[Numpy-discussion] numpy.spacing question

[Alok Singhal]

On Thu, Dec 4, 2014 at 4:25 PM, Ryan Nelson <rnelsonchem at gmail.com> wrote:
>
> I guess I'm a little confused about how the spacing values are calculated.

np.spacing(x) is basically the same as np.nextafter(x, np.inf) - x,
i.e., it returns the minimum positive number that can be added to x to
get a number that's different from x.

> My expectation is that the first logical test should give an output array
> where all of the results are the same. But it is also very likely that I
> don't have any idea what's going on. Can someone provide some clarification?

For 1e-10, np.spacing() is 1.2924697071141057e-26.  1e-10 * eps is
2.2204460492503132e-26, which, when added to 1e-10 rounds to the
closest number that can be represented in a 64-bit floating-point
representation.  That happens to be 2*np.spacing(1e-10), and not
1*np.spacing(1e-10).

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