> 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)
> 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)
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