array and pointers

[junnytony]

Hi,

I have a few questions:

1. How do I define a pointer to an array of pointers?
2. How do I define a 1-D array and a 2-D array as part of a struct and then initialize them later with the values passed into a function.
3. How do I malloc for these arrays.

Any help is appreciated!!

 

[ArkM]

1.

type* array[]; /* see sacramental char* argv[] in main() */

2. The specification (request) is not complete. Initialization what: extents or elements (or both)? Let's suppose it's extent(s) initialization. Declare the structure as a container for two pointers (the 1st: to 1D array, the 2nd: to 2D array of rows with knonws size), for example:

#define COLS	10

typedef int  Row[COLS];
typedef Row* Matrix;
/* It's two pointers container */
typedef struct
{
  int*   arr1d; /* Pointer to 1D array of int */
  Matrix arr2d; /* Pointer to 2D array of int */
                /*  array of int[COLS] arrays */
} DynArr;

/* Initialize the structure, allocate it if pdyn == 0 */
DynArr* InitDynArr(DynArr* pdyn, int n1d, int n2d)
{
  int sz1d;
  int sz2d;
  int i, j;
  sz1d = n1d*sizeof(int);
  sz2d = n2d*sizeof(Row);
  if (!pdyn) /* dynamic allocation */
  {
    pdyn = (DynArr*)malloc(sizeof(DynArr)); /*cast for C++*/
  }
  pdyn->arr1d = (int*)malloc(sz1d);   /* cast for C++ */
  pdyn->arr2d = (Matrix)malloc(sz2d); /* cast for C++ */
  /* Initialization */
  for (i = 0; i < n1d; ++i)
      pdyn->arr1d[i] = 0;
  for (i = 0; i < n2d; ++i)
  for (j = 0; j < COLS; ++j)
      pdyn->arr2d[i][j] = 0;
  /* See (uncomment):
  {
  printf("Row:%u Matrix: %u Matrix*: %u %u %u\n",
	sizeof(Row),sizeof(Matrix),sizeof(pdyn->arr2d[0],sz1d,sz2d);
	
  int*	pi;
  Row*	pp;
  pi = &pdyn->arr2d[0][1];
  pp = pdyn->arr2d;
  printf("%p %p\n",pp,pi);
  }
  */
  return pdyn;
}

/* Free arrays, not a structure */
void ClearDynArr(DynArr* pdyn)
{
  if (pdyn)
  {
     free(pdyn->arr1d);
     pdyn->arr1d = 0;
     free(pdyn->arr2d);
     pdyn->arr2d = 0;
  }
}
/* Free a structure with its contents */
void FreeDynArr(DynArr* pdyn)
{
  if (pdyn)
  {
     ClearDynArr(pdyn);
     free(pdyn);
  }
}

int main()
{
  DynArr* p;
  DynArr  local;
  p = InitDynArr(0,10,10);
  printf("%u\n",sizeof(DynArr));
  InitDynArr(&local,100,100);
  FreeDynArr(p);
  ClearDynArr(&local);
  return 0;
}

You can't declare 2D array with (both) dynamic extents in classic C. You can't place dynamic members in a structure (except the last, but I think it's not a good practice too).

So the C language is not a proper tool for 2D totally dynamic arrays handling (with comfort. Switch to C++ (or remember unforgettable PL/I or be ready to write cumbersome (but sometimes effective codes in C...

3. See the snippet above...

 

[xwb]

ArkM - maybe you ought to put this in a FAQ. The 2D/3D array question keeps on popping up in the C and C++ forums.

 

[ArkM]

xwb, it's a very important program case, I agree (especially in C).
But FAQ writing is much more responsible task than to write brief notes or to propose a good solution.
Alas, my broken English puts obstacles in this way...