De-structure results from broadcasting

General Usage
1

Hello!

When not using broadcasting, there is a way to destructure results from a function:

julia> f(x) = (x, x^2);

julia> (y1, y2) = f(5);

julia> y1
5

julia> y2
25

However, this does not work while broadcasting:

julia> x = 1:10
1:10

julia> (z1, z2) = f.(x);

julia> z1
(1, 1)

julia> z2
(2, 4)

Is there any way to do the same, and have in z1 and z2 arrays with the collection of the first and second arguments of the result respectively?

2

Sadly, no. The feature you want is unzip. There is a PR here which has not gotten merged into Base.

There is a package for this, though: Unzip.jl.

1 Like
3

notice this produces:

julia> f.(1:10)
10-element Vector{Tuple{Int64, Int64}}:
 (1, 1)
 (2, 4)
 (3, 9)
 (4, 16)
 (5, 25)
 (6, 36)
 (7, 49)
 (8, 64)
 (9, 81)
 (10, 100)

which is not ideal storage in memory to begin with if you’re interested in the “columns” (in this display). you can do:

julia> [x^y for x in 1:10, y in 1:2]
10Ă—2 Matrix{Int64}:
  1    1
  2    4
  3    9
  4   16
  5   25
  6   36
  7   49
  8   64
  9   81
 10  100

Sadly, there’s no syntax for multi dimensional comprehension (probably would be too confusing)

4

You can also do this with StructArrays.jl:

julia> using StructArrays

julia> using StructArrays: components

julia> z1, z2 = components(StructArray(f.(1:10)))
([1, 2, 3, 4, 5, 6, 7, 8, 9, 10], [1, 4, 9, 16, 25, 36, 49, 64, 81, 100])

edit: this naturally suggests

unzip(f, x) = components(StructArray(f(xᵢ...) for xᵢ in x))

which works well for multidimensional iterators:

julia> g(x, y) = x*y, y/x
g (generic function with 1 method)

julia> v1, v2 = unzip(g, Iterators.product(1:3, 1:4))
([1 2 3 4; 2 4 6 8; 3 6 9 12], [1.0 2.0 3.0 4.0; 0.5 1.0 1.5 2.0; 0.3333333333333333 0.6666666666666666 1.0 1.3333333333333333])

julia> v1
3Ă—4 Matrix{Int64}:
 1  2  3   4
 2  4  6   8
 3  6  9  12
1 Like
5

There is also Destruct.jl:

using Destruct
f(x) = (x, x^2)
x = 1:10
z1, z2 = f.(x) |> destruct

This yields

z1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
z2 = [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]