Overloading Operators

[oonej]

Hey guys,

New to the forums here. I'm a student taking C++ and need some help with an overloaded method. I've tried looking up everything but confused. Maybe someone here can shed some light on my problem

I have a class that dynamically allocates a 2d array. This array is like a matrix holding doubles. The problem i'm having is with the header file and cpp file associated with the class.

I need to overload the method * and multiply the 2d array. Here is a few parts of it:

My Header file::

 
class Matrix
{
 public:
   Matrix(); //Default Constructor
   //cut out some of my functions that aren't necessary for this problem
   Matrix operator*(const Matrix& a1, const double vector);
 private:     
   void initialize(); //initializes the matrix to 0;
   double **arrayMatrix;
   int row;
   int col;
};

My CPP code::

   
Matrix Matrix::operator*(const Matrix& a1, const double vector)
   {
       for(int i = 0; i < row; i++){
           for(int j = 0; j < col; j++){
               arrayMatrix[i][j] = arrayMatrix[i][j] * vector;  //multiply the matrix by vector         
           }
       }         
    }

I'm getting the following errors:
16 Matrix.h `void Matrix::operator*(const Matrix&, double)' must take either zero or one argument
102 Matrix.cpp `void Matrix::operator*(const Matrix&, double)' must take either zero or one argument

[therockon7throw]

Hi,

Instead of overloading * in your way
Matrix operator*(const Matrix& a1, const double vector);

you need to define it following way

Matrix operator*(const double vector)

each overload function of type( +, - , * , / , or == and so on ) has the current object this as it first parameter.

And the function must return an object of type Matrix, in most case a temporary object which holds the result

so your code should be like following

class Matrix
{
 public:
   Matrix(); //Default Constructor
    Matrix(int rowSize, int colSize); 
   //cut out some of my functions that aren't necessary for this problem
   Matrix operator*( const double vector);
 private:     
   void initialize(); //initializes the matrix to 0;
   double **arrayMatrix;
   int row;
   int col;
};

and CPP file

Matrix Matrix::operator*(const double vector)
   {
       Matrix tmp( row, col ) ;
       for(int i = 0; i < row; i++){
           for(int j = 0; j < col; j++){
               tmp.arrayMatrix[i][j] = arrayMatrix[i][j] * vector;  //multiply the matrix by vector         
           }
       }
   return tmp;        
    }

You should also implments copy-constructor Matrix(const Matrix& other ) , assigment operator = operator and destructor ~Matrix()

Hope it helps

[oonej]

I have a destructor , haven't gotten to the assignment operator yet or copy-constructor..

I ran into some problems with your code.

[Linker error] undefined reference to `operator*(Matrix const&, double)'

Never mind on the error, forgot i had to recompile all o.0

[oonej]

Actually getting a crash when running your code... here is the use of those functions:

int main(int argc, char *argv[])
{
    Matrix t1(5, 5);

    double count = 1;

    //put some numbers in the matrix
    for(int i = 0; i < 5; i++){
      for(int j = 0; j < 5; j++){     
         t1.set_element(i, j, count);
         count++;

      }
   }
       
    t1 = t1*3;
    t1.printMatrix();
    
    system("PAUSE");
    return EXIT_SUCCESS;
}

[oonej]

Got it ;)

figured i needed the assignment operator which i just created to make this work!

Thanks a ton!

[therockon7throw]

If your class has any dynamic members, let say it here **arrayMatrix, so you have to implement assigment operator, copy constructor and destructor, otherwise it wont work.

a possible assigment operator

Matrix& operator=( const Matrix& other ){
    if( this == &other )
		return *this; 
   if( row != other.getRow() || col != other.getCol() ){
            clean() ; //  it is the destructor, implement it to destroy the current object
             row = other.getRow();// you should implement getters
             col   = other.getCol(); //
            arrayMatrix= new double*[  row ];
	for(int i = 0 ; i < row ; i++){
		arrayMatrix[i] = new double[ col];
    }
     // here a copy values
      for(int i = 0; i < row; i++)
           for(int j = 0; j < col; j++)
          arrayMatrix[i][j] = other[i][j]; // [] operator should be implemented as well
    return *this;
}

And about the error change the method param to double vector, it should work than

Matrix operator*(double vector)

and it should be this way, the safer way
t1 = t1* (double )3;

[oonej]

Thanks, question on making an operator multiplication of another class member
header file:
Matrix operator*(const Matrix test);

This is to multiply 2 matrices
cpp:

    Matrix Matrix::operator*(const Matrix test)
{
    Matrix tmp(row, test.num_cols());
     //some code here
    return tmp;
}
 

[therockon7throw]

are you asking a question or giving the answer ? excuse me I do not understand you.

