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

From: Dave Steffen (dgsteffen_at_[hidden])
Date: 2007-02-16 12:41:42

Hugo Duncan writes:
 > 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.

 Well, IIRC BOOST_CHECK_CLOSE does a check for _relative error_.  And
 you can't do relative error if one of the numbers is zero.  From a
 conceptual point of view, relative error says that two values differ
 by an amount that is small relative to their magnitude.  Zero has no
 magnitude, e.g. no information about what's small relative to it, so
 the question is meaningless.

 I'm wrapping a bunch of the BOOST_CHECK macros for my own use, and
 will probably come up with something else to check for "close to
 zero", e.g. an _aboslute_ tolerance; maybe "BOOST_CHECK_SMALL" or
 something.

----------------------------------------------------------------------
Dave Steffen, Ph.D.       Fools ignore complexity.
Software Engineer IV      Pragmatists suffer it.
Numerica Corporation      Some can avoid it.
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dgsteffen_at_[hidden]            -- Alan Perlis
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