# GPU issues: scalar indexing is disallowed on sum(::ITensor) and traces of products involving diagonal itensors **URL:** \ **Category:** ITensor Julia Questions\ **Created:** [June 23, 2024, 4:43pm UTC](https://itensor.discourse.group/t/gpu-issues-scalar-indexing-is-disallowed-on-sum-itensor-and-traces-of-products-involving-diagonal-itensors/1828 "2024-06-23T16:43:25Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![ste](https://avatars.discourse-cdn.com/v4/letter/s/76d3ee/32.png) [@ste](https://itensor.discourse.group/u/ste)\ **Post date:** [June 23, 2024, 4:43pm UTC](https://itensor.discourse.group/t/gpu-issues-scalar-indexing-is-disallowed-on-sum-itensor-and-traces-of-products-involving-diagonal-itensors/1828/1 "2024-06-23T16:43:25Z")
hi, I’m posting here for better bookkeeping: while playing with diagonal and GPU tensors I stumbled upon a few more odd things, 1. I’m getting “Scalar indexing is disallowed” errors when I calculate the `sum()` of a cuda tensor, and also when computing a product of the form `Aij*Dij`, where D is a diagonal cuda tensor (whereas it works if D is not diagonal) 2. Probably unrelated, I saw there is a `tr()` function for ITensors, but that doesn’t work for me (neither for cpu nor gpu tensors). Since I couldn’t find any doc for it, I imagine it’s not recommended for use - what I found strange is that there seems to be a method for tr(::ITensor), but then the function fails in some inner call. A MWE recapping all this is: ```auto using ITensors, CUDA gpu = NDTensors.cu ij = (Index(3), Index(3)) jk = (ij[2], Index(3)) aij= random_itensor(ij) bjk = random_itensor(jk) cij = random_itensor(ij) djk = diag_itensor([1,2,3.0+im],jk) cua = gpu(aij) cub = gpu(bjk) cuc = gpu(cij) cud = gpu(djk) cua * cub # ok, rank2 cua * cuc # ok, rank0 cua * cud # ok, rank2 cub * cud #ERROR: Scalar indexing is disallowed. # Sum(): sum(aij) # ok, scalar sum(djk) # ok, scalar sum(cua) #ERROR: Scalar indexing is disallowed. sum(cud) #ERROR: Scalar indexing is disallowed. # tr(): tr(aij) # ERROR: MethodError: no method matching checkdims_perm(::NDTensors.DenseTensor{Float64, 3, Tuple{…}, NDTensors.Dense{…}}, ::NDTensors.DenseTensor{Float64, 2, Tuple{…}, NDTensors.Dense{…}}, ::Tuple{Int64, Int64, Int64}) tr(djk) # ERROR: MethodError: no method matching checkdims_perm(::NDTensors.DenseTensor{…}, ::NDTensors.DenseTensor{…}, ::Tuple{…}) tr(cua) # ERROR: MethodError: no method matching checkdims_perm(::NDTensors.DenseTensor{Float64, 3, Tuple{…}, NDTensors.Dense{…}}, ::NDTensors.DenseTensor{Float64, 2, Tuple{…}, NDTensors.Dense{…}}, ::Tuple{Int64, Int64, Int64}) tr(cud) #ERROR: MethodError: no method matching checkdims_perm(::NDTensors.DenseTensor{…}, ::NDTensors.DenseTensor{…}, ::Tuple{…}) ``` thanks! ---
**Author:** ![mtfishman](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/mtfishman/32/11_2.png) [@mtfishman](https://itensor.discourse.group/u/mtfishman)\ **Post date:** [June 23, 2024, 4:58pm UTC](https://itensor.discourse.group/t/gpu-issues-scalar-indexing-is-disallowed-on-sum-itensor-and-traces-of-products-involving-diagonal-itensors/1828/2 "2024-06-23T16:58:04Z")
Thanks for the report. The issues with `Diag * Dense` contraction and `sum(::ITensor)` on GPU are being fixed in [[NDTensorsGPUArraysCoreExt] Fix nonuniform Diag-Dense contractions on GPU by mtfishman · Pull Request #1511 · ITensor/ITensors.jl · GitHub](https://github.com/ITensor/ITensors.jl/pull/1511). Regarding `tr`, right now it only works for tracing over pairs of primed and unprimed indices, i.e. these should work: ```julia using ITensors: Index, diag_itensor, random_itensor using NDTensors: NDTensors using CUDA: CUDA device = NDTensors.cu i = Index(2) a = device(random_itensor(i, i')) tr(a) a = device(diag_itensor(randn(2), i, i')) tr(a) ``` As you noticed, it isn’t documented right now since we weren’t certain that is the right behavior, and wanted to consider how it might generalize beyond tensors with pairs of primed and unprimed indices before making it public. As an alternative, you can use contractions with `delta` tensors: ```julia using ITensors: Index, diag_itensor, random_itensor using NDTensors: NDTensors using CUDA: CUDA device = NDTensors.cu i, j = Index.((2, 2)) a = device(random_itensor(i, j)) (a * delta(eltype(a), i, j))[] a = device(diag_itensor(randn(2), i, j)) (a * delta(eltype(a), i, j))[] ``` ---
**Author:** ![ste](https://avatars.discourse-cdn.com/v4/letter/s/76d3ee/32.png) [@ste](https://itensor.discourse.group/u/ste)\ **Post date:** [June 24, 2024, 9:47am UTC](https://itensor.discourse.group/t/gpu-issues-scalar-indexing-is-disallowed-on-sum-itensor-and-traces-of-products-involving-diagonal-itensors/1828/3 "2024-06-24T09:47:57Z")
awesome, thanks Matt! regarding the `tr()` yes, I usually use just use `delta` tensor to trace, I just stumbled upon it while looking around trying to investigate the `sum()` issues and thought I’d mention it. As a matter of fact, I expected a syntax like `tr(::ITensor, (pairs of indices to trace on))` or maybe just something accepting rank-2 ITensors, so I was a bit puzzled - anyway I see the idea behind it now, thanks! ---
**Author:** ![mtfishman](https://yyz2.discourse-cdn.com/free1/user_avatar/itensor.discourse.group/mtfishman/32/11_2.png) [@mtfishman](https://itensor.discourse.group/u/mtfishman)\ **Post date:** [June 24, 2024, 12:48pm UTC](https://itensor.discourse.group/t/gpu-issues-scalar-indexing-is-disallowed-on-sum-itensor-and-traces-of-products-involving-diagonal-itensors/1828/4 "2024-06-24T12:48:07Z")
Yes, ideally we would have something like that for `tr` where you can specify pairs of indices to trace over, but we didn’t work out the syntax for it yet. ---
**Author:** ![system](https://global.discourse-cdn.com/free1/uploads/itensor/original/1X/d3072b13e047cd06df7f3981547c0917d940dfa4.png) [@system](https://itensor.discourse.group/u/system)\ **Post date:** [July 4, 2024, 12:48pm UTC](https://itensor.discourse.group/t/gpu-issues-scalar-indexing-is-disallowed-on-sum-itensor-and-traces-of-products-involving-diagonal-itensors/1828/5 "2024-07-04T12:48:19Z")
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