[Numpy-discussion] numpy.spacing question

Ryan Nelson rnelsonchem at gmail.com
Fri Dec 5 08:29:50 EST 2014


Alok Singhal <alok <at> edgestreamlp.com> writes:

> 
> 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).
> 

Thanks Nathaniel and Alok. Your explanations were very helpful. I was 
expecting that all of those logical tests would come out True. It might 
have been the example in the doc string for 
`assert_array_almost_equal_nulp` that was throwing me off a little bit. 
The precision test in that function is `np.abs(x-y) <= ref`, where `ref` 
is the spacing for the largest values in the two arrays (which is `y` in 
my case). In the doc string, this function is run comparing x to (x*eps 
+ x), which seems like it shouldn't throw an error given the logical 
test in the function. For example, if you change the following `x = 
np.array([1., 1e-9, 1e-20])`, then the assert test function does not 
throw an error for that example.

Anyway, I guess that is the problem with working at the last unit of 
precision in these numbers... Pesky floating point values...






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