two-dimensional dynamical array of char

[Cawas]

Simple like that, is what i want.

On VB, it would be something like (dunno if it really works):

Dim i, j As Integer
Dim arr As String
 
i = 5
j = 10
arr = New Array(i, j)

On C, I'm trying to do similar by the following:

int i, j;
char **arr;

char = (char**) malloc(sizeof(char) * i * j);

But it isn't working! I must be doing something wrong, I'm not used to C pointers / array stuff... It's really confusing. :P

[parksie]

Did you set the sizes of i and j first?

[Cawas]

Ya, I forgot to mention on my code above, but I did.

I did even try this way:

char **arr;

arr = (char**) malloc(sizeof(char) * 40 * 40);

[Cawas]

Oh, yeah, the erro I got isn't on compiling.

It builds the EXE file, with my BCC5, free command line compiler from Borland.

But when I try to run the EXE on my Windows 2k, it crashes. I get 2 memmory fail and one fatal error message, in sequence, at the same second I run it.

[parksie]

I think something's seriously broken there. Are you creating the proper executable type?

Just out of interest, what happens if you try and allocate a 3x3 block?

[Cawas]

Here.

I just wrote a whole file just to test it alone:

#include <stdio.h>
#include <alloc.h>

int main() {
	char **arr;
	int i, j;

	// Dynamicaly setting the array
	arr = (char**) malloc(sizeof(char) * 10 * 10);

	// Filling the array
	for (i = 0; i < 10; i++) {
		for (j = 0; j < 10; j++) {
			if (i == 0 || j == 0 || i == 9 || j == 9) {
				arr[i][j] = '#';
			} else {
				arr[i][j] = ' ';
			}
		}
	}

	// Showing the array
	for (i = 0; i < 10; i++) {
		for (j = 0; j < 10; j++) {
			printf("%c", arr[i][j]);
		}
	}

	return(0);
}

It compiles and doesnt works.
I called it "arrtest.c".

I can compile other Cs, if I don't try to use this kind of declaration.

[CornedBee]

This has never worked and never will...

The problem is that indexing into an array is resolved at compile-time, but in two-dimensional dynamic arrays one dimension has no precisely defined size:
int ar[10][10];
-->
ar[0] has the size sizeof(int)*10 (because it's type is int[10]), so the compiler knows how much to increment the pointer if you say
ar[1].

But when you do
int **ar = (int**)malloc(sizeof(int)*10*10);
and you want to use this as a two-dimensional array, ar[0] has the size sizof(int)*??? because the compiler has no way of knowing how large you want this dimension to be. Actually you are allocating an array of 100 int* here, but those pointers have random values. And if int happens not to match the size of void* (as is the case on 64-bit systems), you have no idea how many pointers were allocated in the first place.

There are some possibilities:
a) Do:

int ** ar = (int**)malloc(sizeof(int*)*10);
for(int i=0; i < 10; i++)
  ar[i] = (int*)malloc(sizeof(int)*10);

which is extremly tedious at removal because you have to call free for every call to malloc.

b) Have the inner dimension fixed:
int (*ar)[10] = (int (*)[10])malloc(sizeof(int[10])*10);
which has a damn confusing syntax.

c) The VB array approach. A VB array internally is a structure of type SAFEARRAY, which works like this (not exactly can't remember now)

typedef struct tagDIMENSION {
  DWORD dwSize;
} DIMENSION, *LPDIMENSION;

typedef struct tagSAFEARRAY {
  void *pData;
  DWORD dwDimensions;
  DWORD dwDataSize;
  DIMENSION dims[1];
} SAFEARRAY, *LPSAFEARRAY;

Gotta go now, I'll explain the workings of this tomorrow. A SAFEARRAY is ALWAYS part of a VARIANT...

[Cawas]

I just improved the arrtest.c

#include <stdio.h>
#include <alloc.h>

// Dynamic set, not working
void test1 (void) {
	char **arr;
	int i, j;

	// Dynamicaly setting the array
	arr = (char**) malloc(sizeof(char) * 10 * 10);

	// Filling the array
	for (i = 0; i < 10; i++) {
		for (j = 0; j < 10; j++) {
			if (i == 0 || j == 0 || i == 9 || j == 9) {
				arr[i][j] = '#';
			} else {
				arr[i][j] = ' ';
			}
		}
	}

	// Showing the array
	for (i = 0; i < 10; i++) {
		for (j = 0; j < 10; j++) {
			printf("%c", arr[i][j]);
		}
	}
}

// Non dynamic set, working
void test2 (void) {
	char arr[10][10];
	int i, j;

	// Filling the array
	for (i = 0; i < 10; i++) {
		for (j = 0; j < 10; j++) {
			if (i == 0 || j == 0 || i == 9 || j == 9) {
				arr[i][j] = '#';
			} else {
				arr[i][j] = ' ';
			}
		}
	}

	// Showing the array
	for (i = 0; i < 10; i++) {
		for (j = 0; j < 10; j++) {
			printf("%c", arr[i][j]);
		}
	}
}

int main() {
	int op;

	printf("Choose the test by the number\n");
	scanf("%d", &op);

	if (op == 1) {
		printf("Test 1 selected");
		test1();
	}

	if (op == 2) {
		printf("Test 2 selected");
		test2();
	}

	return(0);
}

[Cawas]

This is wonderfull Bee.

I'll try first option, I liked it.

I didn't even see the third yet, but I will read and re-read everysingle word as soon as I make the first option work here.

