], otherwise the behavior is undefined.
Contents |
[edit]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, conststd::valarray<std::size_t>& sizes, conststd::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.
[edit]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
[edit]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) |
[edit]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) |
