Julia 中的嵌套列表推导

问题描述

在 python 中，我可以进行嵌套列表推导，例如，我可以将以下数组展平:

In python I can do nested list comprehensions, for instance I can flatten the following array thus:

a = [[1,2,3],[4,5,6]]
[i for arr in a for i in arr]

获取[1,2,3,4,5,6]

如果我在 Julia 中尝试这种语法，我会得到:

If I try this syntax in Julia I get:

julia> a
([1,2,3],[4,5,6],[7,8,9])

julia> [i for arr in a for i in arr]
ERROR: syntax: expected ]

在 Julia 中可以嵌套列表推导式吗?

Are nested list comprehensions in Julia possible?

推荐答案

列表推导在 Julia 中的工作方式有点不同:

List comprehensions work a bit differently in Julia:

> [(x,y) for x=1:2, y=3:4]
2x2 Array{(Int64,Int64),2}:
 (1,3)  (1,4)
 (2,3)  (2,4)

如果 a=[[1 2],[3 4],[5 6]] 是一个多维数组，vec 会将其展平:

If a=[[1 2],[3 4],[5 6]] was a multidimensional array, vec would flatten it:

> vec(a)
6-element Array{Int64,1}:
 1
 2
 3
 4
 5
 6

由于 a 包含元组，这在 Julia 中有点复杂.这可行，但可能不是处理它的最佳方法:

Since a contains tuples, this is a bit more complicated in Julia. This works, but likely isn't the best way to handle it:

function flatten(x, y)
    state = start(x)
    if state==false
        push!(y, x)
    else
        while !done(x, state) 
          (item, state) = next(x, state) 
          flatten(item, y)
        end 
    end
    y
end
flatten(x)=flatten(x,Array(Any, 0))

然后，我们可以运行:

> flatten([(1,2),(3,4)])
4-element Array{Any,1}:
 1
 2
 3
 4

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