# Macro ThN2O() myN2O

**URL:** <https://community.freefem.org/t/macro-thn2o-myn2o/1033>\
**Category:** General Discussion\
**Created:** [June 14, 2021, 10:36am UTC](https://community.freefem.org/t/macro-thn2o-myn2o/1033 "2021-06-14T10:36:05Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![yongxing](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/yongxing/32/3632_2.png) [@yongxing](https://community.freefem.org/u/yongxing)\
**Post date:** [June 14, 2021, 10:36am UTC](https://community.freefem.org/t/macro-thn2o-myn2o/1033/1 "2021-06-14T10:36:06Z")

</div>

Dear Pierre,

Following up your talk: [Pierre Jolivet introduction to parallel FreeFEM part 1 - YouTube](https://www.youtube.com/watch?v=-Aw2O46V2bo) [Pierre Jolivet introduction to parallel FreeFEM part 2 - YouTube](https://www.youtube.com/watch?v=Hx57gxTOnvE)

fespace VhG(Thglobal, [P2, P2, P1]);  
fespace Vh(Th, [P2, P2, P1]);

int[int] myN2O;  
macro ThN2O() myN2O //

int[int] subIdx = restrict(Vh, VhG, myN2O);

**My question is:**

Once I remesh (after 10 time steps), both Th and Thglobal would change, so do the FEM space VhG and Vh. I realise there is an error for: **int[int] subIdx = restrict(Vh, VhG, myN2O);**

I did cout\<\<"===="\<\<Vh.ndof\<\<"===="\<\<VhG.ndof\<\<endl; and found that both **Vh.ndof** and **VhG.ndof** were different after remeshing. However, It seems I am not allowed to redefine the following, even inside a bracket {…}  
int[int] myN2O;  
macro ThN2O() myN2O //

Please let me know what is the correct thing to do after remeshing?

Best,  
Yongxing

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<div class="post-metadata">

**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\
**Post date:** [June 14, 2021, 11:48am UTC](https://community.freefem.org/t/macro-thn2o-myn2o/1033/2 "2021-06-14T11:48:09Z")

</div>

I don’t have any error with this code.

```auto
include "macro_ddm.idp"

mesh Th = square(40, 40);
mesh Thglobal = Th;
fespace VhG(Thglobal, [P2, P2, P1]);
fespace Vh(Th, [P2, P2, P1]);

int[int] myN2O;
macro ThN2O() myN2O //

buildDmesh(Th);
int[int] subIdx = restrict(Vh, VhG, myN2O);

Th = square(80, 80);
Thglobal = Th;

buildDmesh(Th);
subIdx.resize(Vh.ndof);
subIdx = restrict(Vh, VhG, myN2O);

```

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<div class="post-metadata">

**Author:** ![yongxing](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/yongxing/32/3632_2.png) [@yongxing](https://community.freefem.org/u/yongxing)\
**Post date:** [June 15, 2021, 7:28pm UTC](https://community.freefem.org/t/macro-thn2o-myn2o/1033/5 "2021-06-15T19:28:34Z")

</div>

Thank you prj, I have now solved my problem using subIdx.resize(Vh.ndof) as you suggested. Can you please help me to look at another problem about **savevtk()**

I need to call savevtk() twice to save a **velocity filed** on the whole mesh, at the same I need to save a **displacement field** on a truncated solid mesh. First, if I only save results based on the local mesh, i.e. the following code works well:

```
if (n % saveEach == 0)
	savevtk("velocity.vtu",, Th, ux, uy, p, dataname="ux uy p", order=orderOut,append=true);

```

However, if I try to save the solid displacement as follows, the result are messed up.

```
if (n % saveEach == 0){
	savevtk("velocity.vtu", Th, ux, uy, p, dataname="ux uy p", order=orderOutVel,append=true);
	
	Wh [sxscaled,syscaled]=[sx,sy];
	sxscaled[].*= M.D; [wGx,wGy]=[0,0];[wRx,wRy]=[0,0];
	wRx[](subIdxW)=sxscaled[];
	mpiAllReduce(wRx[], wGx[], mpiCommWorld, mpiSUM);	

	if(mpirank == 0){
		mesh Ths=trunc(Thglobal,region==solid);
		fespace Sh(Ths, PV2);
		Sh [sgx,sgy];
		[sgx,sgy]=[wGx,wGy];
		savevtk("displacement.vtu", Ths, sgx, sgy, dataname="dx dy", order=orderOutDisp,append=true);	
	}
}

```

In the above **Th is a local mesh** and **Ths is a global solid mesh** which is truncated from global mesh Thglobal.

So what is the correct idea to save results in my case: one (fluid+solid) mesh which is distributed to different processes; at the same time, I also try to visualise displacement on a solid mesh (truncated from the whole/global mesh)?

Thank you very much!  
Yongxing.

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<div class="post-metadata">

**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\
**Post date:** [June 16, 2021, 5:56am UTC](https://community.freefem.org/t/macro-thn2o-myn2o/1033/6 "2021-06-16T05:56:34Z")

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> However, if I try to save the solid displacement as follows, the result are messed up.

Which results? Is `wGx` correct after the `mpiAllReduce`? On which FE space is `M` defined (`Wh` or `Sh`)?

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<div class="post-metadata">

**Author:** ![yongxing](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/yongxing/32/3632_2.png) [@yongxing](https://community.freefem.org/u/yongxing)\
**Post date:** [June 16, 2021, 11:54am UTC](https://community.freefem.org/t/macro-thn2o-myn2o/1033/7 "2021-06-16T11:54:37Z")

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Thank you Pierre, all the results are correct. It seems because the implementation of savevtk().

When I remove if(mpirank == 0){ }, and just run savevtk(“displacement.vtu”, Ths,…) at all the processors, it saves the results files correctly.

Do you expect this? the problem has been solved anyway. Thank you very much!

Best,  
Yongxing.

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<div class="post-metadata">

**Author:** ![prj](https://avatars.discourse-cdn.com/v4/letter/p/ecae2f/32.png) [@prj](https://community.freefem.org/u/prj)\
**Post date:** [June 16, 2021, 11:58am UTC](https://community.freefem.org/t/macro-thn2o-myn2o/1033/8 "2021-06-16T11:58:45Z")

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Ah, of course, I missed that. Yes, it is expected. If you want to plot “sequential” solution, you need to use the additional parameter `communicator = mpiCommSelf` in `savevtk`.