CODEWITHSUNDEEP
for-loopmodulescopejulia
Clemens Watzenböck
Clemens Watzenböck

Reputation: 23

How does the scope of variables in julia modules work?

Running this code

module mod1
export foo

function foo()
    i=0
    for ii in 1:10
        global i+=ii
    end
    i
end

end

mod1.foo()

gives UndefVarError: i not defined.

It seems that the way to do it is by adding a global before the variable i:

module mod2
export bar

function bar()
    global i=0
    for ii in 1:10
        global i+=ii
    end
    i
end

end

mod2.bar()

This gives: 55

Why does the first method not work? As I understand the for introduces a new scope. Therefore I need the global inside the loop. But why do I also need it outside of the loop?

(I am using julia 1.5.0)

Upvotes: 2

Views: 298

Answers (1)

François Févotte
François Févotte

Reputation: 20248

The correct way to write this would be:

module mod1
export foo

function foo()
    i=0
    for ii in 1:10
        i+=ii
    end
    i
end

end

julia> mod1.foo()
55

There are 3 scopes introduced here (from outermost to innermost):

  1. global scope of module mod1
  2. local scope of function foo
  3. local scope of for body

The rules for scoping are explained in the manual. In this particular case: when i is referred to in the for body, we first look for a variable named i defined in that same scope. Since none is found, we look for it in the enclosing scope which is the local scope of foo... and find the variable declared using i=0.

If no variable named i had been found in the local foo scope, we'd have had to look into the enclosing scope, which is a global scope. And in this case, you'd have had to put the global keyword to explicitly allow it (because it has performance implications).

Upvotes: 2

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