run np.empty for the second time

Question

In the Scipy documents written that :

The function zeros creates an array full of zeros, the function ones creates an array full of ones, and the function empty creates an array whose initial content is random and depends on the state of the memory. By default, the dtype of the created array is float64.

So I was ran this code :

import numpy as np
np.empty((1,2))

And it's return :

array([[  6.92892901e-310,   8.42664136e-317]])

So it's return a random numbers and all of things are great .

But, when I was running that code for the second time (in that shell) it's return a zero array !

np.empty((1,2))
array([[ 0.,  0.]])

And here is the question, why it's return zero array at the second time (instead of random number) ?

Answer 1

It's not random, it depends on what was saved in the bytes in memory that your computer gave NumPy when it requests some space for the array. If there is something other than zeros in there then these will be interpreted with the requested dtype (seemingly random but a better word would be unpredictable).

In your example you didn't save the first array so the memory for the first array immediatly reused.

>>> import numpy as np
>>> print(id(np.empty((20))))
2545385324992
>>> print(id(np.empty((20))))
2545385324992

Now comes the amazing part: It seems Python (or NumPy or your OS) zeros that memory before it gives it to NumPy again.

If you create a bigger array than it won't be "zero" because it's taken from somewhere else:

>>> print(np.empty((1, 2)))
[[  1.25757479e-311   1.25757479e-311]]
>>> print(np.empty((1, 3)))
[[  4.94065646e-324   9.88131292e-324   1.25757705e-311]]

Answer 2

The wording of the docs seems a bit unfortunate in this case. They do not mean random in the sense of a proper random number generator. If the latter is what you need you can use one of the functions in numpy.random or scipy.stats.

Describing numpy.empty a better word would be "undefined" meaning you the user can't make any assumptions on the values initially in the returned array. empty is the cheapest way of creating an array if you know you will overwrite its content anyway. The computer will just grab some memory for you. If that memory was not yet used in that session chances are it will appear random. But your computer also recycles memory.

I have to admit I don't really know what recycled memory looks like but two plausible possibilities would be

• it contains what the program that used it before happened to write
• or perhaps the OS has overwritten it with zeros for security reasons

Either possibility would explain what you are seeing.