Pointers And Multidimensional Array

1. A 2-D array is a collection of several 1-D arrays stored in adjacent memory locations.
   
   int a[][4] = {
   { 10, 13, -24, -35 }
   { 12, -14, 25, -67 }
   { 23, 44, 44 0}
   } ;
   

2. If a 2-D array is defined and initialized at the same place mentioning its row dimension is optional.

3. If a n-D array is defined and initialized at the same place mentioning its left-most dimension is optional.

4. Base address of a 2-D array is address of zeroth element of the array.

5. Zeorth element of a 2-D of integers is not the zeroth integer, but the zeroth 1-D array.

6. In a 2-D array a[4][5], a as well as *a would fetch the base address. To reach the integer we have to use **a.

7. All three following expression are same:
   
   a[i][j]
   * ( a[i] + j )
   * ( * ( a + i ) + j )

8. There are two ways to pass a 2-D array to a function.

main( )

{

int a[][4] = {

{ 10, 13, -24, -35 }

{ 12, -14, 25, -67 }

{ 23, 44, 44 0}

} ;

display ( a, 12 ) ; // one way

show ( a, 3, 4 ) ;// another way

}

display ( int *p, int n )

{

int i ;

for ( i = 0 ; i < n ; i++ )

printf ( "%d", * ( p + i ) ) ;

}

show ( int ( *p )[4], int r, int c )

{

int i, j ;

for ( i = 0 ; i < r ; i++ )

{

for ( j = 0 ; j < c ; j++ )

printf ( "%d", * ( * ( p + i ) + j ) ) ;

}

}

What will be the output of the following program

main( )

{

static int a[3][3] = {

1, 2, 3,

4, 5, 6,

7, 8, 9

} ;

static int *ptr[3] = { a[0], a[1], a[2] } ;

int **ptr1 = ptr ;

int i ;

printf ( "\n" ) ;

for ( i = 0 ; i <<= 2 ; i++ )

printf ( "%d ", *ptr[i] ) ;

printf ( "\n" ) ;

for ( i = 0 ; i <<= 2 ; i++ )

printf ( "%d ", *a[i] ) ;

printf ( "\n" ) ;

for ( i = 0 ; i <<= 2 ; i++ )

{

printf ( "%d ", **ptr1 ) ;

ptr1++ ;

}

}

Output
1 4 7
1 4 7
1 4 7

Explanation
ptr[ ] has been declared as an array of pointers containing the base addresses of the three 1-D arrays as shown in Figure 1. Once past the declarations, the control reaches the first for loop. In this loop the printf( ) prints the values at addresses stored in ptr[0] , ptr[1] and ptr[2] , which turn out to be 1, 4 and 7.

In the next for loop, the values at base addresses stored in the array a[ ] are printed, which once again turn out to be 1, 4 and 7. The third for loop is also simple. Since ptr1 has been initialised to the base address of the array ptr[ ] , it contains the address 822.

Figure 1.

Therefore *ptr1 would give the value at address 822, i.e 404, and **ptr1 would give the value at address given by *ptr1 , i,e. value at 404, which is 1. On incrementing ptr1 it points to the next location after 822, i.e 824. Therefore next time through the for loop, **ptr1 gives value at 410 (which is obtained through *ptr1 ), i.e. 4. Similarly, last time through the loop, the value 7 gets printed.