assign values to arrays with eval and symbols in Julia - arrays

Question

I have a dictionary that has 3 key, each key has arbitrary 2d arrays. And I want to assign each of these values(2d arrays) to newly created arrays. So I wrote this :
levels = (:easy, :medium, :hard)
easy = []; medium = []; hard = [];
curriculum = Dict((k=>rand(3,3) for k in levels)...); # dictionary with 3 keys - each key is a symbol
for level in levels
eval(level) = curriculum[level]
end
There is no problem during execution but when I check the easy, medium or hard arrays after, there are no matrices inside.
What is the proper way of doing this?
B.R.
(edited)
What I understand is that during the loop iteration eval(level) behaves like local binding. Thus, at the end of the iteration it does not remember anything. But why? And how to convert to global again ?

Answer 1

There is no problem during execution but when I check the easy, medium or hard arrays after, there are no matrices inside.
What I understand is that during the loop iteration eval(level) behaves like local binding. Thus, at the end of the iteration it does not remember anything. But why? And how to convert to global again ?
The problem is that you are defining and then redefining an eval function in each iteration inside the for loop, instead of doing assignments as you believe.
easy = []; medium = []; hard = []
1.- There is no need to predefine this variables.
eval(level) = curriculum[level]
2.- Warning: This is being interpreted as a short style function definition!
julia> eval(level) = curriculum[level]
eval (generic function with 1 method)
julia> levels = :easy, :medium, :hard
(:easy, :medium, :hard)
julia> curriculum = Dict((k => rand(3, 3) for k in levels)...)
Dict{Symbol,Array{Float64,2}} with 3 entries:
:medium => [0.230877 0.244128 0.205712; 0.649405 0.887568 0.957849; 0.245076 0.476088 0.160561]
:hard => [0.0424943 0.157261 0.798607; 0.590535 0.78125 0.704322; 0.555827 0.675355 0.74026]
:easy => [0.715595 0.914093 0.517973; 0.750787 0.489784 0.491666; 0.1966 0.133281 0.249144]
julia> for level in levels
#eval $level = curriculum[$(Meta.quot(level))]
end
julia> easy
3×3 Array{Float64,2}:
0.715595 0.914093 0.517973
0.750787 0.489784 0.491666
0.1966 0.133281 0.249144
3.- Inside the for loop, in the first iteration, $level is interpreted as the easy variable identifier, while $(Meta.quot(level)) is interpreted as the symbol :easy, for which the extra level of quoting is needed.
julia> Meta.quot(:foo)
:(:foo)
julia> Expr(:quote, :foo)
:(:foo)

Answer 2

Edit there is a better way as pointed out in the accepted answer and comment by Ismael. Namely that:
global level = level is not necessary, see my answer, the issue is
that if you want to interpolate a symbol into an expression as a
symbol instead of as an identifier, then you need to wrap the symbol
into another quote expression.
Warning: this is (almost certainly) not the best way of setting out to accomplish whatever you want to do. Metaprogramming in julia is super powerful and useful but you should very rarely need to use eval. You can read more about it here. If you ask another question with the details of your end-goal you might get a better answer :)
Nonetheless:
If you want to do the assignment this way you'll have to create an Expression Expr that assigns the matrix object to the variable with the name you want. But because eval always executes the statement in the global scope you'll need to make the variable you want to use available in the global scope.
for level in levels
global level = level
eval(:($level = curriculum[level]))
end
or using the #eval macro:
for level in levels
global level = level
#eval $level = curriculum[level]
end