Printing 2D Arrays with Pointers

[Super KHB]

I know that I am close, but it wont work. HELP!!!!

void PrintA( int * , int);

void main()
{

int num[3][5] = {
{1, 2,3,4,5},
{6,7,8,9,10},
{11,12,13,14,15 },
};
int *num_ptr;
num_ptr = &num[0][0];

PrintA( num_ptr, 5 );
}

void PrintA( int *a, int cols )
{
for (int r = 0; r < 3 ; r++)
{for ( int c = 0; c < 5; c++ )
cout << *(*(a + r) + c) << " ";
cout << endl;
}
}

[Siemel Naran]

Re: Printing 2D Arrays with Pointers

"Super KHB" <look@message.e nd> wrote in message news:Jx0gd.4908 3
[color=blue]
> void PrintA( int *a, int cols )
> {
> for (int r = 0; r < 3 ; r++)
> {for ( int c = 0; c < 5; c++ )
> cout << *(*(a + r) + c) << " ";
> cout << endl;
> }
> }[/color]

Should the line
[color=blue]
> cout << *(*(a + r) + c) << " ";[/color]

be

cout << *(*(a + 5*r) + c) << " ";


Also, instead of the hardcoded constant "5", do you mean to use the variable
"cols"?

[Karl Heinz Buchegger]

Re: Printing 2D Arrays with Pointers

Super KHB wrote:[color=blue]
>
> I know that I am close, but it wont work. HELP!!!!
>
> void PrintA( int * , int);
>
> void main()
> {
>
> int num[3][5] = {
> {1, 2,3,4,5},
> {6,7,8,9,10},
> {11,12,13,14,15 },
> };[/color]

Here you have a memory layout looking like this:

num
+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
+----+----+----+----+----+----+----+----+----+----+----+----+----+----+----+

and the compiler keeps track of calulating the correct offset from the beginning
when given 2 indices
[color=blue]
> int *num_ptr;
> num_ptr = &num[0][0];
>
> PrintA( num_ptr, 5 );
> }
>
> void PrintA( int *a, int cols )
> {
> for (int r = 0; r < 3 ; r++)
> {for ( int c = 0; c < 5; c++ )
> cout << *(*(a + r) + c) << " ";[/color]

Here you assume a memory layout like this:

num
+------+ +----+----+----+----+----+
| o--------->| 1 | 2 | 3 | 4 | 5 |
| | +----+----+----+----+----+
+------+ +----+----+----+----+----+
| o------------------>| 6 | 7 | 8 | 9 | 10 |
| | +----+----+----+----+----+
+------+ +----+----+----+----+----+
| o------------>| 11 | 12 | 13 | 14 | 15 |
| | +----+----+----+----+----+
+------+

( an array of pointers, where each pointer points to an array of int )

Totally different.



--
Karl Heinz Buchegger
kbuchegg@gascad .at

[Venki]

Re: Printing 2D Arrays with Pointers

> cout << *(*(a + r) + c) << " ";

try this cout << *(a+((cols * r) + c))<< " ";

[Old Wolf]

Re: Printing 2D Arrays with Pointers

"Siemel Naran" <SiemelNaran@RE MOVE.att.net> wrote:[color=blue]
> "Super KHB" <look@message.e nd> wrote:
>[color=green]
> > void PrintA( int *a, int cols )
> > {
> > for (int r = 0; r < 3 ; r++)
> > {for ( int c = 0; c < 5; c++ )
> > cout << *(*(a + r) + c) << " ";
> > cout << endl;
> > }
> > }[/color]
>
> Should the line[color=green]
> > cout << *(*(a + r) + c) << " ";[/color]
> be
> cout << *(*(a + 5*r) + c) << " ";[/color]

We are dealing with a pointer to int so there should be
exactly one indirection. I would write it as:
a[r * cols + c]

(NB. Actually I wouldn't write this because I think
it is UB. However we have just had a big discussion
with no conclusion on that topic in another thread
so I don't want to start it up again here).