I'm trying to understand the scope rules in Julia. I've tried a for loop with z as a regular integer variable:
z = 2
for i = 1:4
z += 1
end
println(z)
and it gives me an UndefVarError: z not defined error, unless I put global z inside of the loop.
However, if I make z a 1x1 array it works perfectly fine without global:
z = [2]
for i = 1:4
z .+= 1
end
println(z)
$ julia test.jl
[6]
What's the difference between scopes of arrays and variables?
The contents of an array assigned to a global variable are constant in type:
julia> a = [1]
1-element Array{Int64,1}:
1
julia> push!(a, "s")
ERROR: MethodError: Cannot `convert` an object of type String to an
object of type Int64
And const typed globals are OK to be referenced within loops.
const x = 0; for i in 1:4; x +=1 end would work, but it doesn't.const x = 0; x = 1. It just prints a warning.for loop as long as the variable's type and memory reference don't change. This is what the manual means when it says that "in a local scope, all variables are inherited from its parent global scope block unless ... an assignment would result in a modified global variable...". Thus "modified" boils down to the memory reference changing, which happens when you change the value of a constant scalar, but doesn't happen when you change the values of elements in an array.