28 Numerics library [numerics]

28.9 Basic linear algebra algorithms [linalg]

28.9.13 BLAS 1 algorithms [linalg.algs.blas1]

28.9.13.11 Index of maximum absolute value of vector elements [linalg.algs.blas1.iamax]

template<in-vector InVec>
  typename InVec::extents_type vector_idx_abs_max(InVec v);
template<class ExecutionPolicy, in-vector InVec>
  typename InVec::extents_type vector_idx_abs_max(ExecutionPolicy&& exec, InVec v);

[Note 1:

These functions correspond to the BLAS function IxAMAX[bib].

— end note]

Let T be decltype(abs-if-needed(real-if-needed(declval<typename InVec::value_type>())) + abs-if-needed(imag-if-needed(declval<typename InVec::value_type>())))

Mandates: declval<T>() < declval<T>() is a valid expression.

Returns:

(4.1)
numeric_limits<typename InVec::size_type>::max() if v has zero elements;
(4.2)
otherwise, the index of the first element of v having largest absolute value, if InVec::value_type is an arithmetic type;
(4.3)
otherwise, the index of the first element $v_{e}$ of v for which abs-if-needed(real-if-needed( $v_{e}$ )) + abs-if-needed(imag-if-needed( $v_{e}$ )) has the largest value.