[Cawas]

One more, before I erase the file arrtest.c

// Dynamic set, WORKING! Thanks CornedBee
void test3 (void) {
	char **arr;
	int i, j;

	// Dynamicaly setting the array
	arr = (char**) malloc(sizeof(char*) * 10);
	if (arr == NULL) {
		free(arr);
		return;
	}

	for (i = 0; i < 10; i++) {
		arr[i] = (char*) malloc(sizeof(char) * 10);
		if (arr == NULL) {
			for (j = 0; j < 10; j++)
				free(arr[j]);
			return;
		}
	}

	// Filling the array
	for (i = 0; i < 10; i++) {
		for (j = 0; j < 10; j++) {
			if (i == 0 || j == 0 || i == 9 || j == 9) {
				arr[i][j] = '#';
			} else {
				arr[i][j] = ' ';
			}
		}
	}

	// Showing the array
	for (i = 0; i < 10; i++) {
		for (j = 0; j < 10; j++) {
			printf("%c", arr[i][j]);
		}
	}
}

int main() {
	int op;

	printf("Choose the test by the number\n");
	scanf("%d", &op);

	if (op == 1) {
		printf("Test 1 selected");
		test1();
	}

	if (op == 2) {
		printf("Test 2 selected");
		test2();
	}

	if (op == 3) {
		printf("Test 2 selected");
		test3();
	}

	return(0);
}

[CornedBee]

3rd option renewed. Forget everything I wrote above about the third option.

Here is how you implement an array similar to the one that you use in VB, but rewritten by me.
Using these two data structures, you can make a dynamically sized dynamically dimensionized arbitrary indexed array:
In plain C, if I got nothing better to do I might write a nice C++ class that does the same thing. Here's the API:

#define SA_PTR 0x80000000
#define SA_I16 1
#define SA_I32 2
#define SA_CHAR 3
#define SA_SINGLE 4
#define SA_DOUBLE 5
#define SA_ARRAY 0x80000006
#define SA_BOOL 7

typedef struct tagDIMENSION
{
  long lIndexStart;
  unsigned long ulSize;
} DIMENSION;

typedef struct tagSTRANGEARRAY
{
  unsigned long ulDims;
  unsigned long ulSize;
  unsigned long ulType;
  void *pvData;
  DIMENSION dims[1];
} STRANGEARRAY;

STRANGEARRAY *CreateStrangeArray(unsigned long type,
    unsigned long ulDims, ...);

void *ElemAt(const STRANGEARRAY *ar, ...);

void DestroyStrangeArray(STRANGEARRAY *ar);

I'll implement the function and get back to you...

[Cawas]

Ah, good I didn't read it then.

Is the thir option, even now, for C++ only?

I'm using C plain only, for now, cause of class on college.

After finishing the whole class, I'll head to C++.

[CornedBee]

No, I'm currently writing the third option in plain C.
But it's hard to manage in C, C++ would make it easier.

[CornedBee]

Here's the implementation:

// starting from here in a source file

size_t getitemsize(unsigned long type)
{
	if(type & SA_PTR)
	{
		return sizeof(void*);
	}
	else
	{
		switch(type)
		{
		case SA_CHAR:
			return sizeof(char);
		case SA_I16:
			return sizeof(short);
		case SA_BOOL:
		case SA_I32:
			return sizeof(long);
		case SA_SINGLE:
			return sizeof(float);
		case SA_DOUBLE:
			return sizeof(double);
		}
	}
	return 0;
}

// This function allocates memory for a strange array
STRANGEARRAY *CreateStrangeArray(unsigned long type,
								 unsigned long ulDims, ...)
{
	va_list l;
	int i;
	unsigned long size = 1;
	// may not be zero dimensions
	if(ulDims <= 0)
		return NULL;
	STRANGEARRAY *ptr = (STRANGEARRAY *)malloc(
		sizeof(STRANGEARRAY) + (ulDims-1)*sizeof(DIMENSION));
	if(ptr == NULL)
		return NULL;
	ptr->ulDims = ulDims;
	// got the mem, now find out about dimension info
	va_start(l, ulDims);
	for(i=0; i<ulDims; ++i) {
		ptr->dims[i].ulSize = va_arg(l, unsigned long);
		size *= ptr->dims[i].ulSize;
		ptr->dims[i].lIndexStart = va_arg(l, long);
	}
	ptr->ulSize = size;
	// now get the type
	ptr->ulType = type;
	size *= getitemsize(type);
	ptr->pvData = malloc(size);
	if(ptr->pvData == NULL)
	{
		free(ptr);
		return NULL;
	}
	// array is now ready
	return ptr;
}

void *ElemAt(const STRANGEARRAY *ar, ...)
{
	// find the element following the info in the structure
	int i, j;
	unsigned long off = 0;
	va_list l;
	va_start(l, ar);

	for(i = 0; i < ar->ulDims; ++i)
	{
		unsigned long dimsize = 1;
		long index = va_arg(l, long);
		index -= ar->dims[i].lIndexStart;
		if(index >= ar->dims[i].ulSize || index < 0)
			return NULL;	// out of bounds
		for(j = i-1; j >= 0; --j)
		{
			dimsize *= ar->dims[j].ulSize;
		}
		off += (index*dimsize);
	}
	off *= getitemsize(ar->ulType);
	
	return (void*)(((char*)ar->pvData) + (offset-1));
}

void DestroyStrangeArray(STRANGEARRAY *ar)
{
	free(ar->pvData);
	free(ar);
}

It's quite complicated. I will not explain it until you ask me to do so.