[9fans] printing floating point numbers

[9fans] printing floating point numbers

[Tristan Plumb]

I'm working on (re)writing some utilies for geospatial work on Plan9,
there are lots of floating point numbers (a file outlining the USGS
quadrangles for the state of Maine has about 60k). With the current
implementation this takes a long time. Maybe I should switch to a binary
format (but that's part of what I'm trying to get away from!).

Basically, do folks consider it possible to improve speed while keeping
or increasing the precision? A very simple implementation is accurate to
~15 digits, the current one is to 18 digits (though there are some
rounding errors).

In ways it would be nice to print stored numbers exactly. Yet numbers
like 2⁻¹⁰²²+2⁻¹⁰⁷⁴ (767 digits) are stored exactly...

2.2250738585072018771558785585789482407880088486837041956131300312119688603996006965297904292212628858639037013670281908017171296072711910355127227413175152199055740043138804567803233377539881639177387328959246074229270113078053813397081653361296447449529789521218979090783852583365901851789618799885150427514782636076021680436220311292700454832073964845713103912225963935608322440623896907276890186717054549275173986589324810401738228328251245795065655738191038008646911615828719989708647293221449796971546706720399791990809160347625980385995424739847678861180095072511543762389603716215171729816011544604359531284325406441938645324905389137795680915804792405099227413854274942620542640408839836919187418172987793340279242767544565229087538682506419718265533447265625e-308

The current implementation just prints the first 18 digits.

Thoughts?
Tristan

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Re: [9fans] printing floating point numbers

[Russ Cox]

> The current implementation just prints the first 18 digits.

18 digits are sufficient to reconstruct exactly the same bit
representation when you run the text back through strtod.
All those extra digits may be precise but they're not necessary
and probably not accurate.

Russ