# Scoping in loops and multi-threading

**URL:** <https://discourse.julialang.org/t/scoping-in-loops-and-multi-threading/71675>\
**Category:** New to Julia\
**Tags:** multithreading\
**Created:** [November 17, 2021, 5:32pm UTC](https://discourse.julialang.org/t/scoping-in-loops-and-multi-threading/71675 "2021-11-17T17:32:53Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![rubaiyat](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rubaiyat/32/27423_2.png) [@rubaiyat](https://discourse.julialang.org/u/rubaiyat)\
**Post date:** [November 17, 2021, 5:32pm UTC](https://discourse.julialang.org/t/scoping-in-loops-and-multi-threading/71675/1 "2021-11-17T17:32:53Z")

</div>

Hello everyone,  
I just started using multi-threading, and have a basic question about multi-threading with loops. This might be a bit of a duplicate on some other questions here (like this: [PSA: Reasoning about scope rules and multithreading](https://discourse.julialang.org/t/psa-reasoning-about-scope-rules-and-multithreading/56967/1)) but I was still a bit confused. It would be great to get some advice on this!

Here is a rough outline of the code I’m running:

```julia
@threads for i = 1:N
data = full_data[agent_id .== i,:] 
# thus subset of data used varies by iterations
iter = 1.0
tol = 1e6

   while ( (iter < 5000) && (tol > 1e-8) )
    #-----some code here using data subset --------#
    #-----create new_vec------#

    tol = norm(new_vec - old_vec)
    iter = iter + 1
   end

end

```

You can think that within each iteration of the for-loop, I’m running a while loop. In each iteration of the while loop, I’m creating a vector called `new_vec`, and comparing that with the previous while-loop’s created vector (`old_vec`). If the difference is small enough, I stop the while-loop, and move on to the next iteration of for-loop. Note that I am _not_ writing to `data` or `full_data`, I just use them to create `new_vec`.

I’m multi-threading the for-loop. Each iteration of the for-loop does not depend on other iterations, i.e. `new_vec for i = 100` doesn’t influence `new_vec for i = 500`.

My worry is that if I do this, then there could be some sort of data race being created for `iter` and `tol`. That is, the value of `iter` in one `i` could end up in the value of iter in another `i`, and similarly for `tol`. Is this an issue? Or are the scopes here somehow for-loop iteration specific, i.e. `tol` and `iter` in each iteration of for-loop is untouched by others?

I also did the following but I’m not sure if it would help.

```julia
@threads for i = 1:N
data = full_data[agent_id .== i,:] 
# thus subset of data used varies by iterations
local iter = 1.0
local tol = 1e6

   while ( (iter < 5000) && (tol > 1e-8) )
    #-----some code here using data subset --------#
    #-----create new_vec------#

    tol = norm(new_vec - old_vec)
    iter = iter + 1
   end

end

```

Does putting in the `local` at the beginning limit the scope of `tol` and `iter` to that specific for-loop iteration?  
Any suggestions on how to improve the code would be appreciated too. Thanks!

---

<div class="post-metadata">

**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:** [November 17, 2021, 5:35pm UTC](https://discourse.julialang.org/t/scoping-in-loops-and-multi-threading/71675/2 "2021-11-17T17:35:25Z")

</div>

> [@rubaiyat](#):
>
> Or are the scopes here somehow for-loop iteration specific, i.e. `tol` and `iter` in each iteration of for-loop is untouched by others?

that is true even without multithreading

> [@rubaiyat](#):
>
> Does putting in the `local` at the beginning limit the scope of `tol` and `iter` to that specific for-loop iteration?

their scope is already local to each iteration of the loop. You probably want to add a `local old_vec`, beause the same is valid to the iterations of the `while` loop, and if you do not add `local old_vec` (or initialize it with a value) outside that `while` loop, its value won’t be preserved from one iteration to the other.