Is it possible to have the last iterator change the fastest in Iterator.product?

General Usage
1

In Julia’s Iterators.product, the first iterator changes the fastest. Is there an easy way to get elements in an order, so that the last iterator changes the fastest? For example with three sequences, I obtain:

res = Iterators.product([:A, :B, :C], [:X, :Y], [1,2]) |> collect
3×2×2 Array{Tuple{Symbol, Symbol, Int64}, 3}:
[:, :, 1] =
 (:A, :X, 1)  (:A, :Y, 1)
 (:B, :X, 1)  (:B, :Y, 1)
 (:C, :X, 1)  (:C, :Y, 1)

[:, :, 2] =
 (:A, :X, 2)  (:A, :Y, 2)
 (:B, :X, 2)  (:B, :Y, 2)
 (:C, :X, 2)  (:C, :Y, 2)

If I get elements using res[:] I obtain:

julia> res[:]
12-element Vector{Tuple{Symbol, Symbol, Int64}}:
 (:A, :X, 1)
 (:B, :X, 1)
 (:C, :X, 1)
 (:A, :Y, 1)
 (:B, :Y, 1)
 (:C, :Y, 1)
 (:A, :X, 2)
 (:B, :X, 2)
 (:C, :X, 2)
 (:A, :Y, 2)
 (:B, :Y, 2)
 (:C, :Y, 2)

Instead, I want:

(:A, :X, 1)
 (:A, :X, 2)
 (:A, :Y, 1)
 (:A, :Y, 2)
 (:B, :X, 1)
 (:B, :X, 2)
 (:B, :Y, 2)
 (:B, :Y, 2)
(:C, :X, 1)
 (:C, :X, 2)
 (:C, :Y, 2)
 (:C, :Y, 2)

What is the easiest way to get that from the res variable? Thank you!

1 Like
2

May I suggest you use different elements in the three iterators for your example? I think that would make it clearer

1 Like
3 
julia> res = Iterators.map(reverse, Iterators.product([:C, :D], [:C, :D], [:C, :D])) |> collect |> vec
8-element Vector{Tuple{Symbol, Symbol, Symbol}}:
 (:C, :C, :C)
 (:C, :C, :D)
 (:C, :D, :C)
 (:C, :D, :D)
 (:D, :C, :C)
 (:D, :C, :D)
 (:D, :D, :C)
 (:D, :D, :D)
4 Likes
4

One way to do that is to use reverse:

reverse.(res)

But you’ll probably want to first reverse the order of the arguments to product, too.

5

Thank you @Elrod and @lostella. This works for the example I had given earlier. On lostella’s suggestion I changed the example to have different elements. This “reverse” solution does not work for the modified example. Is there a solution that works for this modified example?

Actually using reverse along with reversing the order of arguments to the Iterators.product (as lostella suggested) works well.

Thank you.

1 Like