# How to update a tensor element-wise in ITensors ?

**URL:** <https://itensor.discourse.group/t/how-to-update-a-tensor-element-wise-in-itensors/1814>\
**Category:** ITensor Julia Questions\
**Tags:** julia\
**Created:** [June 18, 2024, 9:26am UTC](https://itensor.discourse.group/t/how-to-update-a-tensor-element-wise-in-itensors/1814 "2024-06-18T09:26:00Z")\
**Posts on this page:** 1\
**Showing post:** 5

<div class="post-metadata">

**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 26, 2024, 2:03pm UTC](https://itensor.discourse.group/t/how-to-update-a-tensor-element-wise-in-itensors/1814/5 "2024-06-26T14:03:13Z")

</div>

Here is a minimal rewrite of your code that demonstrates it should be updating `T2`:

```julia
using ITensors: ITensor, Index, random_itensor
using Random: Random
function main()
  Random.seed!(1234)
  a_1, a_2, i_1, i_2 = Index.((2, 2, 2, 2))
  R2 = random_itensor(a_1, a_2, i_1, i_2)
  T2 = random_itensor(a_1, a_2, i_1, i_2)
  Scaled_R2 = random_itensor(a_1, a_2, i_1, i_2)
  function update_T2!(T2::ITensor, R2::ITensor, Scaled_R2::ITensor)
    fvv = random_itensor(a_1, a_2)
    foo = random_itensor(i_1, i_2)
    nv = 2
    nocc = 2
    shiftp = 0.20
    for a in 1:nv, b in 1:nv, i in 1:nocc, j in 1:nocc
      numerator = R2[a_1 => a, a_2 => b, i_1 => i, i_2 => j]
      denominator = fvv[a_1 => a, a_2 => a] + fvv[a_1 => b, a_2 => b] - foo[i_1 => i, i_2 => i] - foo[i_1 => j, i_2 => j] + shiftp
      Scaled_R2[a_1 => a, a_2 => b, i_1 => i, i_2 => j] = numerator / denominator
    end
    T2 .-= Scaled_R2
    return T2
  end
  @show T2
  update_T2!(T2, R2, Scaled_R2)
  @show T2
  return nothing
end

```

When I run `main()` I get the output:

```julia
julia> main()
T2 = ITensor ord=4
Dim 1: (dim=2|id=592)
Dim 2: (dim=2|id=579)
Dim 3: (dim=2|id=308)
Dim 4: (dim=2|id=949)
NDTensors.Dense{Float64, Vector{Float64}}
 2×2×2×2
[:, :, 1, 1] =
  1.6218076699653694 -0.5347089043694231
 -0.7636889550217101 -0.8371161519624881

[:, :, 2, 1] =
 -0.09989094954741905 -0.240570071975397
 -0.726142325634263 0.9427328405545989

[:, :, 1, 2] =
 0.44588457807333104 -1.4451163776340619
 0.4604268914916924 0.3716847353731026

[:, :, 2, 2] =
 1.9522884266156713 -0.7578371325188665
 0.3237050517386015 1.1693426324343965
T2 = ITensor ord=4
Dim 1: (dim=2|id=592)
Dim 2: (dim=2|id=579)
Dim 3: (dim=2|id=308)
Dim 4: (dim=2|id=949)
NDTensors.Dense{Float64, Vector{Float64}}
 2×2×2×2
[:, :, 1, 1] =
 2.212330480674426 -2.2440191884987355
 1.0922381970413777 -0.82496215138259

[:, :, 2, 1] =
 -0.3416953884869767 8.401621007271215
 -5.33919777706203 0.12228179003341455

[:, :, 1, 2] =
  0.7516874457199619 0.8084381798095097
 -2.353150269542973 -0.21578545586345294

[:, :, 2, 2] =
 2.2143522987879845 0.07399335046884059
 0.3979494267800609 0.276824871640005

```

Note that I am indexing into the ITensors in the loop using the ITensor Indices, which is better practice since otherwise the code can become incorrect if ITensors are input with Indices that are ordered in a different way. I also shortened `T2 .= T2 .- Scaled_R2` to `T2 .-= Scaled_R2`, those are equivalent to each other.

Also functions that change object in-place in Julia have a convention that their name ends in `!`.

So the issue you are seeing is probably coming from a different part of your code.

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