[Python-Dev] Expert floats (original) (raw)

Andrew Koenig ark-mlist at att.net
Wed Mar 31 11:04:12 EST 2004

> You argued against applying the Scheme rules because that would make > marshalling less accurate when the unmarshalling is done on a machine > with longer floats.

I said the 754 committee had that objection.

OK, but either that objection is relevant for Python or it isn't.

This was discussed on David Hough's numeric-interest mailing list at the time Clinger and Steele/White published their float<->string papers, and "phooey" was the consensus of the 754 folks on the mailing list at the time.

Interesting -- I didn't know that. But it would make sense -- they're probably more interested in cross-platform computational accuracy than they are in convenience for casual uses.

I personally don't think decimal strings are a sane way to transport binary floats regardless of rounding gimmicks.

Fair enough.

> But on such a machine, 17 digits won't be good enough anyway.

Doesn't change that 17 digits gets closer then shortest-possible: the art of binary fp is about reducing error, not generally about eliminating error. Shortest-possible does go against the spirit of 754 in that respect.

That's a fair criticism. On the other hand, maybe ?!ng is right about the desirable properties of display for people being different from those for marshalling/unmarshalling.

> That's what I meant. Rather than 0.47 from exact, I meant 0.47 from > the best possible.

Well, you originally said that in response to my saying that the standard doesn't require perfect rounding (and it doesn't), and that the standard has different accuracy requirements for different inputs (and it does). So now I'm left wondering what your original "I thought that ..." was trying to get across.

I don't remember; sorry.

> Hey, I know some people who write C programs that don't rely on the > platform C libraries for anything :-)

Python would love to grab their I/O implementation then <0.8 wink>.

http://www.research.att.com/sw/tools/sfio/

I think the licensing terms are compatible with Python, if you're serious.

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