I needed rather the value of the vector field at that position, i.e. for 
fe_values[N_density].value(i, q_point) the value 300 for all 
q_point-values. Might be that I have to take a look at the tutorials 
again...

Am Samstag, 5. August 2017 16:14:32 UTC+2 schrieb Jean-Paul Pelteret:
>
> Yeah, thats pretty much it. Judging from the output, each of your 
> FEExtractors is vector valued, so fe_values[N_density] returns a 
> vector-valued view and fe_values[N_density].value(i, q_point) is the i'th 
> vector valued *shape function* (not DoF value) associated for the 
> N_density vector field at the i, q_point quadrature point. It is indeed 
> not connected to the solution vector. But value() is just one of the 
> functions that FEValuesViews 
> <https://www.dealii.org/8.5.0/doxygen/deal.II/classFEValuesViews_1_1Vector.html>
>  provides 
> - do any of the other functions appear more helpful in terms of what you're 
> trying to achieve?
>
> On Saturday, August 5, 2017 at 2:27:49 PM UTC+2, Maxi Miller wrote:
>>
>> I think I understand my problem (slowly):
>> Using the project-function means that I write the initial conditions into 
>> my solution-vector. But when I call fe_values[N_density] I get a view on 
>> the finite element-variable, which is not connected to the solution-vector. 
>> Thus I get (in the last line) the value of the i-th degree of freedom at 
>> the q-point-th point in the grid. Now, when iterating over the degrees of 
>> freedom, the first degree of freedom will be in x-direction (if looking in 
>> a cartesian coordinate system), the next one in y-direction and the last in 
>> z-direction, thus being the base vectors for a 3d grid, but they are not 
>> connected to the values set before in the project-function. Is that 
>> correct? Or did I still misunderstand something?
>>
>

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