# Using ForwardDiff results in a stack overflow
**URL:** https://discourse.julialang.org/t/using-forwarddiff-results-in-a-stack-overflow/90399
**Category:** General Usage
**Tags:** forwarddiff
**Created:** [November 17, 2022, 2:06pm UTC](https://discourse.julialang.org/t/using-forwarddiff-results-in-a-stack-overflow/90399 "2022-11-17T14:06:15Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@c\_sell](https://discourse.julialang.org/u/c_sell)
#### Post date: [November 17, 2022, 2:06pm UTC](https://discourse.julialang.org/t/using-forwarddiff-results-in-a-stack-overflow/90399/1 "2022-11-17T14:06:15Z")
Hi, im using the ForwardDiff package. The following code is only a simplification of the real problem.
But I am getting there as well a stack overflow in the line of the LU decomposition.
```julia
using ForwardDiff, LinearAlgebra, SparseArrays
function foo(x)
r = [1,2,1];
c = [1,2,2];
v = [x[1],x[2],x[1]];
rhs = [1.0,2.0];
A = sparse(r,c,v);
LU = lu(A)
y = LU \ rhs;
return y
end
J = ForwardDiff.jacobian(foo,[1.,2.])
```
---
### Author:  [@j-fu](https://discourse.julialang.org/u/j-fu)
#### Post date: [November 17, 2022, 11:52pm UTC](https://discourse.julialang.org/t/using-forwarddiff-results-in-a-stack-overflow/90399/2 "2022-11-17T23:52:32Z")
Interesting that this pops up as a stack overflow error…
The point is that `\` for SparseMatrixCSC is channeled to UMFPACK from SuiteSparse which is implemented in C and thus not able to work with ForwardDiff.Dual numbers.
That said, recently, the package Sparspak.jl was announced which is a pure Julia implementation of the classical Sparspak by George and Liu and thus provides an LU factorization working with general number types. The ExtendableSparse.jl package (shameless plug: I am the author…) provides the ExtendableSparseMatrix type which has a `\` which falls back to Sparspak on number types not supported by SuiteSparse/UMFPACK .
So now we have:
```julia
using ExtendableSparse, ForwardDiff, LinearAlgebra, SparseArrays
function foo(x)
r = [1,2,1];
c = [1,2,2];
v = [x[1],x[2],x[1]];
rhs = [1.0,2.0];
A = ExtendableSparseMatrix(sparse(r,c,v));
LU = lu(A)
y = LU \ rhs;
return y
end
J = ForwardDiff.jacobian(foo,[1.,2.])
2×2 Matrix{Float64}:
-1.0 0.5
0.0 -0.5
```
---
### Author:  [@c\_sell](https://discourse.julialang.org/u/c_sell)
#### Post date: [November 18, 2022, 7:28am UTC](https://discourse.julialang.org/t/using-forwarddiff-results-in-a-stack-overflow/90399/3 "2022-11-18T07:28:14Z")
Hi,
thank you for your quick reply. Somehow I am still getting an Error with your code (I copied your version).
ERROR: MethodError: Cannot `convert` an object of type
Sparspak.SpkProblem.Problem{Int64{},Float64} to an object of type
Sparspak.SpkProblem.Problem{Int64{},ForwardDiff.Dual{ForwardDiff.Tag{typeof(foo), Float64}, Float64, 2}}
---
### Author:  [@j-fu](https://discourse.julialang.org/u/j-fu)
#### Post date: [November 18, 2022, 8:42am UTC](https://discourse.julialang.org/t/using-forwarddiff-results-in-a-stack-overflow/90399/4 "2022-11-18T08:42:37Z")
Strange, I tried this even on Julia 1.6 in a clean environment, and it all works.
The error message indicates that you somehow tried to differentiate through a sparse solve with a sparse matrix already created with Float64 entries instead of ForwardDiff.Dual.
---
### Author:  [@c\_sell](https://discourse.julialang.org/u/c_sell)
#### Post date: [November 18, 2022, 12:16pm UTC](https://discourse.julialang.org/t/using-forwarddiff-results-in-a-stack-overflow/90399/5 "2022-11-18T12:16:46Z")
After i restarted my enviroment everything worked fine.
Thanks for your professional help 🙂