std::gslice

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Numerics library

Common mathematical functions
Mathematical special functions (C++17)
Mathematical constants (C++20)
Basic linear algebra algorithms (C++26)
Floating-point environment (C++11)
Complex numbers
Numeric array (valarray)
Pseudo-random number generation
Factor operations
gcd (C++17)	lcm (C++17)
Interpolations
midpoint (C++20)	lerp (C++20)
Saturation arithmetic
add_sat (C++26) sub_sat (C++26) saturate_cast (C++26)	mul_sat (C++26) div_sat (C++26)
Generic numeric operations
iota (C++11) ranges::iota (C++23) accumulate inner_product adjacent_difference partial_sum	reduce (C++17) transform_reduce (C++17) inclusive_scan (C++17) exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Bit operations
has_single_bit (C++20)     bit_cast (C++20) bit_ceil (C++20) bit_floor (C++20) bit_width (C++20) rotl (C++20) rotr (C++20)	popcount (C++20) countl_zero (C++20) countl_one (C++20) countr_zero (C++20) countr_one (C++20) byteswap (C++23) endian (C++20)

 

std::valarray

Member functions
valarray::valarray valarray::~valarray valarray::operator= valarray::operator[] valarray::swap valarray::size valarray::resize valarray::sum valarray::min valarray::max valarray::shift valarray::cshift valarray::apply	valarray::operator+valarray::operator-valarray::operator~valarray::operator! valarray::operator+=valarray::operator-=valarray::operator*=valarray::operator/=valarray::operator%=valarray::operator&=valarray::operator|=valarray::operator^=valarray::operator<<=valarray::operator>>=
Non-member functions
swap(std::valarray) (C++11) begin(std::valarray) (C++11)   end(std::valarray) (C++11) abs exp log log10 pow sqrt sin cos tan asin acos atan atan2 sinh cosh tanh	operator*operator/operator%operator+operator-operator^operator&operator|operator<<operator>>operator&&operator|| operator==operator!=operator<operator>operator<=operator>=
Helper classes
slice_array gslice_array indirect_array	slice gslice mask_array
Deduction guides (C++17)

 

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.

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, 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.

1) Default constructor. Equivalent to gslice(0, std::valarray<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 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

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)