# Do I have to add a \`global\` before the counting variable in the \`while\` loop?

**URL:** <https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549>\
**Category:** General Usage\
**Tags:** question\
**Created:** [May 29, 2023, 10:18am UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549 "2023-05-29T10:18:56Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![WuSiren](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wusiren/32/42529_2.png) [@WuSiren](https://discourse.julialang.org/u/WuSiren)\
**Post date:** [May 29, 2023, 10:18am UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549/1 "2023-05-29T10:18:56Z")

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```julia
i = 0
while i <= 100
    println(i)
    i += 1
end

```

Write the above code in a julia script `Test.jl` and run it, in the REPL it reports the following error:

```julia
┌ 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.        
└ @ d:\CodeTest\Test.jl:4
ERROR: UndefVarError: `i` not defined
Stacktrace:
 [1] top-level scope
   @ d:\CodeTest\Test.jl:3

```

I know adding a `global` before `i += 1` can solve this problem, but I wonder if this is the typical solution to this kind of error in julia’s usage. In another word, I feel this feature of Julia language brings trouble when programming. Do I have to adapt to Julia, or there are some better ways in Julia to deal with this kind of use case?

Thanks!

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**Author:** ![rfourquet](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rfourquet/32/3610_2.png) [@rfourquet](https://discourse.julialang.org/u/rfourquet)\
**Post date:** [May 29, 2023, 11:10am UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549/2 "2023-05-29T11:10:08Z")

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This kind of use of globals

> [@WuSiren](#):
>
> but I wonder if this is the typical solution to this kind of error in julia’s usage

This is how scoping works, so using `global` is necessary if you don’t want to change the structure of your code. But the general recommendation is to wrap your code within a function.

There is actually a trick to run your code as is, but it’s using julia internals and is not officially supported. As you probably noted, you can run the initial code without a problem if it’s typed in the REPL. You can achieve the same effect when the code is in a file via:

```julia
import REPL
include(REPL.repl_ast_transforms[1], "your-script.jl")

```

---

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**Author:** ![fatteneder](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fatteneder/32/33991_2.png) [@fatteneder](https://discourse.julialang.org/u/fatteneder)\
**Post date:** [May 29, 2023, 11:16am UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549/3 "2023-05-29T11:16:44Z")

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Try

```julia
let
  i = 0
  while i <= 100
      println(i)
      i += 1
  end
end

```

and the warning is gone.  
If you need the value of `i` later on, use

```julia
i = let
  i = 0
  while i <= 100
      println(i)
      i += 1
  end
  i
end

```

---

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**Author:** ![cormullion](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cormullion/32/49131_2.png) [@cormullion](https://discourse.julialang.org/u/cormullion)\
**Post date:** [May 29, 2023, 11:25am UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549/4 "2023-05-29T11:25:59Z")

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> [@WuSiren](#):
>
> Do I have to adapt to Julia

Well, yes… 🙂 If you’re coming from another language, you’ll need to focus on the things which are different. - sometimes you have to unlearn things as well as learn things.

---

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**Author:** ![WuSiren](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wusiren/32/42529_2.png) [@WuSiren](https://discourse.julialang.org/u/WuSiren)\
**Post date:** [May 29, 2023, 11:58am UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549/5 "2023-05-29T11:58:37Z")

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Thanks for erveryone’s kind reply! They are all helpful to me.

Now I have another question. The error report says I have an undefined `i` in the 3rd line of my script, which corresponds to the command `println(i)`. If I add a `global i` line above `println(i)`, there will be no error or warning. This is understandable:

```julia
i = 0
while i <= 100
    global i
    println(i)
    i += 1
end

```

But if I only add the `global` in the 4th line before `i += 1`,

```julia
i = 0
while i <= 100
    println(i)
    global i += 1
end

```

I can’t understand why the `UndefVarError: `i` not defined` corresponding to the 3rd line `println(i)` also disappears. As we can see, I didn’t declare any `global i` before the 3rd line in the `while` loop, then how did `println(i)` recognize the `i` in this case?

I’m not sure if I expressed myself clearly…

---

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**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [May 29, 2023, 12:19pm UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549/6 "2023-05-29T12:19:27Z")

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You **could** write this, which probably is clearer:

```julia
i = 0
while i <= 100
    global i
    println(i)
    i += 1
end

```

thus, just specify that your reference to `i` is about the global variable just at the begining of the loop. (The `while` loop is parsed at all once, as I understand, that’s why you can use `global` at any point).

Yet, **you should not do this** , neither any other alternative that uses a global variable, most of the time. It is better if you write:

```julia
function myloop()
    i = 0
    while i <= 100
        i += 1
        # do your stuff here
    end
end
myloop() # call the function

```

This will allow Julia to compile your `myloop()` function entirely and the code will potentially be much faster. Additionally, if you use `Revise`, you can do

```julia
julia> using Revise

julia> includet("./myscript.jl") # note the t

julia> myloop()

```

and then any modification to the function will be tracked, and you just need to run `myloop()` again in the REPL to see the changes in action.

Concerning scopes, the manual section is this one: [Scope of Variables · The Julia Language](https://docs.julialang.org/en/v1/manual/variables-and-scoping/)

I have some notes here with a simpler and personal account of that: [Scope of loops · JuliaNotes.jl](https://m3g.github.io/JuliaNotes.jl/stable/loopscopes/)

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**Author:** ![WuSiren](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wusiren/32/42529_2.png) [@WuSiren](https://discourse.julialang.org/u/WuSiren)\
**Post date:** [May 29, 2023, 1:51pm UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549/7 "2023-05-29T13:51:06Z")

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Thank you very much for your professional answer! I will spend some time on digesting and absorbing. 🤝

---

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**Author:** ![LeePhillips](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/leephillips/32/205514_2.png) [@LeePhillips](https://discourse.julialang.org/u/LeePhillips)\
**Post date:** [May 29, 2023, 3:11pm UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549/8 "2023-05-29T15:11:54Z")

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> I can’t understand why the `UndefVarError: ` i` not defined` corresponding to the 3rd line `println(i)` also disappears.

A type declaration within a scope block enforces an unchangeable type for the entire block. The declaration can occur anywhere within the block.

---

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**Author:** ![WuSiren](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/wusiren/32/42529_2.png) [@WuSiren](https://discourse.julialang.org/u/WuSiren)\
**Post date:** [May 29, 2023, 3:15pm UTC](https://discourse.julialang.org/t/do-i-have-to-add-a-global-before-the-counting-variable-in-the-while-loop/99549/9 "2023-05-29T15:15:02Z")

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I see. Thanks! 🤝