Multi-Demensional Arrays on Heap

**GrNxxDaY** · 08-10-2002

are multi-demensional arrays on the heap impossible?
try this out on your compiler. get an error dev 4.9.4.1

Code:

int * pInt = new int[2][2];
delete pInt;

my error says Cannot assign to int * from int[*][2] or something very similar

**Salem** · 08-10-2002

Not impossible

Code:

const int max_cols=20;

// use this when rows and columns are truly dynamic
// like int arr[x][y]
void f1 ( int num_rows, int num_cols ) {
    int **p;
    int i;

    p = new int*[num_rows];
    for ( i = 0 ; i < num_rows; i++ ) p[i] = new int[num_cols];

    // use it

    for ( i = 0 ; i < num_rows ; i++ ) delete p[i];
    delete p;
}

// use this if colums is a compile time constant
// like int arr[x][10];
void f2 ( int num_rows ) {
    int (*p)[max_cols];

    p = new int[num_rows][max_cols];

    // use it

    delete p;
}

In both these functions, you can use the allocated array as p[row][col]

**GrNxxDaY** · 08-10-2002

mm that looks very confusing. is there a simpler way? why can't i just type

Code:

int * pInt = new int[2][2];

**Salem** · 08-11-2002

> is there a simpler way?
You mean like my f2 example

int * pInt = new int[2][2];
would be
int (*pInt)[2] = new int[2][2];

> why can't i just type
Because this is for a 1D array of int
int * pInt = new int[2];
You can't make it into a 2D array simply by changing the new operator.

> that looks very confusing
Then I suggest careful study here
Pointer declarations, especially the more complex ones are one of the tricker aspects of C and C++ programming.

**GrNxxDaY** · 08-11-2002

I know the exact size of the array in the program (its going to be a 20x20 array of ROOMS for a text rpg, but i used ints for simplicity), so can I do this?..

Code:

ROOM * create()    // so i don't need this parameter?
{
    ROOM * pRoom[20];      //an array of pointers?

    pRoom = new ROOM[20][20];  // replace with numbers?

    // can i use pRoom as it were a regular pointer to an array now?
    // such as pRoom[2][3]->method();

    return pRoom;
}

the link your gave is for C, and i dont recognize most of the printfs and all that stuff..

could you explain why i needed to declare pRoom as an array of 20 ROOMS?

**Fordy** · 08-11-2002

Code:

#include <iostream>


class Room{ //Your class.....whatever.....
public:
	int member;
};


int main(void)
{
	Room** room; //pointer to a pointer to Room
	int x,y; 

	/*Set up your array*/

	room = new Room*[20];//Set first dimensions

	for(x = 0;x < 20;x++)
		room[x] = new Room[20];//Set second dimensions

	/*Do whatever with it*/

	for(x = 0;x < 20;x++)
		for(y = 0;y < 20;y++)
			room[x][y].member = x+y; //set values...blah blah

	for(x = 0;x < 20;x++)
		for(y = 0;y < 20;y++)
			std::cout << room[x][y].member << " ";//Show results

	/* Dont forget to clean up!*/

	for(x = 0;x < 20;x++)
		delete [] room[x];

	delete [] room;

  return 0;
}

**Salem** · 08-11-2002

> I know the exact size of the array in the program
So why are you even trying to allocate it when you can just say

ROOM rooms[20][20];

**GrNxxDaY** · 08-13-2002

that would declare it either globally or on the stack - which i don't want. i'd like it on the heap.

**GrNxxDaY** · 08-13-2002

Originally posted by Fordy

Code:

#include <iostream> class Room{ //Your class.....whatever..... public: int member; }; int main(void) { Room** room; //pointer to a pointer to Room int x,y; /*Set up your array*/ room = new Room*[20];//Set first dimensions for(x = 0;x < 20;x++) room[x] = new Room[20];//Set second dimensions /*Do whatever with it*/ for(x = 0;x < 20;x++) for(y = 0;y < 20;y++) room[x][y].member = x+y; //set values...blah blah for(x = 0;x < 20;x++) for(y = 0;y < 20;y++) std::cout << room[x][y].member << " ";//Show results /* Dont forget to clean up!*/ for(x = 0;x < 20;x++) delete [] room[x]; delete [] room; return 0; }

i was wondering if you could explain a bit more of how that code works fordy, i'm reluctant to use code that i don't understand

**Salem** · 08-13-2002

Then

ROOM (*rooms)[20] = new ROOM[20][20];

Is all you need to get you
rooms[0][0] to rooms[19][19]

**GrNxxDaY** · 08-13-2002

salem, are you saying i can skip that other code and just use yours & it will work?

**The Dog** · 08-13-2002

From Fordy's code above:

Code:


/*Set up your array*/
room = new Room*[20];//Set first dimensions

This is to allocate space for 20 Room pointers. (20 Room*'s)

Code:


for(x = 0;x < 20;x++)
    room[x] = new Room[20];   //Set second dimensions

Creating 20 Room objects on the heap, using the 20 pointers allocated initially.

Code:


/* Dont forget to clean up!*/
for(x = 0;x < 20;x++)
     delete [] room[x];

delete [] room;

First delete each object, then delete the initial pointer.

**GrNxxDaY** · 08-13-2002

somehow that makes no sense to me.
room holds the address to the first of 20 pointers to ROOMs, which is stored on the heap

then each of those 20 pointers is set up to hold the address of a particular ROOM which is created on the heap again (so far, only 20 ROOMS have been made, not 400)

then you delete all the pointers.

it seems this only declares a single-dimensioned array, not a 2-D array like i'm trying to do am i crazy?

**The Dog** · 08-13-2002

>>it seems this only declares a single-dimensioned array, not a 2-D array like i'm trying to do am i crazy?

You're right there!

Do you want a 2D array of rooms on the heap?
If you do, then you'll need to do something like this:

ROOM **rooms[2];

Then treat each element as in the original code.

**GrNxxDaY** · 08-13-2002

ROOM **rooms[2];

What does that do? Not sure I understand how/why pointers to pointers are used.

What original code?? Everyone has a different idea on how to do it and im not sure how each one works, so im still looking for how to do it.

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