[oonej]

The above code doesn't work, trying to figure out why

I'm trying to computer

matrix = matrix1 * matrix2
I have the logic for the computation, just getting syntax errors with the operator overloading

Error messages:
[Linker error] undefined reference to `Matrix::Matrix(Matrix const&)'

[therockon7throw]

Use reference

header file:

Matrix operator*(const Matrix& test);

CPP:

Matrix Matrix::operator*(const Matrix& test){

// do some thing 

}

by the way which compiler and which IDE do you use ?

[oonej]

using Dev C++ , not sure which compiler it uses

but as for the code you supplied, its not working for me :(

[therockon7throw]

some compiler behaves differen, so what I see from the error, your compiler wants you to define the function as following


header file:

Matrix operator*( Matrix const& test);

instead of

Matrix operator*(const Matrix& test);

and in cpp:

Matrix Matrix:: operator*( Matrix const& test){
// do some thing
}

instead of your current implementation

[oonej]

Thanks :)

[oonej]

Ok last and final need for some help...

I'm just not grasping this overloading :(

.h file

 public:
   Matrix(); //Default Constructor
   Matrix(int row, int col); //Constructor for 2 variables (rows and cols)
   ostream & operator<<(ostream &);
};

.cpp

   ostream& Matrix::operator<<(ostream & output){
    for(int i = 0; i < test.num_rows(); i++){
           for(int j = 0; j < test.num_cols(); j++){
                   output << test.mydata[i][j] << " " ;           
           }
    output << "\n";
    return output;   
   }

errors i'm getting :
19 Matrix.h ISO C++ forbids declaration of `ostream' with no type
19Matrix.h expected `;' before '&' token

[therockon7throw]

Hi again,

declare your ostream & operator<<(ostream &); not as a member function but as normal function in your header just under your class Matrix as following

ostream& operator<<(ostream& out, const Matrix& test){
        //do some thing
}

or your compiler might wish some thing else :-) like following

ostream& operator<<(ostream& out, Matrix const& test){
        //do some thing
}

try it yourself

[oonej]

doing the above i got :

30 Matrix.h expected constructor, destructor, or type conversion before '&' token
30 Matrix.h expected `,' or `;' before '&' token

[therockon7throw]

where did you declare and implement the function? It must not be inside the class, but outside of the class

class Matrix{

   //do some thing
};


ostream& operator<<(ostream& out, const Matrix& test){
        //do some thing
}


// or depending on your compiler



ostream& operator<<(ostream& out, Matrix const& test){
        //do some thing
}

[oonej]

class Matrix
{
 public:
   Matrix(); //Default Constructor
   Matrix(int row, int col); //Constructor for 2 variables (rows and cols)
   //...more functions ...
 private:    
   //...some private functions ...
};

ostream& operator<<(ostream & output, const Matrix & test){
    for(int i = 0; i < test.num_rows(); i++){
         for(int j = 0; j < test.num_cols(); j++){
                   output << test.mydata[i][j] << " " ;           
           }
    output << "\n";
    return output;   
   }

errors:
30 Matrix.h expected constructor, destructor, or type conversion before '&' token
30 Matrix.h expected `,' or `;' before '&' token

line 30 points to

ostream& operator<<(ostream & output, const Matrix & test){ //line 30
    for(int i = 0; i < test.num_rows(); i++){
         for(int j = 0; j < test.num_cols(); j++){
                   output << test.mydata[i][j] << " " ;           
           }
    output << "\n";
    return output;   
   }

[therockon7throw]

your class must implement the const version of num_rows , num_rows and operator []

like following

class Matrix{

public:

   int num_rows(){
   //do some thing
   }
   int num_rows() const{
   // do the same as the normal version of num_rows
    }
};

do the same thing for the two other methods too.
you dont have to implement the methods within the class, you can also implement them in your source file....

[oonej]

they were already declared const..

can you explain why i need to do this?

class Matrix
{
 public:
   Matrix(); //Default Constructor
   Matrix(int row, int col); //Constructor for 2 variables (rows and cols)
   Matrix(const Matrix& other); //Copy constructor
   ~Matrix(); //Default Destructor
   int num_rows() const; //returns the number of rows 
   int num_cols() const; //returns the number of columns
   //.. some more stuff .. 
 private:
   // ... some stuff ...

};