# Consistency checks missing when constructing linear algebra objects
**URL:** https://discourse.julialang.org/t/consistency-checks-missing-when-constructing-linear-algebra-objects/76419
**Category:** Numerics
**Tags:** linearalgebra
**Created:** [February 14, 2022, 10:55am UTC](https://discourse.julialang.org/t/consistency-checks-missing-when-constructing-linear-algebra-objects/76419 "2022-02-14T10:55:41Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@andreasvarga](https://discourse.julialang.org/u/andreasvarga)
#### Post date: [February 14, 2022, 10:55am UTC](https://discourse.julialang.org/t/consistency-checks-missing-when-constructing-linear-algebra-objects/76419/1 "2022-02-14T10:55:41Z")
I have the following question which virtually applies to all defined linear algebra objects in Julia/LinearAlgebra:
**Why there are no consistency checks when constructing linear algebra objects?**
As an example, let’s consider the Schur data type, which is defined as:
```julia
struct Schur{Ty,S<:AbstractMatrix} <: Factorization{Ty}
T::S
Z::S
values::Vector
end
```
This structure contains two matrices `T` and `Z` of the same type and a vector` values`. It has nothing specific related to the Schur decomposition, as claimed by the documentation. This can be easily checked by the following construction:
```julia
julia> F = Schur(rand(3,3),rand(2,1),rand(0))
Schur{Float64, Matrix{Float64}}
T factor:
3×3 Matrix{Float64}:
0.851595 0.622935 0.416998
0.493628 0.782394 0.54265
0.522581 0.995593 0.996112
Z factor:
2×1 Matrix{Float64}:
0.6150198969878795
0.08655354595467768
eigenvalues:
Float64[]
```
To avoid such misconstructions, I am missing the following _minimal_ checks: `T` and `Z` should be both square matrices of the same order, say `n`, and `values` should be a vector of length `n`. More involved tests could also check that `T` is quasi-upper triangular in the general real case or upper triangular in the general complex case.
I would like to add that as far as some linear algebra data types serve only to pack the computed results, this setup could be acceptable. However, if the constructed objects are arguments of other functions, then the situation is different. As an example, this lack of checks is disturbing, especially when in a next step I would like to “reorder” the eigenvalues:
```julia
ordschur(F,falses(5))
** On entry to DTRSEN parameter number 8 had an illegal value
ERROR: ArgumentError: invalid argument #8 to LAPACK call
```
Here `ordchur` accepts the above Schur object without complaints, delivers its elements to the `trsen!` wrapper to the LAPACK routine `DTRSEN`, and, finally, the inconsistency is discovered only at the level of this routine with an error message which still requires further inspection of the LAPACK code.
The inconsistency could even remain undetected, leading to completely nonsense results:
```julia
ordschur(Schur(rand(3,3),rand(3,1),rand(0)),falses(5))
Schur{Float64, Matrix{Float64}}
T factor:
3×3 Matrix{Float64}:
0.848517 0.385575 0.183215
0.0834543 0.938042 0.919729
0.746262 0.886942 0.0102043
Z factor:
3×1 Matrix{Float64}:
0.3533799437577295
0.7928718694559094
0.01924732093261672
eigenvalues:
3-element Vector{ComplexF64}:
0.8485167187308525 + 0.1793818626275508im
0.9380416658902896 + 0.9031867257980865im
0.01020433757434469 - 0.9031867257980865im
```
I would like to hear other opinions on how much checking is necessary to build only meaningful linear algebra objects.
---
### Author:  [@Bardo](https://discourse.julialang.org/u/Bardo)
#### Post date: [February 15, 2022, 3:20pm UTC](https://discourse.julialang.org/t/consistency-checks-missing-when-constructing-linear-algebra-objects/76419/2 "2022-02-15T15:20:00Z")
Opened an issue:
[Consistency checks missing when constructing linear algebra objects · Issue #44191 · JuliaLang/julia (github.com)](https://github.com/JuliaLang/julia/issues/44191)
---
### Author:  [@Bardo](https://discourse.julialang.org/u/Bardo)
#### Post date: [February 15, 2022, 4:52pm UTC](https://discourse.julialang.org/t/consistency-checks-missing-when-constructing-linear-algebra-objects/76419/3 "2022-02-15T16:52:05Z")
LinAlg experts waiting for input:
> [@stevengj](https://github.com/JuliaLang/julia/issues/44191): If we want to have a catch-all issue like this, it really needs to come with a checklist of constructors that need checks, i.e. someone needs to do a survey.
---
### Author:  [@andreasnoack](https://discourse.julialang.org/u/andreasnoack)
#### Post date: [February 15, 2022, 7:22pm UTC](https://discourse.julialang.org/t/consistency-checks-missing-when-constructing-linear-algebra-objects/76419/4 "2022-02-15T19:22:14Z")
You generally shouldn’t construct linear algebra objects by calling the constructors directly and if you do then it’s up to you to ensure that you do it correctly. E.g. by writing your own tests for the functions that end up calling the constructors directly.
However, we do generally try to catch all illegal values in Julia before we call LAPACK so it looks like the checks in our wrapper for `xtrsen` doesn’t have all the required checks.