From: Clifford Heath <no@...>
Date: 2009-02-14T08:33:33+09:00
Subject: Re: Bug in converting float to int?

Jeff Vogt wrote:
> Clifford, can you elaborate on why round is unreliable?

If you've read the Goldberg paper, I doubt I can add anything.

Conversions to/from ASCII are never totally symmetrical.
You can convert to float and back to string and get a
different result, for the simple reason that it's not an
exact one-to-one mapping. For *some* of those patterns,
the change in the last digit will be *propagated* by the
rounding process, resulting in a change to your value.

I recall learning this in 1986 on an HP500. which converted
516.12 to double and back as 516.11 - only five significant
figures.

Clifford Heath.