[Octave-bug-tracker] [bug #54399] function LuAminusSigmaB in eigs-base.cc can be made faster for special case of sigma == 0

[Rik]

Follow-up Comment #19, bug #54399 (project octave):

There are several different templates for the function.  In some of them,
sigma is a real value declared in C++ as a double.  Here is one such example.


static bool
LuAminusSigmaB (const SparseMatrix& m, const SparseMatrix& b,
                bool cholB, const ColumnVector& permB, double sigma,
                SparseMatrix& L, SparseMatrix& U, octave_idx_type *P,
                octave_idx_type *Q, ColumnVector &r)


In this case it is appropriate to compare sigma to 0.0.

In other cases sigma can be complex as in this prototype for the function


static bool
LuAminusSigmaB (const SparseComplexMatrix& m, const SparseComplexMatrix& b,
                bool cholB, const ColumnVector& permB, Complex sigma,
                SparseComplexMatrix& L, SparseComplexMatrix& U,
                octave_idx_type *P, octave_idx_type *Q, ColumnVector &r)
{


In these cases you can use the C++ abs() function to get the magnitude of the
complex number so something like this


if (std::abs (sigma) != 0.0)




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