# Understanding mutating functions correctly

**URL:** <https://discourse.julialang.org/t/understanding-mutating-functions-correctly/30651>\
**Category:** New to Julia\
**Tags:** question\
**Created:** [November 3, 2019, 1:49pm UTC](https://discourse.julialang.org/t/understanding-mutating-functions-correctly/30651 "2019-11-03T13:49:01Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)\
**Post date:** [November 3, 2019, 2:05pm UTC](https://discourse.julialang.org/t/understanding-mutating-functions-correctly/30651/2 "2019-11-03T14:05:32Z")

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> [@1991sig](#):
>
> Am I doing something wrong, or is the `M.Fit.β = β` and same for the other vectors strictly necessary in these cases?

This depends on what `β` is — if it is an array, then `β = M.Fit.β` should be sufficient to make sure they are the same, except that you create new one with

```julia
β = β + Jinv * U

```

(In the future, please include a **self-contained MWE** , which should make it much easier to help you.)

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>
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