"Printing Floating-Point Numbers Quickly and Accurately"

Michael Hudson mwh at python.net
Sat Jul 24 18:01:00 CEST 2004

Tim Peters <tim.peters at gmail.com> writes:

> [Michael Hudson]
> > Does anyone (Tim?) have (ideally Python, but I can cope) code
> > implementing the fixed-format algorithm from the Subject: named paper
> > by Burger & Dybvig?
> I don't, and you're too young if you think anyone else might <wink>.
> The paper gives Scheme code, you know!  

Not for the fixed format algorithm it doesn't.  The paper says:

    The rational arithmetic used in fixed-format printing can wbe
    converted into high-precision integer arithmetic by introducing a
    common denominator as before. Because there are several more cases
    to consider, however, the resulting code is lengthy and has
    therefore been omitted from this paper.

I was hoping someone else had considered all the cases for me :-)

I've already translated the free format code from the paper into
Python, unfortunately it's not what I actually need...

> And there's a Haskell variant here:
>     http://lml.ls.fi.upm.es/~jjmoreno/manual/haskell98-library-report/numeric.html

That's the free format algorithm again, unless I've gone blind.

> As the paper says at the end,
>     [David] Gay ... showed that floating-point arithmetic is sufficiently
>     accurate in most cases when the requested number of digits is small.
>     The fixed-format printing algorithm described in this paper is useful when
>     these heuristics fail.
> IOW, the point of Burger & Dybvig was to run faster than the
> algorithms in the earlier Steele & White paper, but David Gay's code
> *usually* beats everything on speed (if you don't care about speed,
> code for this task is quite simple; if you do care about speed, it's
> mind-numbingly complicated), so there's little incentive to implement
> this algorithm.

I'm probably suffering from an attack of perfectionism.  Burger &
Dubvig's algorithm is so neat!

> Gay's code is written in C, & available from Netlib:
>     http://www.netlib.org/fp/

Now *that* code is over-the-top, even for me :-)


    . <- the point                                your article -> .
    |------------------------- a long way ------------------------|
                                       -- Christophe Rhodes, ucam.chat

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