# Strange behaviour of FE initialization in a loop

**URL:** <https://community.freefem.org/t/strange-behaviour-of-fe-initialization-in-a-loop/3962>\
**Category:** General Discussion\
**Created:** [June 10, 2025, 2:47pm UTC](https://community.freefem.org/t/strange-behaviour-of-fe-initialization-in-a-loop/3962 "2025-06-10T14:47:10Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Loic](https://avatars.discourse-cdn.com/v4/letter/l/e9c0ed/32.png) [@Loic](https://community.freefem.org/u/Loic)\
**Post date:** [June 10, 2025, 2:47pm UTC](https://community.freefem.org/t/strange-behaviour-of-fe-initialization-in-a-loop/3962/1 "2025-06-10T14:47:10Z")

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Dear all,

I am trying to understand the strange behaviour of the small example:

```auto
mesh Th = square(1,1); 

macro def(i)[i, i#B]// EOM // vector field definition
func Pk = [P1, P1]; 

fespace Wh0(Th, Pk); 
Wh0 def(u);

for (int i=0; i< 5; i++){ 

    cout << " FIRST COUT " << endl; 
    cout << u[] << endl; 

    fespace Wh(Th, Pk);
    Wh def(u);
    u[] = 1e-3; 

    cout << " SECOND COUT " << endl; 
    cout << u[] << endl; 
}

```

For each iteration I get

> FIRST COUT  
> 8   
> 0 0 0 0 0  
> 0 0 0   
> SECOND COUT  
> 8   
> 0.001 0.001 0.001 0.001 0.001  
> 0.001 0.001 0.001

So, why the first `cout` is always 0, whereas `u[]` is re-initialize only after the first `cout` ?

Would someone have an explanation about this observation ?

Thank you in advance,

Best regards,

Loïc,

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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 10, 2025, 3:47pm UTC](https://community.freefem.org/t/strange-behaviour-of-fe-initialization-in-a-loop/3962/2 "2025-06-10T15:47:02Z")

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You are `cout`ing uninitialized memory. There is no guarantee that it will always be 0.

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<div class="post-metadata">

**Author:** ![Loic](https://avatars.discourse-cdn.com/v4/letter/l/e9c0ed/32.png) [@Loic](https://community.freefem.org/u/Loic)\
**Post date:** [June 10, 2025, 4:09pm UTC](https://community.freefem.org/t/strange-behaviour-of-fe-initialization-in-a-loop/3962/3 "2025-06-10T16:09:37Z")

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So if I understand well the first `u[]` is not equal to `1e-3` after the first iteration. The two `u[]` are different objects and the first one continue being an object of `Wh0`.

But for example, if I do something as:

```auto
fespace Wh0(Th, Pk); 
Wh0 def(u0);

for (int i=0; i< 5; i++){

* varf with u0 on the RHS

fespace Whnew(Th, Pk);
Whnew def(u);

* Find u by solving varf
* Th = movemesh(Th, ...)
* u0[] = u[]
}

```

My problem is there. I want to define a new u0 living in `Whnew`, but the `u0` that I have used in the `varf` remains the first `u0` that I have defined outside the loop.

How to initialize properly my `u0` , so that I can redefine it at each iteration ?

Best regards,

Loïc,

---

<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 10, 2025, 4:36pm UTC](https://community.freefem.org/t/strange-behaviour-of-fe-initialization-in-a-loop/3962/4 "2025-06-10T16:36:40Z")

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Please use an actual code. Your first snippet is perfectly functioning, and I don’t see what the issue is here. There are multiple such examples for mesh adaptation.

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**Author:** ![fb77](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/fb77/32/3796_2.png) [@fb77](https://community.freefem.org/u/fb77)\
**Post date:** [June 10, 2025, 5:15pm UTC](https://community.freefem.org/t/strange-behaviour-of-fe-initialization-in-a-loop/3962/5 "2025-06-10T17:15:33Z")

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The fact is that when you declare a variable inside a subsection delimited by braces {}, it is a local definition only valid within these braces. When you leave these braces, in your case the name “u” recovers its declaration and values that were valid before the {} (here no value was given thus it gives 0).  
Here you have a loop thus for each value of i you open and close a new subsection {}.  
If you want to keep the value that you compute inside the {}, you must save it in a variable that was declared outside the {}. I think that it is not possible to declare a global variable inside a {}.

But as Pierre says, it works in the context of movemesh: you change Th within the loop, but make no declaration within the loop (in particular you do not declare `fespace Whnew(Th, Pk);`). Only the unknowns are updated. The unknowns are declared outside the loop.

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<div class="post-metadata">

**Author:** ![Loic](https://avatars.discourse-cdn.com/v4/letter/l/e9c0ed/32.png) [@Loic](https://community.freefem.org/u/Loic)\
**Post date:** [June 10, 2025, 6:22pm UTC](https://community.freefem.org/t/strange-behaviour-of-fe-initialization-in-a-loop/3962/6 "2025-06-10T18:22:16Z")

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Thanks, I have understand now.  
I was misinterpreting the `{}` for the `for loop` and the declaration of `fespace`.

Best regards,

Loïc