# Accumulating error in for loop generated by tensor macro vs sum

**URL:** <https://discourse.julialang.org/t/accumulating-error-in-for-loop-generated-by-tensor-macro-vs-sum/21647>\
**Category:** Numerics\
**Created:** [March 8, 2019, 7:14pm UTC](https://discourse.julialang.org/t/accumulating-error-in-for-loop-generated-by-tensor-macro-vs-sum/21647 "2019-03-08T19:14:25Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![grahamas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/grahamas/32/3401_2.png) [@grahamas](https://discourse.julialang.org/u/grahamas)\
**Post date:** [March 8, 2019, 7:14pm UTC](https://discourse.julialang.org/t/accumulating-error-in-for-loop-generated-by-tensor-macro-vs-sum/21647/1 "2019-03-08T19:14:25Z")

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Could anyone give me a pointer on a numerical inaccuracy that’s causing my simulation to explode? Here’s a toy example of the operation I’m running, though in practice it accumulates as `b += S(C)`, where S is a nonlinear function.

```julia
using TensorOperations

A = rand(9,8,7,6)
b = rand(8,6)

# Calculate C as a tensor operation
@tensor C_tensor[to,i] := A[to,from,i,j] * b[from,j]

# Calculate C as a sum-of-matrix-multiplications operation
C_loop = Array{Float64}(undef, 9,7)
for i in 1:7
  C_loop[:,i] .= sum(A[:,:,i,j] * b[:,j] for j in 1:6)
end

all(C_tensor .== C_loop) # false
all(C_tensor .≈ C_loop) # true

```

For just this one step, the difference is very small. However, while the loop code converges to a correct solution in my diffeq system, the tensor/loop code accumulates errors so that the “solution” explodes. Does anyone have any idea what might be the problem with the tensor approach?

TensorOperations doesn’t expand super decipherably, but another tensor contraction library (Einsum) gives the following, which also explodes in exactly the same way:

```julia
for i = 1:min(size(C_tensor, 2), size(A, 3))
  for to = 1:min(size(C_tensor, 1), size(A, 1))
    accum = 0
    for j = 1:size(A, 4)
      for from = 1:size(A, 2)
        accum += A[to, from, i, j] * b[from, j]
      end
    end
    C_tensor[to, i] = accum
  end
end

```

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<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [March 8, 2019, 11:52pm UTC](https://discourse.julialang.org/t/accumulating-error-in-for-loop-generated-by-tensor-macro-vs-sum/21647/2 "2019-03-08T23:52:08Z")

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> [@grahamas](#):
>
> Does anyone have any idea what might be the problem with the tensor approach?

The `sum` function uses pairwise summation, which is more accurate than the simple loop used in your tensor contraction.

However, if what you are doing is this sensitive to small roundoff errors, you might think about how to reformulate your overall approach (i.e. what are you using these sums for?) in a more numerically stable way.