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

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)

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

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))

Then, we can run:

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