multidimensional array and malloc

This is a discussion on multidimensional array and malloc within the C Programming forums, part of the General Programming Boards category; Im trying to use malloc to allocate memory for a multi dimensional array of ints. After this code has finished ...

  1. #1
    zsaniK Kinasz's Avatar
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    Question multidimensional array and malloc

    Im trying to use malloc to allocate memory for a multi dimensional array of ints. After this code has finished i want to be able to address it with plane[x][y]. Does any one know what i'm doing wrong here?

    Code:
    	
                         // Declare variables 	
                         int plane_width = 10, plane_height = 10, x, y; 	
                         int * plane;	 	 	
                         
                         // Allocate memory for first dimension of  plane 	
                         if ( ( ( plane = malloc( sizeof( int * ) * plane_width ) ) == NULL ) ) { 
                            fprintf( stderr, "No memory available\n" );
                            exit( EXIT_FAILURE ); 
               	     }	 	
                         // Allocate memory for second dimension of plane
                         for ( x = 0; x < plane_width; x++ ) {
                 		if ( ( ( plane[x] = ( int * ) malloc( sizeof( int ) * plane_height ) ) == NULL ) ) { 
    			      fprintf( stderr, "No memory available\n" );
           			      exit( EXIT_FAILURE ); 		
                           } 		
                            // Set tile to default value
                            for ( y = 0; y < plane_height; y++ ) { 	
               		  plane[x][y] = TILE_UNINITIALISED; 		
                            } 
    	           }
    "Assumptions are the mother of all **** ups!"

  2. #2
    and the hat of wrongness Salem's Avatar
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    Start with
    int **plane;

    Lose the malloc casts - they're hiding your real problem.

    The general form is T = malloc ( sizeof(*T) * num ), so in your case
    plane = malloc( sizeof( *plane ) * plane_width )
    plane[x] = malloc( sizeof( *plane[x] ) * plane_height )
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
    zsaniK Kinasz's Avatar
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    thanx heaps salem!
    "Assumptions are the mother of all **** ups!"

  4. #4
    Registered User
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    Just on a side note, there's another way of doing the same thing:

    There's been a few times when I've used a library which required the use of the flat[x+w*y] form but where I've wanted the convenience of using two indices like arr2d[y][x].

    Code:
    int * flat;
    int ** arr2d;
    unsigned int i;
    
    flat=malloc(sizeof *flat * w * h);
    if (!flat) {
      /* handle the error */
    }
    
    arr2d=malloc(sizeof *arr2d * h);
    if (!arr2d) {
      /* handle the error */
    }
    
    for (i=0;i<h;i++) {
      arr2d[i]=flat+i*w;
    }
    You can index the flat array using:

    flat[x+w*y]

    but also can use:

    arr2d[y][x]

    to refer to the same element. (Note that the dimensions are reversed.)

    Ian Woods

  5. #5
    Super Moderator VirtualAce's Avatar
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    And if you make the array dimensions a power of 2, then you can access the linear array as a 2D array by using a simple shift.

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