Vector bidimensional & class variables

[AnotherJoe]

Hi to all. :)

I need to create a bidimensional vector and work on it like a bidimensional array. I don't find any example of code and all the implementations of vectors I've seen are in only one dimension. But if it is one of the most powerful data structures of C++ I think I can make it in 2 dimensions.

For the first time I am at work to make a program with classes.
I've tried to declare an int variable in one class and initialize it in the costructor of the class, but compiler give a linking error:
[Linker error] undefined reference to `number'

I can't set the value of the variable in any part of the program, neither in the constructor, neither in standard metods, neither in the main. :(

I really don't know where is the error:

class snake{
      public:
             snake(); 
             int stampaSnake(int righe); 
             static const int COLONNE = 4;
             static int number; 
      };

snake::snake()
{ 
int a =0;
int righe = 4;
number=4; //error
}

if I remove the row "number=4;" the program works correctly. But I need this variable. :(

Thanks for reading!! :)

PS. I'm using Dev-C++ 4.9.9.2

Thanks again.

[Duthomhas]

It is because you did not actually create the variable (which you must do) -- you only declared it as existing with the static keyword. On line 9, add:

int snake::number;

Then you can compile it.

BTW, you should make sure you understand what it is that you are doing by declaring class data members as "static".

Hope this helps.

[AnotherJoe]

Thanks so much, now it works. :D

Static is because at the beginning of program, if I remember correctly, I have written this code:

class snake{
      public:
             snake(); 
             int stampaSnake(int righe); 
             static const int COLONNE = 4;
             int number; 
      };


int main {

snake game;
game.number=3;
cout<<game.number;

return 0;
}

So, the compiler gave me an error like it couldn't compile because number wasn't a static member. I have declared it static and maintained this but, as you know, new errors are incurred :(.

I also read that a static member is visible only in the area where it has been declared. I need it only in the methods of the class, so I think I can mantain it as static (the print in the main has been just a test). :)

No suggestions about bidimensional vector? :(

Thanks :D

[Duthomhas]

For simplicity, just use a vector of vectors. (Sorry to have forgotten about your Q.)

Often, it is worth making your own class to do it. Here is the beginning of a simple "matrix" class you can play with:

#include <iostream>
#include <vector>
using namespace std;

//----------------------------------------------------------------------------
// This is the 'matrix' class.
//
template <typename Number>
struct matrix: vector <vector <Number> >
  {
  matrix( unsigned m, unsigned n, const Number& value = Number() )
    {
    resize( m, vector <Number> ( n, value ) );
    }
  unsigned rows()    const { return this->        size(); }
  unsigned columns() const { return this->front().size(); }
  };

//----------------------------------------------------------------------------
// This is the output operator for the 'matrix' class. Yes, I know it is pretty simple...
//
template <typename Number>
ostream& operator << ( ostream& outs, const matrix <Number> & m )
  {
  for (  unsigned row = 0; row < m.rows();    row++)
    {
    for (unsigned col = 0; col < m.columns(); col++)
      outs << m[ row ][ col ] << " ";
    outs << "\n";
    }
  return outs;
  }

//----------------------------------------------------------------------------
// A quick example of use
//
int main()
  {
  int rows, cols, value;

  cout << "number of rows> ";    cin >> rows;
  cout << "number of columns> "; cin >> cols;
  cout << "initial value> ";     cin >> value;

  matrix <int> M( rows, cols, value );

  cout << "Initial matrix value:\n" << M << endl;

  return 0;
  }

Currently, there is no protection against inappropriate modifications to the matrix -- you can add that kind of stuff if it is needed. Also, you'll notice there is no input for it. All these things are an exercise left to you ;-)

Hope this helps.