Multi-Demensional Arrays on Heap

This is a discussion on Multi-Demensional Arrays on Heap within the C++ Programming forums, part of the General Programming Boards category; are multi-demensional arrays on the heap impossible? try this out on your compiler. get an error dev 4.9.4.1 Code: int ...

  1. #1
    Registered User GrNxxDaY's Avatar
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    Lightbulb Multi-Demensional Arrays on Heap

    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
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  2. #2
    and the hat of int overfl Salem's Avatar
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    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]

  3. #3
    Registered User GrNxxDaY's Avatar
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    mm that looks very confusing. is there a simpler way? why can't i just type
    Code:
    int * pInt = new int[2][2];
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  4. #4
    and the hat of int overfl Salem's Avatar
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    > 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.

  5. #5
    Registered User GrNxxDaY's Avatar
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    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?
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  6. #6
    &TH of undefined behavior Fordy's Avatar
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    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;
    }
    Last edited by Fordy; 08-11-2002 at 04:05 AM.

  7. #7
    and the hat of int overfl Salem's Avatar
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    > 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];

  8. #8
    Registered User GrNxxDaY's Avatar
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    that would declare it either globally or on the stack - which i don't want. i'd like it on the heap.
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  9. #9
    Registered User GrNxxDaY's Avatar
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    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
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  10. #10
    and the hat of int overfl Salem's Avatar
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    Then

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

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

  11. #11
    Registered User GrNxxDaY's Avatar
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    salem, are you saying i can skip that other code and just use yours & it will work?
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  12. #12
    Registered User The Dog's Avatar
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    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.

  13. #13
    Registered User GrNxxDaY's Avatar
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    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?
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  14. #14
    Registered User The Dog's Avatar
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    >>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.

  15. #15
    Registered User GrNxxDaY's Avatar
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    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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