[Numpy-discussion] Float128 integer comparison

Aronne Merrelli aronne.merrelli@gmail....
Sat Oct 15 14:42:11 CDT 2011


On Sat, Oct 15, 2011 at 1:12 PM, Matthew Brett <matthew.brett@gmail.com>wrote:

> Hi,
>
> Continuing the exploration of float128 - can anyone explain this behavior?
>
> >>> np.float64(9223372036854775808.0) == 9223372036854775808L
> True
> >>> np.float128(9223372036854775808.0) == 9223372036854775808L
> False
> >>> int(np.float128(9223372036854775808.0)) == 9223372036854775808L
> True
> >>> np.round(np.float128(9223372036854775808.0)) ==
> np.float128(9223372036854775808.0)
> True
>
>
I know little about numpy internals, but while fiddling with this, I noticed
a possible clue:

>>> np.float128(9223372036854775808.0) == 9223372036854775808L
False
>>> np.float128(4611686018427387904.0) == 4611686018427387904L
True
>>> np.float128(9223372036854775808.0) - 9223372036854775808L
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'numpy.float128' and 'long'
>>> np.float128(4611686018427387904.0) - 4611686018427387904L
0.0


My speculation - 9223372036854775808L is the first integer that is too big
to fit into a signed 64 bit integer. Python is OK with this but that means
it must be containing that value in some more complicated object. Since you
don't get the type error between float64() and long:

>>> np.float64(9223372036854775808.0) - 9223372036854775808L
0.0

Maybe there are some unimplemented pieces in numpy for dealing with
operations between float128 and python "arbitrary longs"? I could see the ==
test just producing false in that case, because it defaults back to some
object equality test which isn't actually looking at the numbers.

Aronne
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