[Numpy-discussion] Nice float -> integer conversion?
Matthew Brett
matthew.brett@gmail....
Sat Oct 15 14:20:51 CDT 2011
Hi,
On Tue, Oct 11, 2011 at 7:32 PM, Benjamin Root <ben.root@ou.edu> wrote:
> On Tue, Oct 11, 2011 at 2:06 PM, Derek Homeier
> <derek@astro.physik.uni-goettingen.de> wrote:
>>
>> On 11 Oct 2011, at 20:06, Matthew Brett wrote:
>>
>> > Have I missed a fast way of doing nice float to integer conversion?
>> >
>> > By nice I mean, rounding to the nearest integer, converting NaN to 0,
>> > inf, -inf to the max and min of the integer range? The astype method
>> > and cast functions don't do what I need here:
>> >
>> > In [40]: np.array([1.6, np.nan, np.inf, -np.inf]).astype(np.int16)
>> > Out[40]: array([1, 0, 0, 0], dtype=int16)
>> >
>> > In [41]: np.cast[np.int16](np.array([1.6, np.nan, np.inf, -np.inf]))
>> > Out[41]: array([1, 0, 0, 0], dtype=int16)
>> >
>> > Have I missed something obvious?
>>
>> np.[a]round comes closer to what you wish (is there consensus
>> that NaN should map to 0?), but not quite there, and it's not really
>> consistent either!
>>
>
> In a way, there is already consensus in the code. np.nan_to_num() by
> default converts nans to zero, and the infinities go to very large and very
> small.
>
> >>> np.set_printoptions(precision=8)
> >>> x = np.array([np.inf, -np.inf, np.nan, -128, 128])
> >>> np.nan_to_num(x)
> array([ 1.79769313e+308, -1.79769313e+308, 0.00000000e+000,
> -1.28000000e+002, 1.28000000e+002])
Right - but - we'd still need to round, and take care of the nasty
issue of thresholding:
>>> x = np.array([np.inf, -np.inf, np.nan, -128, 128])
>>> x
array([ inf, -inf, nan, -128., 128.])
>>> nnx = np.nan_to_num(x)
>>> nnx
array([ 1.79769313e+308, -1.79769313e+308, 0.00000000e+000,
-1.28000000e+002, 1.28000000e+002])
>>> np.rint(nnx).astype(np.int8)
array([ 0, 0, 0, -128, -128], dtype=int8)
So, I think nice_round would look something like:
def nice_round(arr, out_type):
in_type = arr.dtype.type
mx = floor_exact(np.iinfo(out_type).max, in_type)
mn = floor_exact(np.iinfo(out_type).max, in_type)
nans = np.isnan(arr)
out = np.rint(np.clip(arr, mn, mx)).astype(out_type)
out[nans] = 0
return out
with floor_exact being something like:
https://github.com/matthew-brett/nibabel/blob/range-dtype-conversions/nibabel/floating.py
See you,
Matthew
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