[Numpy-discussion] Large numbers into float128

[Matthew Brett]

Hi,

On Sat, Oct 29, 2011 at 3:55 PM, Matthew Brett <matthew.brett at gmail.com> wrote:
> Hi,
>
> Can anyone think of a good way to set a float128 value to an
> arbitrarily large number?
>
> As in
>
> v = int_to_float128(some_value)
>
> ?
>
> I'm trying things like
>
> v = np.float128(2**64+2)
>
> but, because (in other threads) the float128 seems to be going through
> float64 on assignment, this loses precision, so although 2**64+2 is
> representable in float128, in fact I get:
>
> In [35]: np.float128(2**64+2)
> Out[35]: 18446744073709551616.0
>
> In [36]: 2**64+2
> Out[36]: 18446744073709551618L
>
> So - can anyone think of another way to assign values to float128 that
> will keep the precision?

To answer my own question - I found an unpleasant way of doing this.

Basically it is this:

def int_to_float128(val):
    f64 = np.float64(val)
    res = val - int(f64)
    return np.float128(f64) + np.float128(res)

Used in various places here:

https://github.com/matthew-brett/nibabel/blob/e18e94c5b0f54775c46b1c690491b8bd6f07eb49/nibabel/floating.py

Best,

Matthew