[Numpy-discussion] Re: sqrt and divide

Stefan van der Walt stefan at sun.ac.za
Wed Apr 12 13:50:16 CDT 2006


On Wed, Apr 12, 2006 at 01:14:54AM -0500, Robert Kern wrote:
> Stefan van der Walt wrote:
> > Why is the square root of -1 not equal to the square root of -1+0j?
> > 
> > In [5]: N.sqrt(-1.)
> > Out[5]: nan
> > 
> > In [6]: N.sqrt(-1.+0j)
> > Out[6]: 1j
> 
> It is frequently the case that the argument being passed to sqrt() is expected
> to be non-negative and all of their code strictly deals with numbers in the real
> domain. If the argument happens to be negative, then it is a sign of a bug
> earlier in the code or a floating point instability. Returning nan gives the
> programmer the opportunity for sqrt() to complain loudly and expose bugs instead
> of silently upcasting to a complex type. Programmers who *do* want to work in
> the complex domain can easily perform the cast explicitly.

The current docstring (specified in generate_umath.py) states

    y = sqrt(x) square-root elementwise.

It would help a lot if it could explain the above constraint, e.g.

    y = sqrt(x) square-root elementwise. If x is real (and not complex),
	the domain is restricted to x>0.

> > In [9]: N.divide(1.,0)
> > Out[9]: inf
> 
> x/y ?

On my system, x/y (for x=0., y=1) throws a ZeroDivisionError.  Are
the two divisions supposed to behave the same?

Thanks for your feedback!

Regards
Stéfan




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