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
jd
j).
For example, a gslice with starting index 3, strides {19,4,1} and lengths {2,4,3} generates the following set of 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 + 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) |
| startsizestride | returns the parameters of the slice (public member function) |
std::gslice::gslice
gslice() | ||
gslice( std::size_t start, const std::valarray<std::size_t>& sizes,
const std::valarray<std::size_t>& strides );
| ||
gslice( const gslice& other ); |
Constructs a new generic slice.
gslice(0, std::valarray<std::size_t>(), std::valarray<std::size_t>()). This constructor exists only to allow construction of arrays of slices.start, sizes, strides.other.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 rows, int cols, int planes)
{
for(int r=0; r<rows; ++r) {
for(int c=0; c<cols; ++c) {
for(int z=0; z<planes; ++z)
std::cout << v[r*cols*planes + c*planes + z] << ' ';
std::cout << '\n';
}
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 << "After column operations: \n";
test_print(v, 2, 4, 3);
}Output:
Initial 2x4x3 array: 111 112 113 121 122 123 131 132 133 141 142 143 211 212 213 221 222 223 231 232 233 241 242 243 After column operations: 1 -1 113 1 -1 123 1 -1 133 1 -1 143 1 212 213 1 222 223 1 232 233 1 242 243
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) |
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