How are 3D arrays stored in C?

Question

I understand that arrays in C are allocated in row-major order. Therefore, for a 2 x 3 array:

0  1
2  3
4  5

Is stored in memory as

Answer 1

Yes, they're are just stored in consecutive order. You can test that like this:

#include 

int main (int argc, char const *argv[])
{
  int numbers [2][3][4] = {{{1,2,3,4},{5,6,7,8},{9,10,11,12}}
                          ,{{13,14,15,16},{17,18,19,20},{21,22,23,24}}};

  int i,j,k;

  printf("3D:\n");
  for(i=0;i<2;++i)
    for(j=0;j<3;++j)
      for(k=0;k<4;++k)
        printf("%i ", numbers[i][j][k]);

  printf("\n\n1D:\n");
  for(i=0;i<24;++i)
    printf("%i ", *((int*)numbers+i));

  printf("\n");

  return 0;
}

That means that accesses to a multiindexed array with dimensions (N,M,L) are transformed to onedimensional accesses like this:

array[i][j][k] = array[M*L*i + L*j + k]