Stupid question about pointers (it is driving me crazy). Pls help!!!

[Eusebio Salgado]

Ok, let's imagine an stupid program working with matrix.


{
...
int matriz[N][N];
...
int **puntero2;
...
}

as defined above matriz is a matrix of NxN elements, an array of N
pointers to N arrays of N elements of type int, isn't it?
So "matriz" itself is a pointer to pointer to int, isn't it?
So I can do

puntero2=matriz;

Why is compiler giving me this warning?
prueba_con_matrices.c:21: warning: assignment from incompatible pointer type

Problem is that I have a main like:

int main (void)
{
...
int matriz[10][10];
...


lets_give_a_value_to_matriz(10,matriz,5)
}

void lets_give_a_value_to_matriz(int n, int **p,int value)
{
int i,j;
for (i=0;i<n;i++)
for (j=0;j<n,j++)
p[i][j]=value;
}

This gives a warning like this:
prueba_con_matrices.c:40: warning: passing arg 2 of `matriz_a_valor'
from incompatible pointer type

and I obtain an "Segmentation fault" if I try to execute.

I have tried same with vectors. It is:

int vector[10];
int *pointer;

And it works fine. So I can't undestarnd what the problem is...

Please, any suggestion is wellcomed.
BR,

Eu

Pd.- I'm using a linux box with
gcc version 3.2.2 (Mandrake Linux 9.1 3.2.2-3mdk)

[Jens.Toerring@physik.fu-berlin.de]

Eusebio Salgado <eu_soy_yo@terra.es> wrote:

> Ok, let's imagine an stupid program working with matrix.

> {
> ...
> int matriz[N][N];
> ...
> int **puntero2;
> ...
> }

> as defined above matriz is a matrix of NxN elements, an array of N
> pointers to N arrays of N elements of type int, isn't it?
> So "matriz" itself is a pointer to pointer to int, isn't it?

No, it isn't. It is a two-dimensional array, no pointers involved.

> So I can do

> puntero2=matriz;

> Why is compiler giving me this warning?
> prueba_con_matrices.c:21: warning: assignment from incompatible pointer type

You can't because matriz without the '[]' stuff is treated as a
simple pointer to int.

In memory the elements of matriz are stored as a flat array, first all
the elements of the first column, then the elements of the second etc.
And matriz without the '[]' is a pointer to the first element of the
first row.

By defining matriz as matriz[10][10] you tell the compiler how it can
calculate where a certain element is. If, for example, you have
matriz[7][3] the compiler uses this information to convert this to

matriz[7][3] -> *( matriz + 10 * 7 + 3 )

But if you pass matriz as a pointer to a function this information
gets lost and you have to do the above conversion all by yourself.

> Problem is that I have a main like:

> int main (void)
> {
> ...
> int matriz[10][10];
> ...
>

> lets_give_a_value_to_matriz(10,matriz,5)
> }

> void lets_give_a_value_to_matriz(int n, int **p,int value)

You need here

void lets_give_a_value_to_matriz( int n, int *p, int value )

> {
> int i,j;
> for (i=0;i<n;i++)
> for (j=0;j<n,j++)
> p[i][j]=value;

This can't work because the compiler has no way of figuring out that
the pointer you passed to the function was originally a matrix. So
you must do the calculation of the memory location you want to change
by yourself:

*( p + n* i + j ) = value;

> }

Since the elements of the matrix are stored as a flat array, you
can even simplify the function to

void lets_give_a_value_to_matriz( int n, int *p, int value )
{
int i;

for ( i = 0; i < n * n; i++ )
p[ i ] = value;
}
Regards, Jens
--
_ _____ _____
| ||_ _||_ _| [email]Jens.Toerring@physik.fu-berlin.de[/email]
_ | | | | | |
| |_| | | | | | [url]http://www.physik.fu-berlin.de/~toerring[/url]
\___/ens|_|homs|_|oerring

[Jens.Toerring@physik.fu-berlin.de]

[email]Jens.Toerring@physik.fu-berlin.de[/email] wrote:

> In memory the elements of matriz are stored as a flat array, first all
> the elements of the first column, then the elements of the second etc.
> And matriz without the '[]' is a pointer to the first element of the
> first row.

Sorry, that was meant to be: first all the elements of the first *row*,
then the elements of the second *row* etc. (it's FORTRAN where the
elements are stored in a column after column fashion).

Regards, Jens
--
_ _____ _____
| ||_ _||_ _| [email]Jens.Toerring@physik.fu-berlin.de[/email]
_ | | | | | |
| |_| | | | | | [url]http://www.physik.fu-berlin.de/~toerring[/url]
\___/ens|_|homs|_|oerring