[Numpy-discussion] LaTeX version of boolean indexing

[Andras Deak]

On Thu, Oct 11, 2018 at 7:45 PM Matthew Harrigan
<harrigan.matthew at gmail.com> wrote:
>
> What do you mean by indicator?
>

I mostly meant what wikipedia seems to call "set-builder notation"
(https://en.wikipedia.org/wiki/Set-builder_notation#Sets_defined_by_a_predicate).
Since your "input" is `{x_i | i in [0,1,2]}` but your output is a `y_j
for j in [0,1]`, the straightforward thing I could think of was
defining the set of valid `y_j` values (with an implicit assumption of
the order being preserved, I guess). This would mean you can say
something like
`y_i \in {x_j | m_j}` (omitting the \left/\right/\vert fluff for
simplicity here) where `m_j` are the elements of the boolean mask
(say, `m = [True, False, True]`). In this context I'd understand it
that `m_j` is the predicate and `x_j` are the corresponding values,
however the notation isn't entirely ambiguous (see also a remark on
the above wikipedia page) so you can't really get away with omitting
further explanation in order to resolve ambiguity. Though I guess
calling `m_j` elements of a mask would do the same thing.
The other option that comes to mind is to define the auxiliary indices
`n_i` for which `m_j` are True, then you of course denote the result
with integer indices: `y_i = x_{n_i}` where `i` goes from 0 to the
number of `True`s in `m_j`. But then you have the same difficulty
defining `n_i`.
All in all I'm not sure there's an elegant and concise notation for
boolean masking.

András