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## [ff3d-users] Re: new problems

 From: Stephane Del Pino Subject: [ff3d-users] Re: new problems Date: Thu, 22 Jan 2004 17:19:48 +0100

```Robert Li wrote:
>
> Dear Stephane Del Pino,
>  I have new problems with ff3d.
>
> 1.In parse.ff.yy, you used abs(), because class
> ExpressionStdFunction needs a real function
> pointer,should we change it into fabs()?
>
> | ABS '(' functionexp ')'
> {
>   \$\$ = new
> FunctionExpressionUnaryOperator<ExpressionStdFunction<FunctionExpression,
>     std::abs> >(\$3);
> }
Dear Robert,
Since the code is C++ (not C), using abs() is the right way to do it.
C++ allows polymorphism so the correct abs function will be called
depending on the parameter. Note also that you can compile ff3d using
float or double (default) using a ./configure option. If you replace abs
by fabs it will no longer work for floats which require fabsf. We could
also add a 'long double' option which requires just a few lines (look at
the utils/Types.hpp file).

> 2.In parse.ff.yy, although I can find dx,dy,dz
> keywords,I can not find dt keyword. I want to solve a
> heat transfer PDE problem. Unknown T is a
> function(x,y,z,t). And I need to express
> deltT=T[i]-T[i-1]. Could you give me a example about
> how to write them in input file?
Yes. As the manual may say, in ff3d the time discretization is let to
the user.
For instance use the following euler scheme to solve
| dt(T)-Laplace(T) = 0
| T(0) = T0;
| T(t) = 0 on d(Omega)

function Tn = T0;
double n = 0;
double dt = 0.1;
do {
solve(T) in Omega by M {
pde(T)
T = 0 on M;
}
// you can compute deltaT here
un = u;
} while (n<10);