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Loop over cells slowing down the code

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Old   February 6, 2013, 02:57
Default Loop over cells slowing down the code
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Guillaume
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Hi everybody,

I try to create a field for my variable alphath. The value I want to set for this variable depends on space.
Finally, the algorithm should look like :

For all cells
If ( x < xmax and x > xmin) alphath = f(x)

In order to go step by step in the coding process (I'm not really a senior coder in openFoam ...), for the moment, I just try to impose, for all cells,
alphath(x,y,z) = x

I coded this stuff like that :

forAll (mesh.cells(),celli)
{
alphath[celli] = mesh.C().component(vector::X)()[celli];
}

Adding these lines in the code REALLY slows it down ...
As a simple test, I replaced "mesh.C().component(vector::X)()[celli]" by "1.0" and the code is quick again.

Do you know why the added lines slow the code down ?
Would you have an alternative coding ?

Thanks in advance for your help.

Guillaume
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Old   February 6, 2013, 03:13
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ngj
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Niels Gjoel Jacobsen
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Hi Guillaume,

I have a small guess, since you are first extracting the x-coordinate from the cell centres and then you get the i'th index (checked the code and a new volScalarField is constructed). This actually means that you make a copy of all of the x-values every time you only need one; so in the end of the day you have a process, which scales with N^2 rather than N.

This should be faster:

Code:
forAll( alphath, celli )
     alphath[celli] = mesh.C()[celli].x();
or in this particular case, you could merely have written:

Code:
alphath.internalField() = mesh.C().component(vector::X);
Kind regards,

Niels
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Old   February 6, 2013, 04:15
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Guillaume
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Hey Niels,

thanks a lot for having taken some of your time for my problem !
Your answer was quick, detailed and (after a test just performed) very efficient !

Thanks again.

Kind regards,
Guillaume

Last edited by Gloq; February 6, 2013 at 04:42.
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Old   February 6, 2013, 04:25
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ngj
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Niels Gjoel Jacobsen
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Good!

/ Niels
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