# How do I use eigen! with my input matrix?
**URL:** https://discourse.julialang.org/t/how-do-i-use-eigen-with-my-input-matrix/49997
**Category:** Numerics
**Created:** [November 11, 2020, 6:16pm UTC](https://discourse.julialang.org/t/how-do-i-use-eigen-with-my-input-matrix/49997 "2020-11-11T18:16:01Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Noel\_Araujo](https://discourse.julialang.org/u/Noel_Araujo)
#### Post date: [November 11, 2020, 6:16pm UTC](https://discourse.julialang.org/t/how-do-i-use-eigen-with-my-input-matrix/49997/1 "2020-11-11T18:16:01Z")
Hello
I have an idea, but i don’t know how to implemented it. I would like to pass a temporary variables to store the eigenvectors of `eigen!` - by default `eigen!` uses the original matrix.
I am confident that this is possible to do, because inside `eigen` function, a new matrix to store the eigenvectors must be created. I need some help to point out where this happen, then my naive thoughs would be to create a new function, and pass my matrix as argument.
The rest of algorithms does not need to change, therefore, less probability of issues.
Below I show my expected behavior, if, somehow, it helps.
```julia
using LinearAlgebra
A = rand(4,4)
λ₁, ψ₁ = eigen(A)
λ₂, ψ₂ = eigen!(A)
@show all(λ₁ .≈ λ₂)
@show all(ψ₁ .≈ ψ₂)
A_mine = zeros(A)
λ₃ = eigen!(A, A_mine) ### what I need
@show all(ψ₁ .≈ A_mine) ### of course should be true
```
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [November 11, 2020, 6:44pm UTC](https://discourse.julialang.org/t/how-do-i-use-eigen-with-my-input-matrix/49997/2 "2020-11-11T18:44:49Z")
you might as well just do this
```julia
julia> a= rand(3,3); b=similar(a);
julia> my_eigen(a,b) = b.=eigen(a).vectors
julia> my_eigen(a,b);
julia> x,y = eigen(a);
julia> all(b .≈ y)
true
```
---
### Author:  [@Noel\_Araujo](https://discourse.julialang.org/u/Noel_Araujo)
#### Post date: [November 11, 2020, 6:56pm UTC](https://discourse.julialang.org/t/how-do-i-use-eigen-with-my-input-matrix/49997/3 "2020-11-11T18:56:23Z")
> [@jling](#):
>
> `b.=eigen(a).vectors`
But the function `eigen` will allocate memory somewhere inside it, before we put the content inside `b`.
I would like to avoid the `eigen` to create a new matrix temporarily.
---
### Author:  [@jling](https://discourse.julialang.org/u/jling)
#### Post date: [November 11, 2020, 7:08pm UTC](https://discourse.julialang.org/t/how-do-i-use-eigen-with-my-input-matrix/49997/4 "2020-11-11T19:08:49Z")
I mean it’s about the same where you do the copying:
[https://github.com/JuliaLang/julia/blob/af3cd59e13c93fcb3e3788a79b0168127473d18a/stdlib/LinearAlgebra/src/eigen.jl#L235-L237](https://github.com/JuliaLang/julia/blob/af3cd59e13c93fcb3e3788a79b0168127473d18a/stdlib/LinearAlgebra/src/eigen.jl#L235-L237)
or
[https://github.com/JuliaLang/julia/blob/af3cd59e13c93fcb3e3788a79b0168127473d18a/stdlib/LinearAlgebra/src/eigen.jl#L175](https://github.com/JuliaLang/julia/blob/af3cd59e13c93fcb3e3788a79b0168127473d18a/stdlib/LinearAlgebra/src/eigen.jl#L175)
because what you want is a vector (for eigenvalue) and a matrix (for eigenvectors), which needs additional memory at some point
---
### Author:  [@Noel\_Araujo](https://discourse.julialang.org/u/Noel_Araujo)
#### Post date: [November 11, 2020, 7:42pm UTC](https://discourse.julialang.org/t/how-do-i-use-eigen-with-my-input-matrix/49997/5 "2020-11-11T19:42:35Z")
Hummmmm
I thought something more complex was happning behind the scenes, but `AA` is just a copy of `A` and then the `eigen!` function is called.
Looking here :
```julia
eval, evec = LAPACK.geevx!(permute ? (scale ? 'B' : 'P') : (scale ? 'S' : 'N'), 'N', 'V', 'N', A)[[2,4]] return Eigen(sorteig!(eval, evec, sortby)...)
```
Seems that `evec` comes from LAPACK directly, and I cannot offer an empty matrix as input. Am I right ?