Re: [eigen] is MAP oneway ? (urgent)

[Helmut Jarausch <jarausch@xxxxxxxxxxxxxxxxxxx>]

On 24 Jun, Helmut Jarausch wrote:
> Hi,
> 
> I have to write a function which is called by code which is
> written in C-style (though compiled with g++).
> It delivers me data in a C-array  (double*) and expects me
> to modify this.
> 
> When I tried the following example, I was a bit surprised.
> So, what does MAP do?
> 
> 
> #include <iostream>
> using std::cerr;  using std::endl;
> #include <Eigen/Core>
> // import most common Eigen types 
> USING_PART_OF_NAMESPACE_EIGEN
> 
> int main() {
>   double data[4] = {1, 2, 3, 4};
>   
>   VectorXd D = Eigen::Map<VectorXd>(data,4);
>   cerr << "D before : " << D << endl;
>   data[1]= 7;
>   cerr << "D after  : " << D << endl;  // nothing has changed
> }
> 
> 
> So, do I have to copy all data from D.data() back to data?
> 
> Is there something like "overlaying" an Eigen-Vector onto some raw
> C-array?
> 
> Many thanks for a quick hint,
> Helmut.
> 

Sorry for replying to my own email.
I've forgotten to tell you that I also need the otherway round

add in the above code
  D[1]= 17;
  cerr << "data[1] now: " << data[1] << endl;

-- 
Helmut Jarausch

Lehrstuhl fuer Numerische Mathematik
RWTH - Aachen University
D 52056 Aachen, Germany