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Re: Help with eigs

From: John W. Eaton
Subject: Re: Help with eigs
Date: Mon, 7 Jan 2019 17:35:44 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

On 1/7/19 4:48 PM, Rik wrote:

I've been working through the static analyzer warnings about the Octave
code base and I ran in to a bit of trouble with eigs.

A sample warning is

__eigs__.cc (369) V550     An odd precise comparison: tmp.double_value() !=
0.0. It's probably better to use a comparison with defined precision:
fabs(A - B) > Epsilon.

for which the code is

tmp = map.getfield ("cholB");
if (tmp.is_defined ())
   cholB = tmp.double_value () != 0.0;

Checking the documentation for eigs

      Flag if 'chol (B)' is passed rather than B.  The default is

So, I thought the input was just a true/false and that I could change the
code to

   cholB = tmp.bool_value ();

But now when I run the BIST tests I get

octave:1> test eigs
  A = toeplitz (sparse (1:10));
  B = toeplitz (sparse ([1, 1], [1, 2], [2, 1], 1, 10));
  R = chol (B);
  opts.cholB = R;
  [v, d] = eigs (A, R, 4, "lm", opts);
  for i = 1:4
    assert (A * v(:,i), d(i, i) * B * v(:,i), 1e-12)
!!!!! test failed
octave_base_value::bool_value(): wrong type argument 'sparse matrix'

If I execute the commands individually I find that opts.cholB is not
true/false, but a sparse matrix.

It seems wrong to set opts.cholB to the sparse matrix itself here.

The double_value method for a sparse matrix just returns the first (0,0) element of the matrix. That's pretty strange, so I'm wondering whether this method should be deprecated and removed. There is no corresponding bool_value method for sparse matrices, so you get the error.

But if the intent of this code is correct, the opts.cholB setting would need to be

  opts.cholB = R(0,0) != 0;

to avoid changing the meaning. And that seems odd. Is there something special about chol(B) having a non-zero first element? If this really was intended, it would be helpful to have a comment explaining why. And to still use this code to make the intent explicit rather than passing R and relying on the double_value method to extract the first element.

BTW, this warning from the static analyzer tool thing can generate a lot of noise. There's nothing essentially wrong with testing a floating point value is exactly zero (or not).


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