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Pointer to multidimensional array

Started by IADaveMark Nov 22, 2003 at 3:11 PM 21 replies 4.3k views

Original Post

Nov 22, 2003 at 3:11 PM

What is wrong with the following statement (in my Ctor of a class): MARKET * mDemand = new MARKET[300][300]; It gives the following error: error C2440: ''initializing'' : cannot convert from ''struct MARKET (*)[277]'' to ''struct MARKET *'' Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast Dave Mark - President and Lead Designer Intrinsic Algorithm - "Reducing the world to mathematical equations!"

IADaveMark

Nov 22, 2003 at 3:15 PM

quote:
Original post by Anonymous Poster
MARKET ** mDemand = new MARKET[300][300];

Nope...

"cannot convert from ''struct MARKET (*)[277]'' to ''struct MARKET ** ''
Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast"

Dave Mark - President and Lead Designer
Intrinsic Algorithm - "Reducing the world to mathematical equations!"

jungle

Nov 22, 2003 at 3:41 PM

May I suggest
MARKET (*mDemand)[300]=new MARKET[300][300];

Lektrix

Nov 22, 2003 at 3:42 PM

Either:


MARKET** mDemand = new MARKET*[300];
for (std::size_t idx = 0; idx < 300; ++idx)
   mDemand[idx] = new MARKET[300];

...

for (std::size_t idx = 0; idx < 300; ++idx)
   delete [] mDemand[idx];
delete [] mDemand;

or:


std::vector< std::vector<MARKET> > mDemand( 300, std::vector<MARKET>(300) );

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swiftcoder

Nov 22, 2003 at 3:44 PM

MARKET *mDemand = new MARKET[90000];
Will work just fine, as will:
MARKET mDemand[300][300]
but since the following doesn't work:

MARKET *mDemand = new MARKET[90000];
mDemand[25][30].something()

because the computer doesn't know how large either of the dimensions of the array are, why would you expect:
MARKET * mDemand = new MARKET[300][300];
to work either?????
"Multi-dimensional arrays must be declared multi-dimensional, although a pointer may be passed into a function which expects a multi-dimensional array" From the "Big C++ Book"

Here's to the crazy ones, the misfits, the rebels, the troublemakers, the round pegs in the square holes, the ones who see things differently.

[edited by - swiftcoder on November 22, 2003 4:45:51 PM]

IADaveMark

Nov 22, 2003 at 4:13 PM

quote:
Original post by jungle
May I suggest
MARKET (*mDemand)[300]=new MARKET[300][300];

That seems to work. Now that I have this array declared on the heap in my Ctor, all the references to mDemand aren''t seeing it. I assume that I need to do something in the header file that shows that this bloody thing exists so that the functions can see it as well?

Dave Mark - President and Lead Designer
Intrinsic Algorithm - "Reducing the world to mathematical equations!"

MatrixCubed

Nov 22, 2003 at 4:46 PM

Another way to tackle this is to avoid multiple dimensionality completely:



int max_x = 300;
int max_y = 300;
int* int_array = new int[x * y];

// retrieve element n

int get_element(int x, int y)
{

     return int_array[y * max_x + x];

}

IADaveMark

Nov 22, 2003 at 5:41 PM

My problem now is that I was trying to trick the damn thing (a la a different thread) into allowing me to do 4 dimensions. The struct that you see there is now giving me fits because it contains an array of yet ANOTHER struct with an array. Can I just use the tricks for this 2x array and extend it out to 4x?

Dave Mark - President and Lead Designer
Intrinsic Algorithm - "Reducing the world to mathematical equations!"

IADaveMark

Nov 23, 2003 at 10:08 AM

quote:
Original post by Anonymous Poster
How do you pass a multidimensional array to a function? And how would I access the array inside of the function?

Are you asking me?

Dave Mark - President and Lead Designer
Intrinsic Algorithm - "Reducing the world to mathematical equations!"

Crispy

Nov 23, 2003 at 11:54 AM


void PrintFirst(char ** MultDimArray)
{
  cout << "The first string in the string list is: " << MultDimArray[0] << endl;
}

void main()
{
  //it's a lot more intuitive to do this using malloc

  char** MDA = (char**)malloc(num_strings * string_len * sizeof(char*));  

  PrintFirst(MDA);
}

Didn't try to compile it - so I can't guarantee it'll work... Substitute 'char' with 'MARKET' to get the desired result.

[edited by - crispy on November 23, 2003 12:55:37 PM]

dmounty

Nov 23, 2003 at 4:38 PM

well... if you need dynamically sized arrays, and need higher dimensions... just use more indirection...


int ****array=new int***[10];

for (int x=0;x<10;x++) {
    array[x]=new int**[10];
    for (int y=0;y<10;y++) {
        array[x][y]=new int*[10];
        for (int z=0;z<10;z++) array[x][y][z]=new int[10];
    }
}

This gives us a 4D array, and similarly, when the time comes to delete the whole thing...


for (int x=0;x<10;x++) {
    for (int y=0;y<10;y++) {
        for (int z=0;z<10;z++) delete[] array[x][y][z];
    }
    for (int y=0;y<10;y++) delete[] array[x][y];
}
for (int x=0;x<10;x++) {
    delete[] array[x];
}
delete[] array;

Looking at it, that delete code looks a little strange, but I think it works fine, anyway, I hope you get the general idea.

jungle

Nov 23, 2003 at 6:13 PM


void foo_func(int (*p)[30][40][50]) {
	// zero one element

	p[5][19][12][1]=0;
}

int main()
{
	// allocate 4D array, 100 x 30 x 40 x 50

	int (*arr)[30][40][50]=new int[100][30][40][50];

	// pass to function

	foo_func(arr);

	// delete array

	delete [] arr;
}

jungle

Nov 23, 2003 at 7:44 PM


void PrintMulti(int (*a)[5]) {
	for (int i = 0; i < 5; ++i)	{
		for (int j = 0; j < 5; j++)
			cout << a[i][j] << endl;
	}
}

int main() {
	int a[5][5];
	PrintMulti(a);
	
	system("PAUSE");
	return 0;
}

Fruny

Nov 24, 2003 at 4:38 AM

boost::multi_array - Multidimensional containers and adaptors for arrays of contiguous data.

Oluseyi

May 8, 2004 at 1:20 AM

Why necro a dead thread?

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