# Confused \`spdiagm\` with matlab function

**URL:** <https://discourse.julialang.org/t/confused-spdiagm-with-matlab-function/8525>\
**Category:** Numerics\
**Tags:** linearalgebra\
**Created:** [January 22, 2018, 3:45pm UTC](https://discourse.julialang.org/t/confused-spdiagm-with-matlab-function/8525 "2018-01-22T15:45:46Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![jacob-roth](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jacob-roth/32/1862_2.png) [@jacob-roth](https://discourse.julialang.org/u/jacob-roth)\
**Post date:** [January 22, 2018, 3:45pm UTC](https://discourse.julialang.org/t/confused-spdiagm-with-matlab-function/8525/1 "2018-01-22T15:45:47Z")

</div>

I’m having issues running `cholfact` on a sparse tridiagonal matrix (Julia v0.6) which works in matlab. When I try to factor `L`:

```julia
L = spdiagm((-1.*v,10.*v,-1.*v),(-1,0,1))
println(typeof(L))
cholfact(L)

```

I get:

```julia
SparseMatrixCSC{Float64,Int64}

Base.LinAlg.PosDefException(0)

Stacktrace:
 [1] #cholfact!#8(::Float64, ::Function, ::Base.SparseArrays.CHOLMOD.Factor{Float64}, ::Base.SparseArrays.CHOLMOD.Sparse{Float64}) at ./sparse/cholmod.jl:1360
 [2] (::Base.LinAlg.#kw##cholfact!)(::Array{Any,1}, ::Base.LinAlg.#cholfact!, ::Base.SparseArrays.CHOLMOD.Factor{Float64}, ::Base.SparseArrays.CHOLMOD.Sparse{Float64}) at ./<missing>:0
 [3] #cholfact#10(::Float64, ::Array{Int64,1}, ::Function, ::Base.SparseArrays.CHOLMOD.Sparse{Float64}) at ./sparse/cholmod.jl:1398
 [4] #cholfact#11(::Array{Any,1}, ::Function, ::SparseMatrixCSC{Float64,Int64}) at ./sparse/cholmod.jl:1438
 [5] cholfact(::SparseMatrixCSC{Float64,Int64}) at ./sparse/cholmod.jl:1438
 [6] include_string(::String, ::String) at ./loading.jl:515

```

However, the following works:

```julia
L2 = sparse(rand(5,5))
println(typeof(L2))
L3 = L'*L
println(typeof(L3))
cholfact(L3)

```

and both `L2` and `L3` are of type `SparseMatrixCSC{Float64,Int64}`.

---

<div class="post-metadata">

**Author:** ![antoine-levitt](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/antoine-levitt/32/4008_2.png) [@antoine-levitt](https://discourse.julialang.org/u/antoine-levitt)\
**Post date:** [January 22, 2018, 3:58pm UTC](https://discourse.julialang.org/t/confused-spdiagm-with-matlab-function/8525/2 "2018-01-22T15:58:37Z")

</div>

I assume v = ones(n)? If yes, note that L[end,end] = 0 and thus your matrix is not SPD.

---

<div class="post-metadata">

**Author:** ![Elrod](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/elrod/32/22461_2.png) [@Elrod](https://discourse.julialang.org/u/Elrod)\
**Post date:** [January 22, 2018, 3:58pm UTC](https://discourse.julialang.org/t/confused-spdiagm-with-matlab-function/8525/3 "2018-01-22T15:58:51Z")

</div>

What is `v`? Please provide a minimum working example.

```julia
julia> v = fill(1.0, 3);
julia> L = spdiagm((-1.*v,10.*v,-1.*v),(-1,0,1));
julia> Ld = Array(L)
4×4 Array{Float64,2}:
 10.0 -1.0 0.0 0.0
 -1.0 10.0 -1.0 0.0
  0.0 -1.0 10.0 -1.0
  0.0 0.0 -1.0 0.0

```

Anyway, the problem is easier to see when it prints nicely (as a dense array).  
The last diagonal element is zero; this matrix is not positive definite.

EDIT:  
One suggestion:

```julia
julia> L = spdiagm((-1.*@view(v[2:end]),10.*v,-1.*@view(v[1:end-1])),(-1,0,1))
3×3 SparseMatrixCSC{Float64,Int64} with 7 stored entries:
  [1, 1] = 10.0
  [2, 1] = -1.0
  [1, 2] = -1.0
  [2, 2] = 10.0
  [3, 2] = -1.0
  [2, 3] = -1.0
  [3, 3] = 10.0

julia> Array(L)
3×3 Array{Float64,2}:
 10.0 -1.0 0.0
 -1.0 10.0 -1.0
  0.0 -1.0 10.0

```

---

<div class="post-metadata">

**Author:** ![jacob-roth](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jacob-roth/32/1862_2.png) [@jacob-roth](https://discourse.julialang.org/u/jacob-roth)\
**Post date:** [January 22, 2018, 4:00pm UTC](https://discourse.julialang.org/t/confused-spdiagm-with-matlab-function/8525/4 "2018-01-22T16:00:16Z")

</div>

Woah thank you… Sorry for the Q; can I delete this post since it’s not a Julia issue?

---

<div class="post-metadata">

**Author:** ![simonbyrne](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/simonbyrne/32/19_2.png) [@simonbyrne](https://discourse.julialang.org/u/simonbyrne)\
**Post date:** [January 22, 2018, 5:16pm UTC](https://discourse.julialang.org/t/confused-spdiagm-with-matlab-function/8525/5 "2018-01-22T17:16:33Z")

</div>

> [@jacob-roth](#):
>
> Woah thank you… Sorry for the Q; can I delete this post since it’s not a Julia issue?

We don’t yet have an explicit policy (though are in the process of formulating one), but we generally try to avoid deleting posts unless absolutely necessary.