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Defined in header <valarray> | ||
class gslice; | ||
std::gslice is the selector class that identifies a subset ofstd::valarray indices defined by a multi-level set of strides and sizes. Objects of typestd::gslice can be used as indices with valarray'soperator[] to select, for example, columns of a multidimensional array represented as avalarray.
Given the starting values, a list of stridesij and a list of sizesdj, astd::gslice constructed from these values selects the set of indiceskj=s+Σj(ijdj).
For example, a gslice with starting index3, strides{19,4,1} and lengths{2,4,3} generates the following set of24=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 constructstd::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 ofstd::valarray::operator[], otherwise the behavior is undefined.
Contents |
(constructor) | constructs a generic slice (public member function) |
startsizestride | returns the parameters of the slice (public member function) |
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.
| 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::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.
(none)
The parameters of the slice -- start, sizes and strides respectively.
Constant.
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 213121 122 123 221 222 223131 132 133 231 232 233141 142 143 241 242 243 After column operations:1 -1 113 1 212 2131 -1 123 1 222 2231 -1 133 1 232 2331 -1 143 1 242 243
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) |
| get/set valarray element, slice, or mask (public member function)[edit] | |
| BLAS-like slice of a valarray: starting index, length, stride (class)[edit] | |
| proxy to a subset of a valarray after applying a gslice (class template)[edit] |
