What is the most natural way to filter an array by index in Julia? The simplest example would be to leave off the kth element:
A = [1,2,3,4,5,6,7,8]
k = 4
[getindex(A, i) for i = 1:8 if i != k]
The above works but seems verbose compared to the simple A[-k] available in R. What's the cleanest way to perform this simple task?
Not as terse as the R equivalent, but fairly readable:
A[1:end .!= k]
More importantly, this can be used in multidimensional arrays too, e.g.
B[ 1:end .!= i, 1:end .!= j, 1:end .!= k ]
Have a look at deleteat!. Examples:
julia> A = [1,2,3,4,5,6,7,8]; k = 4;
julia> deleteat!(A, k)
7-element Array{Int64,1}:
1
2
3
5
6
7
8
julia> A = [1,2,3,4,5,6,7,8]; k = 2:2:8;
julia> deleteat!(A, k)
4-element Array{Int64,1}:
1
3
5
7
The simplest way is to use "Not".
julia> using InvertedIndices # use ] add InvertedIndices if not installed
julia> A = [1,2,3,4,5,6,7,8]
julia> k = 4;
julia> A[Not(k)]
7-element Array{Int64,1}:
1
2
3
5
6
7
8
You may use filter in conjunction with eachindex
julia> A = collect(1:8); println(A)
[1, 2, 3, 4, 5, 6, 7, 8]
julia> A[ filter(x->!(x in [5,6]) && x>2, eachindex(A)) ]
4-element Array{Int64,1}:
3
4
7
8
If you apply a filter to each dimension of the array, you will need to replace eachindex(A) by indices(A,n) where n is the appropriate dimension, e.g
B[ filter(x->!(x in [5,6])&&x>2, indices(B,1)), filter(x->x>3, indices(B,2)) ]
I think this questions warrants an update. There is now a package for precisely that purpose: