Quoting Tiago de Paula Peixoto (2016-09-26 13:02:03)
> On 26.09.2016 11:06, Patrick Totzke wrote:
> >
> > Hi again,
> >
> > I'm using boolean PropertyMaps to represent subsets of graph vertices
> > in combination with `GraphView(g, vfilt=Mask).vertices()` for iteration.
> > I often need the complement of such sets and would like to complement
> > a mask using `numpy.invert`, for efficiency:
> >
> > """
> > Mask = graph.new_vertex_property('bool')
> >
> > # this fails
> > import numpy
> > notm = numpy.invert(M.ma)
> > NotMask = graph.new_vertex_property('bool', vals=notm)
> >
> > # this works, but is O(|V|)
> > it = imap(lambda x:not Mask[x], graph.vertices())
> > NotMask = graph.new_vertex_property("bool", vals=it)
> > """
> >
> > The issue seems to be that in boolean PropertyMap internally
> > uses a PropertyArray of dtype `uint8`, and not `bool`:
> > In the example above (my graph has 6 nodes), Mask.ma is
> >
> > PropertyArray([0, 0, 0, 0, 0, 0], dtype=uint8)
> >
> > So of course, numpy.invert(M.ma) yields
> >
> > PropertyArray([255, 255, 255, 255, 255, 255], dtype=uint8)
> >
> > and not, as expected, the array with only 1's.
>
> You should use numpy.logical_not() instead.
Great! I overlooked this. Works like a charm :)
> > Is there a reason for using uint8 instead of booleans?
>
> Yes, efficiency. Vector of bools (i.e. vector<bool>) tends to be
> considerably slower.
>
> > And is there a O(1) operation to invert masks?
>
> Of course not, that would be magic. Both logical_lot() and invert() are
> O(N). They are faster than imap because the loops are performed in C,
> not python.
Right, OK. So if I just want to iterate over the nodes of the complement
subgraph, relative to a given vertex mask, I could
use `GraphView.vertices()`, on a graph view after setting
`gv.set_vertex_filter(Mask, inverted=True)`.
Setting this filter is O(1), as is the instantiation of the GraphView.
So far this makes perfect sense to me.
Thanks again for your quick response.
P
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