Hi folks,


i've noticed an problem with conversion from real to float -
this produces ugly values. (in fact it seems to be real->float8)

Maybe its not really an conversion problem, but an bug in sprintf(),
(there were some other float related bugs known in glibc), as these
examples let me suspect:

I noticed this behaviour on postgresql-7.4.1 and 7.4.3 on glibc-2.3.2


BTW: real is an alias to float4 ?

regards,
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Enrico Weigelt <weigelt (AT) metux (DOT) de> writes:
You do realize that float4/real is only good to six decimal places
on most platforms?

You can take the issue up with the glibc boys if you like, but
I think you'll get laughed off.

regards, tom lane

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Tom Lane wrote:
and importantly, are stored as binary fractions. 0.10000 decimal is a
repeating fraction in binary, its like .1100110011.... x 2^-2 or something as a
FLOAT. Float4 has like 22 bits of significants for the mantissa, while Float8
has like 51 bits or something (this is from rusty memory, so I could have
slipped a few bits here).

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John R Pierce <pierce (AT) hogranch (DOT) com> writes:
Right, and conversely a fraction that's cut off at a certain number of
base-2 digits often corresponds to a repeating pattern of base-10
digits. I once had a clue about the exact laws for this, but it was
many years ago :-(

regards, tom lane

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Enrico Weigelt wrote:
Yes.

-Neil

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On Thu, Apr 07, 2005 at 01:02:50AM -0400, Tom Lane wrote:
As with base-10, any denominator that can't be expressed as a power of
the prime factors in the base results in a repeating number. So
basically any denominator that can't be expressed as 2^n turns into a
repeating binary doohicky.

Cheers,
D
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* Tom Lane <tgl (AT) sss (DOT) pgh.pa.us> wrote:
Thats okay for me. I need only four.



cu
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