28 Numerics library [numerics]

28.9 Basic linear algebra algorithms [linalg]

28.9.6 Layouts for packed matrix types [linalg.layout.packed] Observers [linalg.layout.packed.obs]

constexpr index_type required_span_size() const noexcept;
Returns: extents_.extent(0) * (extents_.extent(0) + 1)/2.
[Note 1: 
For example, a 5 x 5 packed matrix only stores 15 matrix elements.
— end note]
template<class Index0, class Index1> constexpr index_type operator() (Index0 ind0, Index1 ind1) const noexcept;
  • is_convertible_v<Index0, index_type> is true,
  • is_convertible_v<Index1, index_type> is true,
  • is_nothrow_constructible_v<index_type, Index0> is true, and
  • is_nothrow_constructible_v<index_type, Index1> is true.
Let i be extents_type​::​index-cast(ind0), and let j be extents_type​::​index-cast(ind1).
Preconditions: i, j is a multidimensional index in extents_ ([mdspan.overview]).
Returns: Let N be extents_.extent(0).
  • (*this)(j, i) if i > j is true; otherwise
  • i + j * (j + 1)/2 if is_same_v<StorageOrder, column_major_t> && is_same_v<Triangle, upper_triangle_t> is true or is_same_v<StorageOrder, row_major_t> && is_same_v<Triangle, lower_triangle_t> is true; otherwise
  • j + N * i - i * (i + 1)/2.
constexpr index_type stride(rank_type r) const noexcept;
  • is_strided() is true, and
  • r < extents_type​::​rank() is true.
Returns: 1.
template<class OtherExtents> friend constexpr bool operator==(const mapping& x, const mapping<OtherExtents>& y) noexcept;
Effects: Equivalent to: return x.extents() == y.extents();