如何连接 Julia 中的两个向量？

Question

Given two vectors a = [1, 2] and b = [3, 4] , how do I obtain the concatenated vector c = [1, 2, 3, 4] ? It seems that hcat or vcat could be used as they work on arrays, but when using vectors to store collections of elements it seems unfitting to first think about the orientation of the data; it's just supposed to be a list of values.

Answer 1

Most Array methods treat arrays as general "tensors" of arbitrary ranks ("data cubes"), so you do need to think about the orientation. In the general case, there's cat(a, b; dims) , of which hcat and vcat are special cases.

There is another class of methods treating Vector s as list like. From those, append! is the method that, well, appends a vector to another. The problem is that it is mutable. So you can, for example, append,(copy(a), b) , or use something like BangBang.NoBang.append (which just selects the right method internally, though).

For the case of more than two vectors to be concatenated, I like the pattern of

reduce(append!, (a, b), init=Int[])

Answer 2

You can write

[a; b]

Under the hood this is the same as vcat , but it's terser, looks better, and is easier to remember, as it's also consistent with literal matrix construction syntax.

An alternative for concatenating multiple vectors is

reduce(vcat, (a, b))