Prevent Julia from automatically converting the ty

问题:

alpha = [1 2 3; 4 5 6]
alpha[:, 1] # Type is Array{Int64, 1}
alpha[:, 1:2] # Type is Array{In64, 2}

I just want to prevent the automatic type conversion, but I am having an amazingly hard time figuring out how to do this. Yeah, I could just go alpha[:, 1]'', but I want to prevent the memory reallocation. There is vec() for going the other direction (1xn matrix) but I can't find a function for keeping a (nx1) matrix a matrix.

回答1:

Use a range of length 1 instead of just an index

Instead of simply specifying the index (Int64) of the desired column, specify a range (UnitRange{Int64}) of length 1: 1:1.

That will trick Julia into preserving the 2D-array type (Array{Int64,2}) instead of returning a vector (Array{Int64,1}).

Edit: the developers discussed this topic here (thanks to Colin for pointing me to it).

julia> alpha = [1 2 3; 4 5 6]
2x3 Array{Int64,2}:
 1  2  3
 4  5  6

julia> alpha[:,1]            # nope
2-element Array{Int64,1}:
 1
 4    

julia> alpha[:,1:1]          # yep
2x1 Array{Int64,2}:
 1
 4