Array of pointers to an array of fixed size

Question

I tried to assign two fixed-size arrays to an array of pointers to them, but the compiler warns me and I don\'t understand why.

int A[5][5];
int B[5][5];
int         


        

Answer 1

Either you should declare the third array like

int A[5][5];
int B[5][5];
int ( *C[] )[N][N] = { &A, &B };

that is as an array of pointers to two-dimensional arrays.

For example

#include 

#define N   5

void output( int ( *a )[N][N] )
{
    for ( size_t i = 0; i < N; i++ )
    {
        for ( size_t j = 0; j < N; j++ ) printf( "%2d ", ( *a )[i][j] );
        printf( "\n" );
    }
}

int main( void )
{
    int A[N][N] =
    {
        {  1,  2,  3,  4,  5 },
        {  6,  7,  8,  9, 10 },
        { 11, 12, 13, 14, 15 },
        { 16, 17, 18, 19, 20 },
        { 21, 22, 23, 24, 25 }
    };
    int B[N][N] =
    {
        { 25, 24, 23, 22, 21 },
        { 20, 19, 18, 17, 16 },
        { 15, 14, 13, 12, 11 },
        { 10,  9,  8,  7,  6 },
        {  5,  4,  3,  2,  1 }
    };

/*
    typedef int ( *T )[N][N];
    T C[] = { &A, &B };
*/

    int ( *C[] )[N][N] = { &A, &B };

    output( C[0] );
    printf( "\n" );

    output( C[1] );
    printf( "\n" );
}        

The program output is

 1  2  3  4  5 
 6  7  8  9 10 
11 12 13 14 15 
16 17 18 19 20 
21 22 23 24 25 

25 24 23 22 21 
20 19 18 17 16 
15 14 13 12 11 
10  9  8  7  6 
 5  4  3  2  1 

or like

int A[5][5];
int B[5][5];
int ( *C[] )[N] = { A, B };

that is as an array of pointers to the first elements of two-dimensional arrays.

For example

#include 

#define N   5

void output( int ( *a )[N] )
{
    for ( size_t i = 0; i < N; i++ )
    {
        for ( size_t j = 0; j < N; j++ ) printf( "%2d ", a[i][j] );
        printf( "\n" );
    }
}

int main( void )
{
    int A[N][N] =
    {
        {  1,  2,  3,  4,  5 },
        {  6,  7,  8,  9, 10 },
        { 11, 12, 13, 14, 15 },
        { 16, 17, 18, 19, 20 },
        { 21, 22, 23, 24, 25 }
    };
    int B[N][N] =
    {
        { 25, 24, 23, 22, 21 },
        { 20, 19, 18, 17, 16 },
        { 15, 14, 13, 12, 11 },
        { 10,  9,  8,  7,  6 },
        {  5,  4,  3,  2,  1 }
    };

/*
    typedef int ( *T )[N];
    T C[] = { A, B };
*/

    int ( *C[] )[N] = { A, B };

    output( C[0] );
    printf( "\n" );

    output( C[1] );
    printf( "\n" );
}        

The program output is the same as above

 1  2  3  4  5 
 6  7  8  9 10 
11 12 13 14 15 
16 17 18 19 20 
21 22 23 24 25 

25 24 23 22 21 
20 19 18 17 16 
15 14 13 12 11 
10  9  8  7  6 
 5  4  3  2  1 

depending on how you are going to use the third array.

Using typedefs (shown in the demonstrative program as commented) ssimplifies the arrays' definitions.

As for this declaration

int*** C = {&A, &B};

then in the left side there is declared a pointer of type int *** that is a scalar object while in the right side there is a list of initializers that have different type int ( * )[N][N].

So the compiler issues a message.