Why does the soft scope warning not appear in a nested loop?

New to Julia
1

Hello, I am confused because this basic loop yields a warning:

i = 5
for j in 1:5
    # ...
    i = "other_value"
    # ...
end

┌ Warning: Assignment to `i` in soft scope is ambiguous because a global variable by the same name exists: `i` will be treated as a new local. Disambiguate by using `local i` to suppress this warning or `global i` to assign to the existing global variable.
└ @ main.jl:4

… which I understand. But the following yields no warning:

for i in 1:5
    # ...
    for j in 1:5
        # ...
        i = "other_value"
        # ...
    end
end

… and I’ve just struggle hard to find a bug because I inadvertently wrote something like this X) Is there a reason for the warning to exist in the former case, but not the latter?

2

In that case i belongs to the scope of the outer loop (thus a local scope).

The problem with the first one is that having a global i has a whole set of implications for type inference and performance (because i is global), and for that it has to be deal with differently.

2 Likes
3

Oh, okay, so there is something really special with the global scope, and this is what this warning is about. It is not just about “i belongs to the outer scope”. I understand now, thank you :slight_smile:

2 Likes
4

Yes, that is why it is highly recommended to avoid the use of global variables. If you wrap everything in a function, or a let block, for example, which define their own scope, you won’t get those warnings either.

2 Likes
5

Well, in this case I would rather have had the warning instead of my error silently passing X)

6

Uhm… not sure if I understand. In that case there would be no “error”, the variable would be modified as intended:

julia> function f()
           i = 1
           for j in 1:2
               i += 1
           end
           @show i
       end
f (generic function with 1 method)

julia> f()
i = 3
3

julia> let 
           i = 1
           for j in 1:2
               i += 1
           end
           @show i
       end
i = 3
3

Take a look at this: Scope of loops · JuliaNotes.jl

7

Haha yes exactly, but my intent was not to modify it :slight_smile: Picking i for the inner loop variable was an unfortunate name clash in my case. I’ll be more careful in the future ^ ^

1 Like
8

You can always declare a variable as local to avoid it picking up any previous definitions:

function f()
   i = 100
   for j in 1:2
       local i = 0
       i = i + j
   end
   println(i)
end

julia> f()
100
7 Likes