unexpected behaviour inside for loop

[thudjx]

I am trying to use @tullio inside a loop to construct an array part by part:

julia> x = zeros(2,2,3)
julia> for k in 1:3
               z = reshape(range((k-1)*4+1, stop=k*4), 2,2)
               @tullio x[i,j,k] = z[i,j]
       end

The result is kind of surprising:

julia> x
2×2×3 Array{Float64, 3}:
[:, :, 1] =
  9.0  11.0
 10.0  12.0

[:, :, 2] =
  9.0  11.0
 10.0  12.0

[:, :, 3] =
  9.0  11.0
 10.0  12.0

This means in @tullio x[i,j,k] = z[i,j], k is taken as a new variable ranging from 1 to 3 rather than the predefined k. And I should use view to get what I want:

julia> for k in 1:3
           z = reshape(range((k-1)*4+1, stop=k*4), 2,2)
           x_catch = view(x, :,:,k)
           @tullio x_catch[i,j] = z[i,j]
       end

julia> x
2×2×3 Array{Float64, 3}:
[:, :, 1] =
 1.0  3.0
 2.0  4.0

[:, :, 2] =
 5.0  7.0
 6.0  8.0

[:, :, 3] =
  9.0  11.0
 10.0  12.0

Is it designed like this? Or maybe you should give some warning as in Einsum.jl?

[mcabbott]

Yes this is intentional. Every index is a new local variable, and is unaffected by whether one of the same name exists outside -- as it would be if you wrote for i in 1:2, k in 1:3 by hand.

But there is syntax for treating a particular index as a constant instead, using a dollar sign. So I think @tullio x[i,j,$k] = z[i,j] does what you want here, and introduces loops over only i and j.