防止 Julia 自动转换一维矩阵切片的类型 [英] Prevent Julia from automatically converting the type of a 1D matrix slice

问题描述

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

我只是想阻止自动类型转换，但我很难弄清楚如何做到这一点.是的，我可以直接使用 alpha[:, 1]''，但我想防止内存重新分配.vec() 用于另一个方向(1xn 矩阵)，但我找不到将 (nx1) 矩阵保持为矩阵的函数.

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.

推荐答案

使用长度为 1 的范围，而不仅仅是索引

不要简单地指定所需列的索引 (Int64)，而是指定 长度 1 的范围 (UnitRange{Int64}): 1:1.

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.

这将诱使 Julia 保留二维数组类型 (Array{Int64,2})，而不是返回向量 (Array{Int64,1}).

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

编辑:开发者在这里讨论了这个话题(感谢 科林指点我).

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

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