Thread: Matrix on stack to int**... How?

Originally Posted by Dave Evans

TO make the parameter a pointer to an array[10][20] of ints, it should be:

Code:

void func(int (*arr2d)[10][20]);

Of course, functions defined like this will only work for that particular size array (which, I think, will not help whatever it is that the Original Poster is trying to accomplish).

D.

That the same (essentially) as this:

Code:

void func(int arr2d[][10][20]);

Passing an 2d array by reference like that makes sense, because if you have a required 2nd detention, chances are you have a required first detention. That syntax provides a way to force that requirement.

Also, you can alternatively make it a template like this. Beware that this will not allow heap arrays to be passed as arguments:

Code:

template <typename T, int I> 
void func(T (&array)[I]);

Note: that I in brackets should be capital. For some reason the forum prevents that.

Originally Posted by laserlight

I think that your example offers no advantage over King Mir's example

It isn't my example; I was just correcting what I thought was a (typographical) syntax error from the previous post.

[edit]I was mistaken about the intent of the previous post. I regret my error.[/edit]

In fact, in my experience, functions that deal with 2-D matrices almost always use pointers or pointers-to-pointers except for very specialized applications that have fixed-size matrices.

Even fixed-size matrix functions (3x3 graphics manipulations, for example) might be more efficient with pointers, since element access is just a couple of indirect address operations instead of using the 2-D matrix calculations involving an addition and a multiplication.

D.

Originally Posted by Dave Evans

It isn't my example; I was just correcting the (typographical) syntax error from the previous post.

In fact, in my experience, functions that deal with 2-D matrices almost always use pointers or pointers-to-pointers except for very specialized applications that have fixed-size matrices.

Even fixed-size matrix functions (3x3 graphics manipulations, for example) might be more efficient with pointers, since element access is just a couple of indirect address operations instead of using the 2-D matrix calculations involving an addition and a multiplication.

D.

It may be worthwhile to pass a 2d stack by creating pointers to each row, and passing that as an array. that would mean doing something like this:

Code:

void func(int **array); //same as int *array[] 
...
int main(){
    int array2d[3][10];
    int array2dasPtr={array2d[0],array2d[1],array2d[2]}
    func(array2dasPtr);
}

But that is not either of our codes were doing.
To use your prototype you must do this:

Code:

void func(int (*arr2d)[3][10]);
...
int main(){
    int array2d[3][10];
    int array3d[5][3][10]
    func(&array2d);
    func(array3d);
}

To use my prototype you must do this:

Code:

void func(int (&arr2d)[3][10]);
...
int main(){
    int array2d[3][10];
    int array3d[5][3][10]
    func(array2d);
    func(array3d[0]);
}