multidimensional array and malloc

[Kinasz]

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?

	
                     // 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; 		
                        } 
	           }

[Salem]

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 )

[Kinasz]

thanx heaps salem!

[hairyian]

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].

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

[VirtualAce]

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.