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

Tim Peters tim.one at comcast.net
Tue Mar 30 20:22:46 EST 2004

[Andrew Koenig]

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. 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. The current incarnation of that committee appears to be in favor of perfect rounding all the time (so was the older incarnation, but it wasn't believed to be practical then), but I don't know what they think about shortest-possible (the older incarnation disliked that one).

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

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.

I thought that 754 requires input and output to be no more than 0.47 LSB away from exact.

No ... - for nearest/even rounding, it requires no more than 0.47 ULP error beyond that allowed for perfect nearest/even conversion (which has a max error of 0.5 ULP on its own)

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.

...

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>.

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