[pda]

Hello,

I would like to have a volScalarField variable which contains the x-position of each cell centre. After looking around for hours, I don't know how to do this. I tried with cellCentres and others, but I cannot find the commands I am looking for.

Any help is greatly appreciated,

PJ

[lr103476]

the following should work:

const volScalarField& xPos = mesh.C().component(vector::X)

Frank

[hjasak]

Close (the above was a memory violation):


volScalarField xPos = mesh.C().component(vector::X);

Hrv

[lr103476]

Hi Hrv,

I thought:

1) A reference &, since you don't want a real copy of the cell centres, only the right to have a look at it.

2) A const, since you're not allowed to change the value of the cell centres.

Why memory violation?

Regards, Frank

[hjasak]

All you said is correct, but you cannot decide on the return type: the function does that for you. Function signature says:


//- Return a component of the field
tmp<geometricfield<cmpttype,> > component
(
const direction
) const;

In other words, the function will return a tmp<volscalarfield>. If you take a reference and not a copy, nobody is holding the original object and tmp will delete the memory in its destructor. You are now left holding something called a reference to a temporary, ie. looking at a piece of memory that has been deleted and is waiting for re-use. If somebody else uses that memory, you get garbage: memory violation.

Clear?

Hrv

[lr103476]

Perfectly clear, but one more thing.... I just remembered a piece of code which you showed at the lunch training in Zagreb (with const and a ref &):

const volVectorField& centres = mesh.C();
point origin(0.5,0.5,0.05);
vector axis(0,0,-1);
U.internalField() = axis ^ (centres.internalField() - origin);

What is the difference? Is it the component function?

Frank

[hjasak]

Easy. Have a look in the library and find out what the return type is for a function mesh.C(). The file is fvMesh.H and the function signature is:


//- Return cell centres as volVectorField
const volVectorField& C() const;


As you can see, this one returns a const volVectorField& and I can accept it as such. In the other, I have no choice - I have to take a copy.

Clear?

Hrv

[lr103476]

thanks, totally clear!

Frank