[Numpy-discussion] array of random numbers fails to construct

Matthew Brett matthew.brett at gmail.com
Sun Dec 6 16:07:25 EST 2015


Hi,

On Sun, Dec 6, 2015 at 12:39 PM, DAVID SAROFF (RIT Student)
<dps7802 at rit.edu> wrote:
> This works. A big array of eight bit random numbers is constructed:
>
> import numpy as np
>
> spectrumArray = np.random.randint(0,255, (2**20,2**12)).astype(np.uint8)
>
>
>
> This fails. It eats up all 64GBy of RAM:
>
> spectrumArray = np.random.randint(0,255, (2**21,2**12)).astype(np.uint8)
>
>
> The difference is a factor of two, 2**21 rather than 2**20, for the extent
> of the first axis.

I think what's happening is that this:

np.random.randint(0,255, (2**21,2**12))

creates 2**33 random integers, which (on 64-bit) will be of dtype
int64 = 8 bytes, giving total size 2 ** (21 + 12 + 6) = 2 ** 39 bytes
= 512 GiB.

Cheers,

Matthew



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