# Strange behavior inside a for-loop with ArrayFire

**URL:** <https://discourse.julialang.org/t/strange-behavior-inside-a-for-loop-with-arrayfire/21187>\
**Category:** GPU\
**Created:** [February 25, 2019, 9:19pm UTC](https://discourse.julialang.org/t/strange-behavior-inside-a-for-loop-with-arrayfire/21187 "2019-02-25T21:19:39Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![Noel\_Araujo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/noel_araujo/32/4862_2.png) [@Noel\_Araujo](https://discourse.julialang.org/u/Noel_Araujo)\
**Post date:** [February 25, 2019, 9:19pm UTC](https://discourse.julialang.org/t/strange-behavior-inside-a-for-loop-with-arrayfire/21187/1 "2019-02-25T21:19:39Z")

</div>

Hello everyone, I have a issue and I don’t even know the exactly cause.

I am using ArrayFire to speed up some matrix multiplications. However, I cannot retrieve the data _unless_ I do some useless operation into my data. Here a basic example of working code:

```julia
using ArrayFire
screen = rand(100,100,3)
r = rand(50,3)
β = rand(50)
k₀ = 1
function getE_scat_GPU(screen, r, β, k₀)
    nAtoms = size(r,1)
    nΘ = size(screen,1)
    nΦ = size(screen,2)

    E_scat_gpu = AFArray(zeros(Complex{Float32},nΘ, nΦ ))
    distance = similar(E_scat_gpu)
    xₛ = AFArray(zeros(Float32,nΘ, nΦ ))
    yₛ = similar(xₛ)
    zₛ = similar(xₛ)
    Rₛ = similar(xₛ)
    temp_exp = similar(E_scat_gpu)
    all_xₛ = AFArray(screen[:,:,1])
    all_yₛ = AFArray(screen[:,:,2])
    all_zₛ = AFArray(screen[:,:,3])
    for j=1:nAtoms
        xₛ = all_xₛ - Float32(r[j,1])
        yₛ = all_yₛ - Float32(r[j,2])
        zₛ = all_zₛ - Float32(r[j,3])

        Rₛ = sqrt( xₛ^2 + yₛ^2 + zₛ^2 )
        distance = im*Float32(k₀)*Rₛ
        temp_exp = exp(distance)
        E_scat_gpu += β[j]*temp_exp/distance
        dummy = sum(E_scat_gpu) # comment this line
    end
    E_scat = Array(E_scat_gpu)
    return E_scat
end

result_GPU = getE_scat_GPU(screen, r, β, k₀)

```

Don’t worry about the whole code, just focus on the line with variable called `dummy` in the end of the loop. It is a operation unnecessary for the the logic of my program.

However, I need this last line to be able to retrieve the information inside the function - Actually I don’t need to do this operation, if I use e.g. `println(sum(E_scat_gpu))`, works as well.

If I remove this lines, I get a matrix full of `NaN`.

Any clues ???

---

<div class="post-metadata">

**Author:** ![anon43133060](https://avatars.discourse-cdn.com/v4/letter/a/57b2e6/32.png) [@anon43133060](https://discourse.julialang.org/u/anon43133060)\
**Post date:** [February 26, 2019, 2:53am UTC](https://discourse.julialang.org/t/strange-behavior-inside-a-for-loop-with-arrayfire/21187/2 "2019-02-26T02:53:53Z")

</div>

could be a bug in your version of arrayfire library, I get no error with this line commented out.

```julia
> afinfo()
ArrayFire v3.6.2 (CUDA, 64-bit Linux, build dc38ef13)
Platform: CUDA Toolkit 10, Driver: 410.79
```

---

<div class="post-metadata">

**Author:** ![Noel\_Araujo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/noel_araujo/32/4862_2.png) [@Noel\_Araujo](https://discourse.julialang.org/u/Noel_Araujo)\
**Post date:** [February 26, 2019, 9:11am UTC](https://discourse.julialang.org/t/strange-behavior-inside-a-for-loop-with-arrayfire/21187/3 "2019-02-26T09:11:15Z")

</div>

I updated the ArrayFire, and the error continues 😥

```julia
julia> afinfo()
ArrayFire v3.6.2 (OpenCL, 64-bit Linux, build dc38ef1)
[0] AMD: gfx900, 8176 MB

```

---

<div class="post-metadata">

**Author:** ![anon43133060](https://avatars.discourse-cdn.com/v4/letter/a/57b2e6/32.png) [@anon43133060](https://discourse.julialang.org/u/anon43133060)\
**Post date:** [February 26, 2019, 4:58pm UTC](https://discourse.julialang.org/t/strange-behavior-inside-a-for-loop-with-arrayfire/21187/4 "2019-02-26T16:58:50Z")

</div>

Use `sync()` instead of `sum()` to work around this bug, and open a ticket with [Issues · arrayfire/arrayfire · GitHub](https://github.com/arrayfire/arrayfire/issues) ?