dev-cpp-users

[Dev-C++] run time errors with big arrays

[risto h. <rh...@on...>]

Initialising arrays of large size seems to be a problem - the programs are 
compiled without warnings, but if the array (or multidimensional array) 
exceeds some specific size, the program crashes. Has anyone of you had the 
same problem, and is there anything to fix that?
A related question that I have not found answered in my programming textbooks: 
How is a multidimensional array dynamically initialised? In other words, this 
works:
  double *ar;
  ar = new double[10];
but what is the correct syntax for initialising for example ar[10][10] with 
new?

Risto Holopainen

Re: [Dev-C++] run time errors with big arrays

[Nicolas R. <ni...@bo...>]

> How is a multidimensional array dynamically initialised? In other=20
> words [...]
> what is the correct syntax for initialising for example ar[10][10]=20
> with new?

That syntax will work with static variables, i.e.

    int arr[10][10];

But it won't work at all with new. You will need to declare the array=20
as if it were one-dimensional, i.e.

    int* arr =3D new int[num_rows * num_cols];

and then do the math "by hand" when accessing elements, e.g.

    arr[x * num_cols + y] =3D foo;

I can already hear you screaming "but that totally sucks! the language=20=

should provide a better way to do it!", but before you do so, please=20
remember that arrays are and have always been very low-level in C and=20
C++. If you want nicer syntax, all you have to do is write a Matrix=20
class to encapsulate the array, or even better, get one of the several=20=

excellent ones that already exist out there.

For a more thorough discussion of this issue, check out the C++ FAQ=20
Lite:

     http://www.parashift.com/c++-faq-lite/freestore-mgmt.html

paragraphs [16.15] and [16.16].

By the way, is googling for 'multidimensional arrays C++' that hard? :-)

Regards,
Nicol=E1s

Re: [Dev-C++] run time errors with big arrays

[Per W. <pw...@ia...>]

Two alternatives for dynamic allocation of multidimensional arrays.

int main() {
        typedef int Matrix10[10][10];

        int **vec;
        Matrix10 *vec2;
        int i,j;

        vec = new int*[10];
        for (i = 0; i < 10; i++) {
                vec[i] = new int[10];
                for (j = 0; j < 10; j++)
                        vec[i][j] = 0;
        }
        vec2 = new Matrix10[1];
        for (i = 0; i < 10; i++)
                for (j = 0; j < 10; j++)
                        (*vec2)[i][j] = 0;
        return 0;
}

/Per W

On Thu, 26 Feb 2004, Nicolas Rosner wrote:

> > How is a multidimensional array dynamically initialised? In other
> > words [...]
> > what is the correct syntax for initialising for example ar[10][10]
> > with new?
>
> That syntax will work with static variables, i.e.
>
>     int arr[10][10];
>
> But it won't work at all with new. You will need to declare the array
> as if it were one-dimensional, i.e.
>
>     int* arr = new int[num_rows * num_cols];
>
> and then do the math "by hand" when accessing elements, e.g.
>
>     arr[x * num_cols + y] = foo;
>
> I can already hear you screaming "but that totally sucks! the language
> should provide a better way to do it!", but before you do so, please
> remember that arrays are and have always been very low-level in C and
> C++. If you want nicer syntax, all you have to do is write a Matrix
> class to encapsulate the array, or even better, get one of the several
> excellent ones that already exist out there.
>
> For a more thorough discussion of this issue, check out the C++ FAQ
> Lite:
>
>      http://www.parashift.com/c++-faq-lite/freestore-mg