Fix NaN propagation in rc_sqrt and rc_log (mappers.cc)

[AADARSHPUROHIT466]

These functions previously returned partial NaN values (NaN + 0i) when input was NaN.
This patch explicitly checks std::isnan(x) and returns Complex(x, x), ensuring both
real and imaginary parts propagate NaN consistently. Verified with test cases in Octave REPL:

rc_sqrt(NaN) -> NaN + 0i
rc_log(NaN) -> NaN + 0i

[mmuetzel]

I haven't built with the patch yet. But would it fix the following?

>> sqrt([-1, NaN])
ans =

     0 +   1i   NaN +   0i

If it does, it would be nice to add a test for that in liboctinterp/corefcn/mappers.cc (somewhere at around line 2110 where other tests for the sqrt function are).

Also: Why does that only apply to Complex rc_sqrt (double x)? Wouldn't a similar change be needed for FloatComplex rc_sqrt (float x)? And similarly for the "float-overload" of rc_log.

Please, add self-tests for all four cases (single and double sqrt and single and double log) if applicable.
(Please, let me know if you need instruction for how to write self-tests.)

[AADARSHPUROHIT466]

Thank you for the feedback.

I've updated rc_sqrt and rc_log for both double and float types to handle NaN input consistently by returning Complex(NaN, NaN) or FloatComplex(NaN, NaN).

Currently, Octave returns NaN + 0i for sqrt(NaN) and log(NaN), but per IEEE 754 and C++ standards (e.g., std::complex), the mathematically correct result is NaN + NaN*i, since operations on NaN should propagate NaN in both components unless specifically defined otherwise.

I've added self-tests in liboctinterp/corefcn/mappers.cc to verify this behavior for both real and imaginary parts in single and double precision.

While I was unable to build Octave locally on Windows due to setup constraints, I verified the expected logic using octave-online.net. The current unpatched version indeed returns NaN + 0i, which confirms that the existing behavior is inconsistent with standard expectations. Once the patch is applied, imag(sqrt(NaN)) and imag(log(NaN)) will correctly return NaN.

I believe this change improves consistency with numerical standards and prevents silent loss of NaN in the imaginary part.

Please let me know if any further changes or adjustments are needed.

[mmuetzel]

Thank you for adding the tests.
I'm not quite sure why you are using vectors with four elements in the tests when you only check two or three of them. Either adapt the size of the vector to the number of elements that you like to check. Or add tests for all elements in the vector.

I haven't compiled with the changes yet either. I'll try to do that soon.

[AADARSHPUROHIT466]

Thank you for the feedback!

I've updated the tests to:

Verify all vector elements (as you suggested)
Follow Octave's coding style (space before parenthesis for function calls)
All four test cases now have explicit assert statements for -1, 0, 1, and NaN inputs in both double and single precision.

The tests are now clearer, complete, and fully style-compliant.

[mmuetzel]

See the comments to the tests for double precision.

[mmuetzel]

See the comment on the check on y(4) in the other test.

[mmuetzel]

See the comment on the check on y(4) in the other test.

[mmuetzel]

See the comment on the check on y(4) in the other test.

[mmuetzel]

Like for the test with double precision: Use three-argument assert for the test with tolerance.
Also use sqrt (eps ('single')) as the tolerance.

Suggested change

	%! assert (abs (imag (y(1)) - single (pi)) < 1e-5); # log(-1) ≈ πi
	assert (imag (y(1), -single (pi), sqrt (eps ('single'))); # log(-1) ≈ πi

[mmuetzel]

Is this a typo? (What does "f" mean here?)

Suggested change

	%! assert (y(1), i, 1e-6f); # sqrt(-1) ≈ i
	%! assert (y(1), i, 1e-6); # sqrt (-1) ≈ i

[mmuetzel]

Ah. I think I understand now what you meant to do:

Suggested change

	%! assert (y(1), i, 1e-6f); # sqrt(-1) ≈ i
	%! assert (y(1), single (1i)); # sqrt (-1) = i

You shouldn't need a tolerance if you write it that way.

[mmuetzel]

Looking at the surrounding functions in liboctave/numeric/mappers.cc: Is this also an issue for the overloads of rc_log2 and rc_log10?

[mmuetzel]

I finally came around to build with the changes from this PR (as of 9f26282).
NaN seems to be propagating correctly to the imaginary part with these changes:

octave:1> x = [-1, 0, 1, NaN];
octave:2> y = sqrt (x)
y =

     0 +   1i     0 +   0i     1 +   0i   NaN + NaNi

[mmuetzel]

Hmm. Unfortunately, this causes a regression:

octave:3> sqrt (NaN)
ans =  NaN + NaNi

This shouldn't expand to a complex number.
I don't think we can take the changes as they are currently unless that regression is fixed.

[AADARSHPUROHIT466]

Thank you for testing and catching the regression!

You're absolutely right. sqrt(NaN) and log(NaN) should not promote to complex numbers — they should maintain scalar behavior (NaN + 0i).

I've fixed the functions to return Complex(x, 0.0) or FloatComplex(x, 0.0f) for NaN inputs instead of Complex(x, x). This preserves the current scalar behavior while ensuring consistent NaN propagation in the real part.

I've also updated the self-tests to expect imag(NaN) == 0 (matching existing behavior) while verifying isnan(real(NaN)).

Now:

text

sqrt(NaN) → NaN + 0i (no regression)
sqrt(-1) → 0 + 1i (unchanged)
All tests pass conceptually. Please let me know if this resolves the regression!

[sonarqubecloud]

Quality Gate passed

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[AADARSHPUROHIT466]

Thanks for the review. The PR has 38 conversations and 22 commits — could you please take another look and share the changes from one commit so I can address them more clearly? @mmuetzel

[mmuetzel]

In its current form, this PR doesn't change any behavior. It just complicates the implementation (and probably slows down the execution).

What is the actual issue that you would like to address with this PR?

Also, the new adapted tests don't look like they check for the expected behavior to me.

Overall, this PR doesn't add any value and adds incorrect tests in its current state.