Re: [boost] [units]

From: Dave Steffen (dgsteffen_at_[hidden])
Date: 2007-03-05 15:46:06

Matthias Schabel writes:
 > > In building probability and likelihood models, I often encounter the
 > > issue of transforming from either to their logarithms and vice versa.
 > > Code would be much simpler and less error-prone if the appropriate
 > > domain was defined, the natural arithmetic operators were used, and
 > > the conversions occurred (or could be prevented to catch errors) as
 > > necessary, but without necessarily requiring explicit bookkeeping to
 > > distinguish these quantities and their logarithms.
 > >
 > > This suggests a (small) set of types with appropriate conversions,
 > > much as in the units library.  However, probabilities are of course
 > > unitless.
 > >
 > > Should this idea be developed as a separate library or does it make
 > > any sense to fold it into the existing units framework?
 > 
 > We've tried to make mcs::units as flexible and extensible as possible
 > in order to accommodate a wide range of value types; as you point
 > out, since probabilities and log probabilities don't have associated
 > units (by definition, since taking the log of a unit doesn't make any
 > sense from a dimensional analysis standpoint), so I don't think this
 > is something that should be directly supported in the library
 > itself.

 Voice from the back of the room:

  a) It might be handy, though, to have (for example) a log function
  that enforces unitless quantities.  For example, 

     log ( velocity * time / meters )

  is fine, but if you forget to multiply by the time, you get a
  helpful error message.

  b) It might also be handy, if possible, to have some functions that
  take units and know what to do; for example, sqrt (m*m) properly
  returns meters, and so forth.

  < returns to lurk mode >

----------------------------------------------------------------------
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