/* plus2.c - Let's look at how + and * work with 2-d arrays in C.
 * The name of the array can be used like a pointer.
 */

#define NUM_ROWS 6
#define NUM_COLS 8

#include <stdio.h>

/* initialize - Put numbers in the array.
 * Unfortunately, C does not provide a way for us to look up the length
 * of an array, so let's put a sentinel value of 0 at the beginning of
 * its last row.  I'm also going to put a 0 at the end of each row.
 *
 * Note that when a function takes a 2-d array as a parameter, we only need to 
 * provide the number of columns.  This is because in order to calculate
 * the address of each element in the array, we need the number of columns,
 * not the number of rows.
 */
void initialize(int a[][NUM_COLS])
{
  int i, j;

  for (i = 0; i < NUM_ROWS; ++i)
  {
    for (j = 0; j < NUM_COLS - 1; ++j)
      a[i][j] = 10 * (i + 1) + (j + 1);
    a[i][NUM_COLS - 1] = 0;             // Put 0 at end of each row.
  }
  a[NUM_ROWS - 1][0] = 0;               // Put 0 in bottom left corner also.
}

/* print_2d - Print a 2-dimensional array.  We will continuously print
 * rows of the array up to and including the "last" row, which we define
 * to be the row that begins with zero.  I'm creating this artificial
 * definition simply because I want to illustrate how + works with 2d arrays.
 */
void print_2d(int a[][NUM_COLS])
{
  int i, j;

  for (i = 0; ; ++i)
  {
    for (j = 0; j < NUM_COLS; ++j)
      printf("%5d", a[i][j]);

    printf("\n");
    if (a[i][0] == 0)
      break;
  }
}

/* print_1d - Print a 1-dimensional array, up to and including the first
 * zero that we encounter.
 *
 * It is interesting to note that when a function takes a 1-d array as a
 * parameter, we do not indicate the size of the array.  It is not necessary,
 * because the size of the array has no bearing on calculating the address
 * of each element of the array.
 */
void print_1d(int a[])
{
  int i;

  for (i = 0; ; ++i)
  {
    printf("%5d", a[i]);
    if (a[i] == 0)
      break;
  }
}

main()
{
  int a[NUM_ROWS][NUM_COLS];

  initialize(a);

  printf("Here is the entire array a:\n");
  print_2d(a);

  printf("\nHere is the whole array starting at row 3:\n");
  print_2d(a + 3);

  printf("\nHere is row 1:\n");
  print_1d(a[1]);

  printf("\nHere is row 3:\n");
  print_1d(*(a + 3));

  printf("\nHere is row 4, starting at its element 2:\n");
  print_1d(*(a + 4) + 2);

  // Here is an alternative way of writing a[i][j]:  *(*(a+i)+j).
  printf("\na[3][2] equals %d\n", *(*(a + 3) + 2));
}

/* Output:
Here is the entire array a:
   11   12   13   14   15   16   17    0
   21   22   23   24   25   26   27    0
   31   32   33   34   35   36   37    0
   41   42   43   44   45   46   47    0
   51   52   53   54   55   56   57    0
    0   62   63   64   65   66   67    0

Here is the whole array starting at row 3:
   41   42   43   44   45   46   47    0
   51   52   53   54   55   56   57    0
    0   62   63   64   65   66   67    0

Here is row 1:
   21   22   23   24   25   26   27    0
Here is row 3:
   41   42   43   44   45   46   47    0
Here is row 4, starting at its element 2:
   53   54   55   56   57    0
a[3][2] equals 43
*/