[Octave-bug-tracker] [bug #59934] Unexpected complex result from acos

[anonymous]

Follow-up Comment #1, bug #59934 (project octave):

This is done for Matlab compatibility. For |x| <= 1, acos(x) and asin(x) will
return their expected values as described in IEC 60559 or IEEE 754. But unlike
the standards, which require a domain error for |x| > 1, Matlab returns a
complex number as described in Example 1 here:
https://ximera.osu.edu/math/complexBook/complexBook/complexTrig/complexTrig

This is probably already known to anyone who came to Octave from a Matlab
background, but yes, it could be documented in acos and asin for new users.
Documentation patch here:


diff -r 7bb6a7cbb65e libinterp/corefcn/mappers.cc
--- a/libinterp/corefcn/mappers.cc      Sat Jan 23 17:28:14 2021 +0100
+++ b/libinterp/corefcn/mappers.cc      Sun Jan 24 14:50:55 2021 -0500
@@ -88,6 +88,7 @@
        doc: /* -*- texinfo -*-
 @deftypefn {} {} acos (@var{x})
 Compute the inverse cosine in radians for each element of @var{x}.
+For |x| > 1, the return value will be complex.
 @seealso{cos, acosd}
 @end deftypefn */)
 {
@@ -292,6 +293,7 @@
        doc: /* -*- texinfo -*-
 @deftypefn {} {} asin (@var{x})
 Compute the inverse sine in radians for each element of @var{x}.
+For |x| > 1, the return value will be complex.
 @seealso{sin, asind}
 @end deftypefn */)
 {



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