Accessing double pointer causes segmentation fault

I am programming Jacobis method for eigenvalue problems in QM and I have just started c++, I want to use double pointers to construct matrices but the physics problem involved requires a lot of code.

I don't want to clutter my main() with unreadible lines (others will have to read this code..) and so wanted to divide the problem into subfunctions. I made a function that takes a double pointer and returns a matrix, but why can't I access it outside the function? My code segfaults (marked below) when I try to. How do I construct a matrix outside main() while still being able to access it in main()?

enter code her    enter code here
int i, j, k;  


//== BEGIN MAIN ==//
int main ()
{
  //Constants and variables          
  double **A;
  double epsilon = pow((double)10, double(-8)); //The convergence limit for jacobis   method
  int N          = 10;                          //Dimension of matrix
  char test[] =  "test";
  cout <<"The inner matrix function:"<<endl;
  makematrix(N, A);
  cout<<endl<<"The outer matrix function:"<<endl;
  //This part segfaults
  for(i=0; i<N; i++)
  {
  cout<<endl;
  for(j=0; j<N; j++)
{
  cout<<A[i][j]<<" ";
}
 }
return 0;
 }
 //== END MAIN ==//



//==Begin function definitions==//
void makematrix(int N, double **A)
{
   //Function for initializing our tridiagonal matrices for jacobis method
    A = new double*[N];
for(i=0; i<N; i++)
{
  A[i] = new double[N];
}
 for(i=0; i<N; i++)
{
  for(j=0; j<N; j++)
{
  A[i][j] = 0;
   }
     }
    //Pr开发者_如何转开发ints the matrix declared here
    for(i=0; i<N; i++)
     {
       cout<<endl;
       for(j=0; j<N; j++)
    {
      cout<<A[i][j]<<" ";
    }
    }
 cout <<endl;
 return;
}

Return it:

double** makematrix(int N) {
    double **A = new double*[N];
    ...
    return A;
}

In main...

double **A = makematrix(N);

//This part segfaults
  for(i=0; i<N; i++)

Because, you are passing double **A by value (which is modified inside makematrix) and not by reference. Change your function signature to following and it should work:

void makematrix(int N, double **&A)
...                            ^^^ pass by reference

Because when you pass A into the function, the function operates on a copy of A. It sets that copy to point at a new array, but that doesn't affect the original A.

One solution is:

double **A;

makematrix(N, &A);  // Pass address of A

...

void makematrix(int N, double ***A)
{
    (*A) = new double*[N];
    // etc.
}

i.e. pass the address of A, so that the function can modify the original.

Note: Any time you end up needing triple pointers, you probably have a design problem. Especially in C++.

I assume you should pass the pointer as parameter by reference. So the function should look like this (I added an &):

void makematrix(int N, double **&A)

This way your variable will be changed, so A is now an output parameter.

To rapidly fix your code, use the pass-by-reference solution, it's easier to read than the triple pointer.

However, if you want to code in real C++, and not in disguised C, a solution to hide the complexity of the double array behind your matrix is to create a class. The C++ FAQ Lite has an extensive description of your problem, and different approaches to solve it. See http://www.parashift.com/c++-faq-lite/operator-overloading.html#faq-13.10 and http://www.parashift.com/c++-faq-lite/operator-overloading.html#faq-13.11