# ModelingToolkit @mtkmodel and loop on equations
**URL:** https://discourse.julialang.org/t/modelingtoolkit-mtkmodel-and-loop-on-equations/113374
**Category:** Modelling & Simulations
**Created:** [April 23, 2024, 7:43am UTC](https://discourse.julialang.org/t/modelingtoolkit-mtkmodel-and-loop-on-equations/113374 "2024-04-23T07:43:08Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@julien-dc](https://discourse.julialang.org/u/julien-dc)
#### Post date: [April 23, 2024, 7:43am UTC](https://discourse.julialang.org/t/modelingtoolkit-mtkmodel-and-loop-on-equations/113374/1 "2024-04-23T07:43:08Z")
Greetings Julia community,
I’m currently exploring the possibility of transitioning from a Modelica/Python workflow to a fully Julia-based workflow for my projects. As I’m relatively new to Julia, I’ve encountered an issue while working with `@mtkmodel`.
The following code snippet demonstrates the problem:
```julia
@mtkmodel Circuit1 begin
@components begin
pipe = [Pipe() for i in 1:8]
end
@equations begin
for i in 1:7
connect(pipe[i].c_out, pipe[i+1].c_in)
end
end
end
@mtkbuild circuit_model = Circuit1()
```
While the above code throws an error: MethodError: Cannot `convert` an object of type Nothing to an object of type Equation
The following code, which manually lists each connection, works as expected:
```julia
@mtkmodel Circuit2 begin
@components begin
pipe = [Pipe() for i in 1:8]
end
@equations begin
connect(pipe[1].c_out, pipe[2].c_in)
connect(pipe[2].c_out, pipe[3].c_in)
connect(pipe[3].c_out, pipe[4].c_in)
connect(pipe[4].c_out, pipe[5].c_in)
connect(pipe[5].c_out, pipe[6].c_in)
connect(pipe[6].c_out, pipe[7].c_in)
connect(pipe[7].c_out, pipe[8].c_in)
end
end
@mtkbuild circuit_model = Circuit2()
```
I’d greatly appreciate any insights or suggestions on how to properly loop over the connections within `@equations` using `@mtkmodel`.
Thank you in advance for your help!
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [April 23, 2024, 10:48am UTC](https://discourse.julialang.org/t/modelingtoolkit-mtkmodel-and-loop-on-equations/113374/2 "2024-04-23T10:48:48Z")
If you want to loop over constructions then you likely want to direct build the ODESystem.
---
### Author:  [@TimoSchubert](https://discourse.julialang.org/u/TimoSchubert)
#### Post date: [April 27, 2024, 1:39pm UTC](https://discourse.julialang.org/t/modelingtoolkit-mtkmodel-and-loop-on-equations/113374/3 "2024-04-27T13:39:58Z")
Hi @ChrisRackauckas , can you give an example? The example above is a typical way of how to create discretized volumes like pipes in Modelica. I wonder what the best way to do this with MTK would look like.
---
### Author:  [@ven-k](https://discourse.julialang.org/u/ven-k)
#### Post date: [April 29, 2024, 5:16am UTC](https://discourse.julialang.org/t/modelingtoolkit-mtkmodel-and-loop-on-equations/113374/4 "2024-04-29T05:16:56Z")
```julia
@mtkmodel Circuit1 begin
@components begin
pipe = [Pipe() for i in 1:8]
end
@equations begin
[connect(pipe[i].c_out, pipe[i+1].c_in) for i in 1:7]...
end
end
@mtkbuild circuit_model = Circuit1()
```
This works.
The `for...end` syntax returns `nothing` resulting the above error; maybe we should add a parser for that as well. For now, please use `[<> for <>]...` syntax.