[SciPy-dev] Numpy example for arctan2 perhaps wrong/misleading/confusing?

[David Huard]

Samuel,

What you want is this :

> [4]: def arctan2(vectors):
   ...:     """Return the angle between the x axis and each vector in
vectors."""
   ...:     v = np.atleast_2d(vectors)
   ...:     return np.arctan2(v[:,0], v[:,1])
   ...:

> [5]: arctan2(([0,0], [1,0]))
< [5]: array([ 0.        ,  1.57079633])


You could also use the numpy.angle function which returns the angle of a
complex argument.

> [15]: np.angle([0, 1j])
 <[15]: array([ 0.        ,  1.57079633])


David


2008/6/9 Samuel John <scipy@samueljohn.de>:

> Hi David!
>
> On Mon, Jun 9, 2008 at 3:56 PM, David Huard <david.huard@gmail.com> wrote:
> > One thing that you should know is that all "ufunc" (universal functions)
> are
> > defined as functions operating on scalars. Numpy then wraps these
> functions
> > such that they can process ndarrays of any shape element by element. This
> is
> > what is explained in the help(arctan2) documentation.
>
> I see the point for arctan and other functions with a single argument, but
> not
> for arctan2, especially. It needs two arguments and if I want to
> compute the result of the vectors a=array([a1,a2]) and b=array([b1,b2])
> like
>
> arctan2(a,b)
>
> what I get is are the two results as if I would have written:
>
> array([ artcan2(a1,b1), arctan2(a2,b2) ])  #Note, this is NOT (a1,a2)
> or (b1,b2) !!
>
> If you know this and distribute your vector entries over two arrays, the
> broadcasting may be very useful. (As it is for many other functions!)
>
> It would be perhaps nice to call arctan2 with just ONE array argument
> and have a broadcasting like this:
>  arctan2( array([[a1,a2],[b1,b2],[c1,c2]]) )
>  --> array([arctan2(a1,a2), arctan2(b1,b2), arctan2(c1,c2) ] )
>
> and arctan2( array([x,y]) ) --> arctan2(x,y)
>
> > As for the example, maybe a way to avoid confusion would be to give an
> > example with scalar arguments first to show the basic behavior, and then
> an
> > example using ndarray broadcasting.
>
> I would argue to show just the scalar version in the example and leave out
> the broadcasting version, because it makes less sense here, doesn't it?
> Is there any fancy matrix-or-whatever notation, that really makes up a
> good example for broadcasting with arctan2?
>
> cheers,
>  Samuel
>
> PS: I am not against the current broadcasting scheme. It is perfectly ok.
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