Thread: a question with pointers

http://67.40.109.61/torek/c/pa.html

Originally Posted by louis_mine

how do I move the pointer to another direction, for example: arr[2][10]

by just manipulating the pointer

Thanks

Suppose you have defined constants NUMROWS and NUMCOLS as the number of rows and number of columns, and you have this

Code:

int i_array[NUMROWS][NUMCOLS];

There are NUMROWS * NUMCOLS contiguous integers in the array, and the address of element i_array[x][y] is equal to this

Code:

&i_array[0][0] + NUMCOLS * x + y;

Note that you must always know the number of columns to access the individual elements.


Try this:

Code:

#include <stdio.h>

int main(void)
{
  int i_array[2][3] = {{1, 2, 3} , {4, 5, 6}}; /* 2 rows, 3 columns */
  int *i_point;
  int j, k;

  for (j = 0; j < 2; j++) {
    for (k = 0; k < 3; k++) {
      printf("i_array[%d][%d] = %d,   ", j, k, i_array[j][k]);
      printf("&i_array[%d][%d] = %p\n", j, k, &i_array[j][k]);
    }
  }

  printf("\n\n");

  /* 
   * for i_array[NumRows][NumCols] the address if element i_array[x][y] is
   * given by this
   *
   *     &i_array[x][y] is equal to &i_array[0][0] + (NumCols * x) + y
   *
   *
   */

  i_point = &i_array[0][0];
  printf("i_point = %p\n", i_point);

  for (j = 0; j < 2; j++) {
    for (k = 0; k < 3; k++) {
      printf("*(i_point + 3 * %d + %d) = %d, ", j, k, *(i_point + 3*j + k));
      printf(" (i_point + 3 * %d + %d) = %p\n", j, k,  (i_point + 3*j + k));
    }
  }

  return 0;
}

This arrangement of the elements is called "row-major" ordering, since the elements of a given row are contiguous in memory. Some other languages (FORTRAN for example) store array elements with "column-major" ordering.

Regards,

Dave

I find it easier to think of it this way:

Code:

char foo[ YSIZE ][ XSIZE ];

And think along the terms of a standard grid. X axis is left to right. Y axis is top to bottom. Z axis would be to the left of the Y axis, and would be the third dimension of movement.

Consider your monitor. The X axis is from the left side to the right side. The Y axis is from top to bottom. It just helps me keep it clear to reverse the variable names from the way you use them.

Quzah.