[Numpy-discussion] Casting to np.byte before clearing values

Chris Barker chris.barker at noaa.gov
Tue Dec 27 14:52:20 EST 2016


On Mon, Dec 26, 2016 at 1:34 AM, Nicolas P. Rougier <
Nicolas.Rougier at inria.fr> wrote:

>
> I'm trying to understand why viewing an array as bytes before clearing
> makes the whole operation faster.
> I imagine there is some kind of special treatment for byte arrays but I've
> no clue.
>

I notice that the code is simply setting a value using broadcasting -- I
don't think there is anything special about zero in that case. But your
subject refers to "clearing" an array.

So I wonder if you have a use case where the performance difference
matters, in which case _maybe_ it would be worth having a ndarray.zero()
method that efficiently zeros out an array.

Actually, there is ndarray.fill():

In [7]: %timeit Z_float[...] = 0

1000 loops, best of 3: 380 µs per loop


In [8]: %timeit Z_float.view(np.byte)[...] = 0

1000 loops, best of 3: 271 µs per loop


In [9]: %timeit Z_float.fill(0)

1000 loops, best of 3: 363 µs per loop

which seems to take an insignificantly shorter time than assignment.
Probably because it's doing exactly the same loop.

whereas a .zero() could use a memset, like it does with bytes.

can't say I have a use-case that would justify this, though.

-CHB






>
> # Native float
> Z_float = np.ones(1000000, float)
> Z_int   = np.ones(1000000, int)
>
> %timeit Z_float[...] = 0
> 1000 loops, best of 3: 361 µs per loop
>
> %timeit Z_int[...] = 0
> 1000 loops, best of 3: 366 µs per loop
>
> %timeit Z_float.view(np.byte)[...] = 0
> 1000 loops, best of 3: 267 µs per loop
>
> %timeit Z_int.view(np.byte)[...] = 0
> 1000 loops, best of 3: 266 µs per loop
>
>
> Nicolas
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>



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