# Some doubts about types

**URL:** <https://discourse.julialang.org/t/some-doubts-about-types/27635>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [August 16, 2019, 11:05pm UTC](https://discourse.julialang.org/t/some-doubts-about-types/27635 "2019-08-16T23:05:41Z")\
**Posts on this page:** 1\
**Showing post:** 10

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**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:** [August 31, 2019, 2:00pm UTC](https://discourse.julialang.org/t/some-doubts-about-types/27635/10 "2019-08-31T14:00:00Z")

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> [@baptiste](#):
>
> My biggest doubt about this approach though, is that ultimately `A` appears in a 3Nx3N linear system `A x = b` (coupled-dipole equations), and if `A` is a NxN matrix of SMatrices I don’t really see how it’s going to work (ultimately some Lapack routine will need the 3Nx3N matrix, won’t it?).

You can do `Ajk = ...some SMatrix..` and then do `A[3j-2:3j, 3k-3:3k] = Ajk` to write the `Smatrix` into the “flat” array `A` in-place.

> [@baptiste](#):
>
> `α_jj = transpose(Rm) * Diagonal(Alpha[jj]) * Rm`

You can do this by `dot(Rm, Alpha[jj] .* Rm)` or `Rm' * (Alpha[jj] .* Rm)`without constructing the `Diagonal` matrix.

Note also that you can just do `foo'` instead of `transpose(foo)` — the two are equivalent for real vectors like your coordinate vectors here.

> [@baptiste](#):
>
> `Ajk = SMatrix(...)`

If you’ve done things correctly, you shouldn’t need the `SMatrix(...)` constructor here… the matrix expression should already give an `SMatrix` (because it’s built out of `SMatrix` expressions like `rjkhat * rjkhat'`).

(You can add a type assertion `::SMatrix` at the end of the expression if you want to check that you did this correctly. A type assertion will throw an error if the expression is not the expected type, but is eliminated at compile-time if the expression has the correct type and type-inference succeeds.)

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