arrays - Julia 截断数组数组的内部维度

Question

给定一个数组数组

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

什么是截断每个内部数组的简洁有效的方法，以便我最终得到

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

有没有像 x[:,1:2] 这样简单的多维数组？

Answer 1

您可以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]

好像比使用map快一点:

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)