Re: [boost] Boost Test Library: general floating point comparisons

From: Dave Steffen (dgsteffen_at_[hidden])
Date: 2007-02-16 12:47:58

Please don't top post.  Rearranged:

Glenn Schrader writes:
 > 
 > Hugo Duncan wrote:
 > > Hi,
 > >
 > > Just to chime in with my problems using BOOST_CHECK_CLOSE ...
 > >
 > > I always have problems when checking for small numbers or zero.
 > > I write something like BOOST_CHECK_CLOSE(0,x,1e-5) and start
 > > wondering why it fails, then rewrite the test using an epsilon
 > > value.
 > >
 > > When I know the value is going to be near zero this is ok, but
 > > sometimes tests are written for computed values.  A possible
 > > solution would be to have a check that uses the maximum of a
 > > relative and an absolute tolerance.
 > >
 > Hmmm.
 > 
 > An absolute difference is:
 > 
 >     BOOST_CHECK_CLOSE( x, y, 1e-5 );
 > 
 > True??

 Uh, no, I don't think so.  IIRC BOOST_CHECK_CLOSE does a relative
 error check.

 > so wouldn't a relative difference be:
 > 
 >     BOOST_CHECK_CLOSE( x / y, 1, 1e-5);
 >     or equivalently
 >     BOOST_CHECK_CLOSE( x, y, abs(y) * 1e-5);
 >     or maybe even
 >     BOOST_CHECK_CLOSE( x, y, max(abs(x),abs(y)) * 1e-5);

 No, that's not what BOOST_CHECK_CLOSE does, unless it's changed since
 1.33.1 (which is what I'm looking at).  The discussion here is, very
 approximately, that since it's not immediately obvious _what_ kind of
 test is being done in BOOST_CHECK_CLOSE because of the name, and that
 this causes confusion.

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Dave Steffen, Ph.D.       Fools ignore complexity.
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