# Inconsistent results when using Threads.@threads in a loop

**URL:** <https://discourse.julialang.org/t/inconsistent-results-when-using-threads-threads-in-a-loop/100134>\
**Category:** General Usage\
**Tags:** multithreading\
**Created:** [June 10, 2023, 9:18am UTC](https://discourse.julialang.org/t/inconsistent-results-when-using-threads-threads-in-a-loop/100134 "2023-06-10T09:18:33Z")\
**Posts on this page:** 1\
**Showing post:** 2

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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:** [June 10, 2023, 10:53am UTC](https://discourse.julialang.org/t/inconsistent-results-when-using-threads-threads-in-a-loop/100134/2 "2023-06-10T10:53:50Z")

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Disclaimer: I am not an expert in `Threads.@threads` or friends.

Applying any of the changes below makes it work correctly on `v1.9.1`:

- add a `local` in front of `v_re, v_im` inside the loop body
- rename `v_re, v_im` inside the loop body
- rename or remove `v_re, v_im` outside the loop

~~Someone correct me, but this smells like a bug in `Threads.@threads`.~~  
EDIT: As clarified in the comments below, this is intended but also bad behavior. What is happening is that by the current scoping rules, using the names `v_re, v_im` also after the loop body makes them being shared between threads, which causes a data race.

* * *

Here is the fixed version:

```julia
function coefs_w_threads(N)
    v = zeros(ComplexF64, N)
    Threads.@threads for k in 1:N-1
        local v_re = 0.5*k
        local v_im = -0.5*k
        v[k+1] = v_re + 1im*v_im
    end
    v_re = 0.25
    v_im = -0.25
    v[1] = v_re + 1im*v_im
    return v
end

function coefs(N)
    v = zeros(ComplexF64, N)
    for k in 1:N-1
        v_re = 0.5*k
        v_im = -0.5*k
        v[k+1] = v_re + 1im*v_im
    end
    v_re = 0.25
    v_im = -0.25
    v[1] = v_re + 1im*v_im
    return v
end

ref = coefs(1000)
threads = coefs_w_threads(1000)
# display(sum(ref) == sum(threads))
display(all(ref .== threads)) # more reliable test

```

EDIT: I had added the `local` to the `coefs` version instead of `coefs_w_threads` …

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_[View the full topic](https://discourse.julialang.org/t/inconsistent-results-when-using-threads-threads-in-a-loop/100134)._