Julia throws undefined error when the variable is defined - julia

Question

I have this simple while loop that uses i = 1 as an index.
global i = 1
n = rows
while i <= n
if prod(isa.(collect((y)[i,:]),Number))==0
delete!(y,i)
x_axis = x_axis[1:end .!= i]
n -= 1
end
i += 1
end
but I'm getting this error:
UndefVarError: i not defined
top-level scope#Local: 23
I even made my i global as per the suggestion on some similar questions on SO but the error persists. I am running this on Pluto.jl so maybe it could be an environment issue.

Answer 1

Firstly, note that if you use Julia v1.5+ then you don't need to make i a global (example below with current stable version v1.5.4):
julia> i = 1
1
julia> while i < 5
println(i)
i += 1 # <----- this works in Julia v1.5+
end
1
2
3
4
However, it seems you are using Julia v1.4 of older, in which case I think Logan Kilpatrick gave you the answer: You need to make i a global from inside the while loop's scope. As Logan mentioned, try adding global where you increment i, like in this example from the while function's docs:
julia> i = 1 ;
julia> while i < 5
println(i)
global i += 1 # <------------ Try this!
end
1
2
3
4
Note also that you don't need to specify it's a global if your while loop is inside a function, as in
julia> function foo(istart)
i = istart
while i < 5
println(i)
i += 1 # <-- 'global' not needed inside a function!
end
end
foo (generic function with 1 method)
julia> foo(1)
1
2
3
4

Answer 2

You have hitted the "ambiguous soft scope case".
In short: the assignment of a local variable inside a (soft) local scope depends if the code is inside a "REPL context"
or not.
For "REPL context" I mean the REPL and all environments that behaves as the REPL in this case, for example
Jupyter:
julia> i = 0
julia> while i < 3
i += 1
#info i
end
[ Info: 1
[ Info: 2
[ Info: 3
Instead code from non interactive context like file, eval and Pluto acts like that:
julia> code = """
i = 0
while i < 3
i += 1
#info i
end
"""
julia> include_string(Main, code)
┌ 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.
└ # string:3
ERROR: LoadError: UndefVarError: i not defined
All of this has been designed to ensure both the convenience of REPL usage and to avoid unwanted side effects of using julia on a large scale.
Full details here.
To fix the problem you may be use global as already suggested or enclose your code inside a function.

Answer 3

Pluto implicitly wraps a cell into a function, see https://github.com/fonsp/Pluto.jl/pull/720, therefore the global annotation or explicit wrapping into a function should not be required.
Putting the following into a Pluto cells works for me:
begin
i = 1
n = 100
while i<=n
if i % 2 == 0
n -= 1
end
i += 1
end
end
The implicit function wrapping is disabled when macros are used inside a cell (which prevents Pluto to gather reactivity information), therefore the following does not work in Pluto due to the Julia scoping rules:
begin
i = 1
n = 100
while i<=n
if i % 2 == 0
n -= 1
end
#show i += 1
end
end
Throws:
UndefVarError: i not defined
top-level scope#Local: 5[inlined]
top-level scope#none:0