Initialize large two dimensional array in C++

Coincidentally, a couple of hours after reading your question, I bumped into a possible solution while looking for something else in the book "C - A Reference Manual" 5th ed., Harbison/Steele (this is a fantastic C reference, by the way).

According to the book,

C99 allows you to name the components of an aggregate (structs, union or array) to be initialized within an initializer list.

... and it gives an example:

int a1[5] = { [2]=100, [1]=3 }; /* eqv. to {0, 3, 100, 0, 0} */

So, depending on the compliance of your compiler and on the size of the non-zero elements in your array, you may be able to use this syntax to init your matrix efficiently. That said, the book doesn't give an example for 2D arrays. Unfortunately, I couldn't test this idea since MSVC++ 2005 doesn't seem to support C99.

When I do this, I use a method to read in data. Generically, it looks like:

extern void ReadElements(string sFile, Matrix a)
{
    int x;
    int y;
    double value;

    ifstream myInFile;

    myInFile.open(sFile, ifstream::in);
    while(!myInFile.eof())
    {
        myInFile >> x >> y >> value;
        a[x][y] = value;
    }

    myInFile.close();
    return;
}

There is no way to assign an int to const array after it's been initialized. So you will have to script it up:

And include your file this way:

class A {
public:
    static const int test[10][10];
};

const int A::test[10][10] = {
#include "data.inc" // points to the file generated by script.
};

Any part of an array which is initialized, that is beyond the initialization, is initialized to 0. Hence:

int const A::test[10][10];           // uninitialized

int const A::test[10][10] = { {0} }; // all elements initialized to 0.

int const A::test[10][10] = {1,2};   // test[0][0] ==1, test[0][1]==2, rest==0

That means all you have to initialize is up to the last non-zero:

int const A::test[10][10] = { 
  {0, 0, 0, 0, 0, 0, 0, 0, 0, 0},  
  {0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, 
  {0, 0, 0, 0, 0, 0, 0, 0, 0, 0}, 
  {0, 0, 0, 7, 7, 7, 7, 0, 0, 0}, 
  {0, 0, 0, 7, 7, 7, 7, 0, 0, 0}, 
  {0, 0, 0, 7, 7, 7, 7, 0, 0, 0}, 
  {0, 0, 0, 7, 7, 7, 7, 0, 0, 0}
};

It is not the best solution, but will save some work.