🚀 go-pugleaf

Terry Heath wrote:
>I have seen many references in text  I have read about multiple
>indirection (i.e. **p); I seen how to initilize them and so on.  The
>question is:

>  Why would it be necessary to use such a pointer, and when?


Consider a case where you need to create a 2 dimensional array whose size
in each of the 2 dimensions is not known at compile time.  Examples are
matrix math functions or arrays of strings.

With double indirection, you can make "anytype **mainPointer" a pointer to
an array of pointers to rows of data, fill each of the pointers to rows of
data with a pointer to a row of data and access the data with bracket
operators as though it were a standard array.

Example:

char **forceField;

void DoLittle( void )
{
    AnyFunction( 10, 20 );
}

void AnyFunction( int rows, int columns )
{
    int allocLoop;

    forceField= (char **)malloc( sizeof( char * ) * rows );
    /* insert check to see you got !NULL, deal with failures */

    for( allocLoop= 0; allocLoop< rows; allocLoop++ )
    {
        forceField[ allocLoop ]= (char *)malloc( sizeof( char ) * columns );
        /* insert check to see you got !NULL, deal with failures */
    }        

    /* now you can do the following */

    forceField[ 5 ][ 6 ]= 1;
    forceField[ 0 ][ 0 ]++;

    /* clean up */
    for( allocLoop= 0; allocLoop< rows; allocLoop++ )
    {
        free( forceField[ allocLoop ] );
    }        
    free( forceField );
}



---
Brian Valters McGroarty -- brianmcg@bix.com
phone/fax (847) 439-7714