Julia truncate inner dimension of array of arrays

Question

Given an array of arrays

x = [[1, 2, 3, 4], [4, 5, 6, 7]] ,

what is the clean & efficient way to truncate each of the inner arrays such that I end up with

[[1, 2], [4, 5]] ?

Is there anything as simple as x[:,1:2] like for multidimensional arrays?

Answer 1

You can broadcast getindex :

julia> x = [[1, 2, 3, 4], [5, 6, 7, 8]];

julia> getindex.(x, (1:2,))
2-element Array{Array{Int64,1},1}:
 [1, 2]
 [5, 6]

It seems to be a bit faster than using map :

julia> foo(xs) = getindex.(xs, (1:2,))
foo (generic function with 1 method)

julia> bar(xs) = map(x -> x[1:2], xs)
bar (generic function with 1 method)

julia> @btime foo($([rand(1000) for _ in 1:1000]));
  55.558 μs (1001 allocations: 101.69 KiB)

julia> @btime bar($([rand(1000) for _ in 1:1000]));
  58.841 μs (1002 allocations: 101.70 KiB)

Answer 2

If you are ok with mutating the input vectors, then this is a very efficient way of doing it:

resize!.(x, 2)

This mutates x in place, so you don't need to assign an output variable.

You can also use view . That doesn't mutate the input, but neither does it allocate a new vector. It's faster than the broadcasted getindex , but not as fast as resize! :

xv = view.(x, Ref(1:2))