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comparisons between floats and ratios
- To: email@example.com
- Subject: comparisons between floats and ratios
- From: firstname.lastname@example.org (Richard Fateman)
- Date: Fri, 24 Jan 86 12:25:47 PST
There is a rule (top of p 194, CLtL) indicating that CL preserves
"as much accuracy as possible" (although not guaranteeing it).
Thus when two numeric values are operated upon, the coercion
is to the larger precision, which tends to preserve more accuracy.
To be consistent with this rule, operations which combine floats of
any precision with ratios should convert floats to ratios, not the
Any float (except possibly IEEE NaN and infinity) can be converted to a
ratio without loss of precision. (and Nan=0/0, inf = 1/0 might even be
used). Of course the rational answer could be re-floated if required.
This would solve the problem of incorrect equality of 0.0 with a
non-zero but small ratio which "underflows".
Now consider comparing the floating point number 0.5 with an integer
sufficiently large as to overflow the exponent range of floats. The
comparison is "an error", according to CLtL (p 194). If one is to
ensure correct handling of the large number of numeric data types
in the language specification, I think the comparison should return
the mathematically correct answer.
(Alternatively, one could consider re-building all built-in functions
for applications like Macsyma.)