# Misunderstood scope rules?
**URL:** https://discourse.julialang.org/t/misunderstood-scope-rules/53097
**Category:** General Usage
**Created:** [January 9, 2021, 10:19pm UTC](https://discourse.julialang.org/t/misunderstood-scope-rules/53097 "2021-01-09T22:19:16Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@austinbean](https://discourse.julialang.org/u/austinbean)
#### Post date: [January 9, 2021, 10:19pm UTC](https://discourse.julialang.org/t/misunderstood-scope-rules/53097/1 "2021-01-09T22:19:16Z")
I think I’m misunderstanding something about variable scopes within functions and I would like some feedback.
Here is a MWE:
```julia
# this function modifies its argument
# and the argument 'stays modified' outside of f1
function f1(x::Array)
for i = 1:10
x .+= 1
end
end
a = zeros(50);
f1(a)
println("a: ", a) # as expected - a = [10.0, ..., 10.0]
```
But this case does **not** work as I expected:
```julia
# this function modifies x and y within f2
# but y does not 'stay modified' outside the scope of f2
function f2(x::Array, y::Array )
for i = 1:2
f1(x)
y += x # this line is not working as I thought it would.
end
end
b = zeros(50);
c = zeros(50);
f2(b,c)
println("c: ", c) # expected c = [30,...,30] but c = [0, 0, ..., 0]
```
What is the correct way to think about about the sum of `y` and `x` _inside_ `f2` ?
Is there a more idiomatic way to do what I want in this case?
As always, I appreciate the feedback!
---
### Author:  [@Iulian.Cioarca](https://discourse.julialang.org/u/Iulian.Cioarca)
#### Post date: [January 9, 2021, 10:29pm UTC](https://discourse.julialang.org/t/misunderstood-scope-rules/53097/2 "2021-01-09T22:29:06Z")
```julia
y .+= x
```
You forgot a dot. Your example creates a local variable y and binds the result of y+x to it. Using the dot modifies in place y.
---
### Author:  [@austinbean](https://discourse.julialang.org/u/austinbean)
#### Post date: [January 10, 2021, 6:27pm UTC](https://discourse.julialang.org/t/misunderstood-scope-rules/53097/3 "2021-01-10T18:27:16Z")
Thanks very much for the reply. An easy fix too!
Is the correct way to understand what’s going on that `y += x` (no dot) creates a _new_ local variable `y` (w/ the same name as the argument of the function), while `y .+= x` does not create a new local variable?
---
### Author:  [@lmiq](https://discourse.julialang.org/u/lmiq)
#### Post date: [January 10, 2021, 8:14pm UTC](https://discourse.julialang.org/t/misunderstood-scope-rules/53097/4 "2021-01-10T20:14:02Z")
Well, be careful. `y += x` is a short notation for
```julia
y = (y + x) # parentheses are just for clarity
```
The sum in parentheses creates a new local, and the equality assigns the label `y` to that new local.
With the dot you have a concise notation for
```julia
for i in 1:length(x)
y[i] = y[i] + x[i]
end
```
Which mutates the values of `y`.
These explanations extracted from previous questions help:
[https://m3g.github.io/JuliaCookBook.jl/stable/assignment/](https://m3g.github.io/JuliaCookBook.jl/stable/assignment/)