That's because the left hand side doesn't match the type of the right
hand side.

The best way to use 2D arrays is to flatten them. Allocate a 1D array of
the size columns * rows:

long * pprime = new long[columns * rows];

and calculate the linearized index on your own, like this:

pprime[row * columns + column] = 0;

You need to take care of deleting your array when it's not used anymore:

delete [] pprime;

If you absolutely need the [row][column] syntax, or if the number of
columns is different in each row, then you have to allocate an array of
pointers, and then for each row allocate an array that holds the columns:

long** matrix = new long[rows];
for(int i = 0; i < rows; ++i)
   matrix[i] = new long[columns];

matrix[row][column] = whatever;

[...]

for(int i = 0; i < rows; ++i)
   delete [] matrix[i];
delete [] matrix;

I strongly recommend that you use std::vector if you program in C++, and
use a 1D vector instead of a 2D one.

Tom