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std::gslice(3) C++ Standard Libary std::gslice(3) NAME std::gslice - std::gslice Synopsis 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 exam- ple, 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+ 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 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 in- dices 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 un- defined. Member functions constructor constructs a generic slice (public member function) start returns the parameters of the slice size (public member function) stride std::gslice::gslice gslice() (1) gslice( std::size_t start, const std::valarray<std::size_t>& sizes, (2) const std::valarray<std::size_t>& strides ); gslice( const gslice& other ); (3) Constructs a new generic slice. 1) Default constructor. Equivalent to gslice(0, std::valar- ray<std::size_t>(), std::valarray<std::size_t>()). This constructor exists only to allow construction of arrays of slices. 2) Constructs a new slice with parameters start, sizes, strides. 3) Constructs a copy of other. Parameters start - the position of the first element sizes - an array that defines the number of elements in each di- mension strides - an array that defines the number of positions between suc- cessive 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 respec- tively. Complexity Constant. Example demonstrates the use of gslices to address columns of a 3D array // Run this code #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 See also operator[] get/set valarray element, slice, or mask (public member function) slice BLAS-like slice of a valarray: starting index, length, stride (class) gslice_array proxy to a subset of a valarray after applying a gslice (class template) http://cppreference.com 2022.07.31 std::gslice(3)
NAME | Synopsis | Member functions | Parameters | Parameters | Return value | Complexity | Example | Output: | See also
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