Nested list comprehensions in Julia

Question

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]
         


        

Answer 1

Any reason why you're using a tuple of vectors? It's much simpler with arrays, as Ben has already shown with vec. But you can also use comprehensions pretty simply in either case:

julia> a = ([1,2,3],[4,5,6],[7,8,9]);
julia> [i for i in hcat(a...)]
9-element Array{Any,1}:
 1
 2
 ⋮

The expression hcat(a...) "splats" your tuple and concatenates it into an array. But remember that, unlike Python, Julia uses column-major array semantics. You have three column vectors in your tuple; is that what you intend? (If they were row vectors — delimited by spaces — you could just use [a...] to do the concatenation). Arrays are iterated through all elements, regardless of their dimensionality.