A little problem in handling 2-D arrays with class

Question

class linklist4x4
{
 private:

  struct node4x4
  {
    double data[4][4];
    node4x4 *link;
  }*p;

 public:

 linklist4x4();
        void append( double* num );
        void add_as_first( double* num );
        void addafter( int c, double* num );
        //void del( double* num );
        void display();
        int count();
 double* getdata(int c);
        ~linklist4x4();
};

The above is my class declaration.

Now below is a function declaration.

void linklist4x4::append(double* num)
{
 node4x4 *q,*t;

 if( p == NULL )
 {
  p = new node4x4;
  for(int i=0;i<4;i++)
   for(int j=0;j<4;j++)
    p->data[i][j]=num[i][j];//error line
  p->link = NULL;
 }
 else
 {
  q = p;
  while( q->link != NULL )
                        q = q->link;

  t = new node4x4;
  for(int i=0;i<4;i++)
   for(int j=0;j<4;j++)
    t->data[i][j]=num[i][j];//error line
  t->link = NULL;
  q->link = t;
 }
}

When i try to compile, it gives me an error saying that "error C2109: subscript requires array or pointer type" at the marked commented lines as shown.

Can anyone please help?

Answer 1

The num passed in is a single double pointer, but you are trying to index it with two subscripts. If you are really passing in a 4x4 double array, change the function signature to indicate that it is such.

Answer 2

You're indexing num as if it were a 2D array or double pointer, while it is only one level deep.

double* num

num[i][j]

Answer 3

You have defined the method append to take a 1-D array of type double. But in the function you are making use of it as a 2-D array.

The data in each of your link list node is a 2-D array of double.

So your all your member functions should accept a 2-D array of double as argument.

Change:

void append( double* num );
void add_as_first( double* num );
void addafter( int c, double* num );

to

void append( double num[][4] );
void add_as_first( double num[][4] );
void addafter( int c, double num[][4] );

Answer 4

"num" has a single indirection so you can't dereference it twice. Declare is as

double (*num)[4] 

if you want it to accept a 2D matrix so that you can apply [] to it twice.

Answer 5

You can also use num[i*4+j] for accessing the data you require.