Re: floating point comparison

[David Ullrich <ullrich@math.okstate.edu>]

Robert Dewar wrote:
[...]

	Well, curiously it turns out that the question of the accuracy of
floating-point operations matters to me more than it did when the thread
started. You and a few others have made points about how in some situations
it can be perfectly appropriate to test for equality. If we're talking
about comparing two numbers that we just came up with somehow fine, I
can believe that.

	But what if, say, an integer of some sort is going to be read from
a file, and then converted to another integer by some algorithm? Maybe we
read a byte b from the file, and then we want to use
n:=  Trunc(Log(b + 1) * 64000) later, "or something like that". It doesn't
matter whether n gets exactly the "right" value or not, but it's crucial
that the same b _always_ leads to the same n, even on different systems.
Surely I can't depend on floating-point operations being standard enough
for this, can I? (My plan at present is to use the floating-point to give
a first approximation, and then verify/tweak n by purely integer operations
until it passes some test defined by integer operations. Doesn't have to
be fast, this only happens once or twice.)

	Seems like there must be _some_ error allowed in the standard
standards, and any error at all means I can't trust the floating-point
here. Please tell me I'm wrong; it would save me some work.

-- 
David Ullrich

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