test failure for mappers.cc

[John W. Eaton]

On  9-Nov-2010, Ben Abbott wrote:

| I'm seeing the failure below with tip 11212
| 
|       octave:2> test mappers.cc
|         ***** assert(arg (single(-1)), single(pi));
|       !!!!! test failed
|       assert (arg (single (-1)),single (pi)) expected
|        3.1416
|       but got
|        3.1416
|       values do not match
| 
| The difference between the actual and expected is ...
| 
|       arg (single (-1)) - single (pi)
|       ans = -2.3842e-07
| 
| I consistently get the *exactly* same difference.
| 
|       for n = 1:10
|         d(n) = arg (single (-1)) - single (pi);
|       endfor
|       std (d)
|       ans = 0
| 
| I assume this came about after your mapper changesets. Do you
| (anyone?) see this as well?

I'm not seeing the difference, but it is probably happening because I
changed the float version of arg from

  float arg (float x) { return atan2 (0.0, x); }

to

  float arg (float x) { return atan2f (0.0f, x); }

It looks like your result is now off by about "2*eps ('single')".
Does adding that as a tolerance avoid the warning?  Even if it does,
should we make the change?  Shouldn't the atan2f function in your math
library recognize -1 as a special case and return precisely the single
precision value for pi?  Should we be tweaking test tolerances to hide
what might be considered bugs in system libraries?

jwe