Question

I want to have static and constant two dimensional array inside a class. The array is relatively large, but I only want to initialize a few elements and others may be whatever compiler initializes them to.

For example, if a class is defined like:

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

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},
  {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}
};

and I am interested only to initialize the elements marked with ‘7’, how do I do this on the same elements, but with array of larger size, like array[1024][1024]?

Answer 1

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.