cppdraft_translate/cppdraft/class/gslice/overview.md

[class.gslice.overview]

29 Numerics library [numerics]

29.6 Numeric arrays [numarray]

29.6.6 The gslice class [class.gslice]

29.6.6.1 Overview [class.gslice.overview]

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namespace std {class gslice {public: gslice(); gslice(size_t s, const valarray<size_t>& l, const valarray<size_t>& d);

size_t start() const; valarray<size_t> size() const; valarray<size_t> stride() const; };}

1

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This class represents a generalized slice out of an array.

Agslice is defined by a starting offset (s), a set of lengths (lj), and a set of strides (dj).

The number of lengths shall equal the number of strides.

2

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Agslice represents a mapping from a set of indices (ij), equal in number to the number of strides, to a single index k.

It is useful for building multidimensional array classes using thevalarray template, which is one-dimensional.

The set of one-dimensional index values specified by a gslice are
k=s+∑jijdj where the multidimensional indices ij range in value from 0 to lij−1.

3

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[Example 1:

Thegslice specificationstart = 3 length = {2, 4, 3} stride = {19, 4, 1} yields the sequence of one-dimensional indices
k=3+(0,1)×19+(0,1,2,3)×4+(0,1,2)×1 which are ordered as shown in the following table:

	(i0,i1,i2,k)=
		(0,  0,  0,   3),    
		(0,  0,  1,   4),    
		(0,  0,  2,   5),    
		(0,  1,  0,   7),    
		(0,  1,  1,   8),    
		(0,  1,  2,   9),    
		(0,  2,  0,  11), 
		(0,  2,  1,  12), 
		(0,  2,  2,  13), 
		(0,  3,  0,  15), 
		(0,  3,  1,  16), 
		(0,  3,  2,  17), 
		(1,  0,  0,  22), 
		(1,  0,  1,  23), 
		…      
		(1,  3,  2,  36)

That is, the highest-ordered index turns fastest.

— end example]

4

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It is possible to have degenerate generalized slices in which an address is repeated.

5

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[Example 2:

If the stride parameters in the previous example are changed to {1, 1, 1}, the first few elements of the resulting sequence of indices will be

	(0,  0,  0,   3),  
	(0,  0,  1,   4),  
	(0,  0,  2,   5),  
	(0,  1,  0,   4),  
	(0,  1,  1,   5),  
	(0,  1,  2,   6),  
	…

— end example]

6

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If a degenerate slice is used as the argument to the non-const version ofoperator[](const gslice&), the behavior is undefined.