# How to set the diagonal part of a GPU Array

**URL:** <https://discourse.julialang.org/t/how-to-set-the-diagonal-part-of-a-gpu-array/16975>\
**Category:** GPU\
**Created:** [October 30, 2018, 3:15pm UTC](https://discourse.julialang.org/t/how-to-set-the-diagonal-part-of-a-gpu-array/16975 "2018-10-30T15:15:50Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![1115](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/1115/32/4465_2.png) [@1115](https://discourse.julialang.org/u/1115)\
**Post date:** [October 30, 2018, 3:15pm UTC](https://discourse.julialang.org/t/how-to-set-the-diagonal-part-of-a-gpu-array/16975/1 "2018-10-30T15:15:50Z")

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Let’s see the code

```julia
julia> using CuArrays;

julia> s
4-element CuArray{Float32,1}:
 3.2211537
 2.1344962
 0.7451281
 0.3308171

julia> v
4×4 CuArray{Float32,2}:
  0.267688 0.355418 -0.311468 -0.839648
 -0.156514 0.844306 -0.296999 0.417663
  0.658483 -0.260938 -0.623603 0.330803
 -0.685745 -0.304527 -0.65261 -0.105441

julia> v + Diagonal(s) # I disabled getindex of cuarrays.
ERROR: scalar getindex is disallowed
...

```

How to make it correct?

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<div class="post-metadata">

**Author:** ![maleadt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/maleadt/32/10097_2.png) [@maleadt](https://discourse.julialang.org/u/maleadt)\
**Post date:** [October 30, 2018, 4:09pm UTC](https://discourse.julialang.org/t/how-to-set-the-diagonal-part-of-a-gpu-array/16975/2 "2018-10-30T16:09:01Z")

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This seems a broadcast issue, please file an issue at CuArrays.jl.

```julia
julia> v + Diagonal(s)
ERROR: scalar getindex is disallowed
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] assertscalar at /home/tbesard/Julia/GPUArrays/src/indexing.jl:8 [inlined]
 [3] getindex(::CuArray{Float32,2}, ::Int64) at /home/tbesard/Julia/GPUArrays/src/indexing.jl:44
 [4] _getindex at ./abstractarray.jl:928 [inlined]
 [5] getindex at ./abstractarray.jl:905 [inlined]
 [6] _broadcast_getindex at ./broadcast.jl:540 [inlined]
 [7] _getindex at ./broadcast.jl:570 [inlined]
 [8] _broadcast_getindex at ./broadcast.jl:546 [inlined]
 [9] getindex at ./broadcast.jl:507 [inlined]
 [10] macro expansion at ./broadcast.jl:838 [inlined]
 [11] macro expansion at ./simdloop.jl:73 [inlined]
 [12] copyto! at ./broadcast.jl:837 [inlined]
 [13] copyto!(::Array{Float32,2}, ::Base.Broadcast.Broadcasted{Base.Broadcast.ArrayConflict,Tuple{Base.OneTo{Int64},Base.OneTo{Int64}},typeof(+),Tuple{CuArray{Float32,2},Diagonal{Float32,CuArray{Float32,1}}}}) at ./broadcast.jl:792
 [14] copy at ./broadcast.jl:768 [inlined]
 [15] materialize at ./broadcast.jl:748 [inlined]
 [16] broadcast at ./broadcast.jl:702 [inlined]
 [17] +(::CuArray{Float32,2}, ::Diagonal{Float32,CuArray{Float32,1}}) at ./arraymath.jl:39
 [18] top-level scope at none:0

```

The `Broadcast.ArrayConflict` looks iffy here, but I’m no expert of the broadcast internals.

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<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [October 30, 2018, 4:18pm UTC](https://discourse.julialang.org/t/how-to-set-the-diagonal-part-of-a-gpu-array/16975/3 "2018-10-30T16:18:37Z")

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> [@maleadt](#):
>
> `Broadcast.ArrayConflict`

Yes, that’s exactly the issue — both structured matrices and CuArrays define custom broadcasting implementations but haven’t decided which one “wins,” so Broadcast falls back to the totally generic builtin version.

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<div class="post-metadata">

**Author:** ![1115](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/1115/32/4465_2.png) [@1115](https://discourse.julialang.org/u/1115)\
**Post date:** [October 30, 2018, 5:14pm UTC](https://discourse.julialang.org/t/how-to-set-the-diagonal-part-of-a-gpu-array/16975/4 "2018-10-30T17:14:27Z")

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Issue has been filed here. Am I supposed to write CUDAnative to set diagonal elements now?

[https://github.com/JuliaGPU/CuArrays.jl/issues/173](https://github.com/JuliaGPU/CuArrays.jl/issues/173)

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<div class="post-metadata">

**Author:** ![maleadt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/maleadt/32/10097_2.png) [@maleadt](https://discourse.julialang.org/u/maleadt)\
**Post date:** [October 31, 2018, 6:50am UTC](https://discourse.julialang.org/t/how-to-set-the-diagonal-part-of-a-gpu-array/16975/5 "2018-10-31T06:50:47Z")

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You can just collect the diagonal and use that, as a temporary workaround:

```julia
julia> v + CuArray(Diagonal(s))
4×4 CuArray{Float32,2}:
 1.66536 0.470219 0.71401 0.319409
 0.44635 1.43419 0.358502 0.810913
 0.667319 0.776175 0.720439 0.154112
 0.109931 0.294766 0.292015 1.41413 

julia> CuArray(Diagonal(s))
4×4 CuArray{Float32,2}:
 0.740219 0.0 0.0 0.0     
 0.0 0.920994 0.0 0.0     
 0.0 0.0 0.0390205 0.0     
 0.0 0.0 0.0 0.968963

```

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<div class="post-metadata">

**Author:** ![mbauman](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mbauman/32/31082_2.png) [@mbauman](https://discourse.julialang.org/u/mbauman)\
**Post date:** [October 31, 2018, 7:12pm UTC](https://discourse.julialang.org/t/how-to-set-the-diagonal-part-of-a-gpu-array/16975/6 "2018-10-31T19:12:42Z")

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Or you can assign directly into the diagonal using this linear indexing trick:

```julia
v[1:size(v, 1)+1:end] .+= s

```

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**Author:** ![yang](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yang/32/5232_2.png) [@yang](https://discourse.julialang.org/u/yang)\
**Post date:** [November 12, 2018, 1:13pm UTC](https://discourse.julialang.org/t/how-to-set-the-diagonal-part-of-a-gpu-array/16975/7 "2018-11-12T13:13:36Z")

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v + Diagonal(s)  
now this works