El viernes, 21 de febrero de 2014 08:25:00 UTC-6, Mauro escribió:
>
> > Given a matrix, which will be large (say 10^5 x 10^5), I need to extract
> > the list of indices (i.e. the pairs (x,y) of positions) of those places
> in
> > the matrix
> > where the value stored satisfies a certain condition. For a minimal
> > example, the condition can just be that the value is greater than 0.5.
>
> Give you a logical array which you can use for indexing:
> r.>.5
> and to get the linear indices
> find(r.>.5)
>
Great, didn't know about find.
Maybe I should just rewrite everything thinking in terms of 1D arrays.
(This is something I have taught in several courses,
but for some reason it didn't occur to me!)
Thanks,
David.
>
> > The code below achieves this, but seems inefficient, since it constructs
> > the whole array of indices, even though there may be only a few places
> > where the condition is satisfied.
> > Is there a more efficient way of doing this?
> > Is there an "if" clause in an array comprehension? -- adding this to the
> > definition of 'indices' would seem to do what I want, but I have not
> found
> > this syntax in the manual for arrays.
>
> There is no if clause for comprehensions but you can always just write
> the loops.
>
> > Thanks,
> > David.
> >
> >
> > julia> L = 5
> > 5
> >
> > julia> r= rand(L, L)
> > 5x5 Array{Float64,2}:
> > 0.705585 0.534721 0.158935 0.343876 0.624299
> > 0.0624089 0.525414 0.131139 0.590439 0.554686
> > 0.190085 0.557751 0.591916 0.485526 0.6307
> > 0.365398 0.943102 0.575083 0.858705 0.105142
> > 0.047924 0.116424 0.756757 0.576293 0.461547
> >
> > julia> indices = [(x,y) for x in 1:L, y in 1:L]
> > 5x5 Array{(Any,Any),2}:
> > (1,1) (1,2) (1,3) (1,4) (1,5)
> > (2,1) (2,2) (2,3) (2,4) (2,5)
> > (3,1) (3,2) (3,3) (3,4) (3,5)
> > (4,1) (4,2) (4,3) (4,4) (4,5)
> > (5,1) (5,2) (5,3) (5,4) (5,5)
> >
> > julia> where = indices[r .> 0.5]
> > 14-element Array{(Any,Any),1}:
> > (1,1)
> > (1,2)
> > (2,2)
> > (3,2)
> > (4,2)
> > (3,3)
> > (4,3)
> > (5,3)
> > (2,4)
> > (4,4)
> > (5,4)
> > (1,5)
> > (2,5)
> > (3,5)
>
> --
>
>
