Thread: Flattening the array

Hi guys,
What do you think about "flattening the array"? Is this legal? For example:

int matrix[6][10];
int *mp;

.....(some code goes here)


mp = &matrix[3][8]; /* so mp points to 3rd row and 8th column */
printf( "First element is %d\n",*mp);/* we are at [3][8]*/
printf( "Second element is %d\n",*++mp);/* next one [3][9]*/
printf( "Third element is %d\n",*++mp);/* [4][0] */

Just read that this technique ia illegal and should be avoided.I don' t quite understand why.After all...aren't multi-dimensional array are really single-dimensional arrays?

What exactly are you flattening?

Looks to me like you are just referring to elements in the array with a 2nd pointer. Since the address and size of the array matrix is constant, then the array and all of its elements are safely bound to storage. Therefore I don't see anything wrong with this "flattening" at all.

They probably are referring to dynamic arrays or something. Then there would be a danger of pointing your second pointer to freed memory and trying to dereference it(bad bad bad).

Oh and yes, multi-dim arrays are really just one-dim arrays in mem.

Multi dimensional arrays are _not_ just flat arrays in memory. In fact, it depends on the compiler, just exactly how they are assembled. That's why it's considered _illegal_, or atleast frowned upon.

Multi-dimensional arrays are usually an array of pointers to the secondary dimension. More dimensions, more pointer layers.

You think you have it bad? We had to write a list flattening function in Lisp!!!