# \[differentialequations.jl\] stuck at initial value when using function

**URL:** https://discourse.julialang.org/t/differentialequations-jl-stuck-at-initial-value-when-using-function/57716
**Category:** Modelling & Simulations
**Created:** [March 22, 2021, 1:51pm UTC](https://discourse.julialang.org/t/differentialequations-jl-stuck-at-initial-value-when-using-function/57716 "2021-03-22T13:51:49Z")
**Posts on this page:** 4
**Page:** 1

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### Author: ![DCT](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@DCT](https://discourse.julialang.org/u/DCT)
#### Post date: [March 22, 2021, 1:51pm UTC](https://discourse.julialang.org/t/differentialequations-jl-stuck-at-initial-value-when-using-function/57716/1 "2021-03-22T13:51:49Z")

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Hi all,  
Being new to Julia, I started playing around a bit with differential equations.  
I started with a simple system, du = A \* u where A is an n\*n array, and u a vector of length n.

when I set up the system as

`f(u,p,t) = A*u`

and provide initial conditions, it works as intended.  
However, when I instead set it up as

```julia
function fun!(du,u,p,t)
   du = A*u
end

```

the solution maintains stuck at the initial value.  
It does work when I write out equations explicitly (du[1] = …, du[2] = …, etc.), but this is not very practical. Also, the size & type of du are exactly the same when I do the matrix multiplication as when I write the equations explicitly. So, I’m a bit puzzled why the matrix multiplication doesn’t work.  
Would appreciate some insights!

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<div class="post-metadata">

### Author: ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [March 22, 2021, 1:59pm UTC](https://discourse.julialang.org/t/differentialequations-jl-stuck-at-initial-value-when-using-function/57716/2 "2021-03-22T13:59:23Z")

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You need `du .= A*u` so that `du` is modified instead of replaced.

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<div class="post-metadata">

### Author: ![DCT](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@DCT](https://discourse.julialang.org/u/DCT)
#### Post date: [March 22, 2021, 2:16pm UTC](https://discourse.julialang.org/t/differentialequations-jl-stuck-at-initial-value-when-using-function/57716/3 "2021-03-22T14:16:28Z")

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The difference a dot can make… 🙂  
Thanks!

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### Author: ![carstenbauer](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/carstenbauer/32/4981_2.png) [@carstenbauer](https://discourse.julialang.org/u/carstenbauer)
#### Post date: [March 22, 2021, 2:58pm UTC](https://discourse.julialang.org/t/differentialequations-jl-stuck-at-initial-value-when-using-function/57716/4 "2021-03-22T14:58:09Z")

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> [@DCT](#):
>
> The difference a dot can make…

> **[More Dots: Syntactic Loop Fusion in Julia](https://julialang.org/blog/2017/01/moredots/)**
>
> More Dots: Syntactic Loop Fusion in Julia | After a lengthy design process (https://github.com/JuliaLang/julia/issues/8450) and preliminary foundations in Julia 0.5 (/blog/2016-10-11-julia-0.5-highlights#vectorized\_function\_calls), Julia 0.6 includes...