Julia abbreviation for index set of array

Question

Consider two matrices of different size

julia> A= ones(Float64,3,3)
3×3 Matrix{Float64}:
 1.0  1.0  1.0
 1.0  1.0  1.0
 1.0  1.0  1.0

julia> B = ones(Float64,2,2)
2×2 Matrix{Float64}:
 1.0  1.0
 1.0  1.0

Is there a shorthand notation for

julia> A[1:size(B)[1],1:size(B)[2]]
2×2 Matrix{Float64}:
 1.0  1.0
 1.0  1.0

in the form of a nice function like "julia> A[indices(B)]" (which unfortunately does not exist ERROR: UndefVarError: indices not defined)?

Answer 1

You can call axes(B) to get a tuple containing the valid indices for B:

julia> A = ones(3, 3)
3×3 Matrix{Float64}:
 1.0  1.0  1.0
 1.0  1.0  1.0
 1.0  1.0  1.0

julia> B = ones(2, 2)
2×2 Matrix{Float64}:
 1.0  1.0
 1.0  1.0

julia> axes(B)
(Base.OneTo(2), Base.OneTo(2))

Thus, you can unpack the tuple and use it for indexing A:

julia> A[axes(B)...]
2×2 Matrix{Float64}:
 1.0  1.0
 1.0  1.0

As Bogumil Kaminski pointed out in the comments, you can also use CartesianIndices:

julia> A[CartesianIndices(B)]
2×2 Matrix{Float64}:
 1.0  1.0
 1.0  1.0