public class TriangularMatrix extends Matrix {
	//Data-elements
	private double[] elements; 
	
	//Constructors
	public TriangularMatrix(int rows,int cols) {
		if(rows!=cols)
			throw new RuntimeException("Illegal matrix dimensions!");
		n=m=rows;
		elements=new double[((1+n)*n)/2];
	}

    public TriangularMatrix(double[][] data) {
    	this(data.length,data[0].length);
        for (int i = 0; i < n; i++)
            for (int j = 0; j < n; j++)
                    setElementAt(i+1,j+1,data[i][j]);
    }

    public TriangularMatrix(Matrix arg) {
    	this(arg.getRows(),arg.getColumns());
        for (int i = 1; i <= n; i++)
            for (int j = 1; j <= n; j++)
                    setElementAt(i,j,arg.elementAt(i,j));
    } 

	//Override
	protected int f(int i,int j) {
		super.f(i,j);
		return (i*(i-1))/2+j-1;        
	}
   	
	//Implementation of abstract methods
	public double elementAt(int i,int j) {
		int k=f(i,j);
		double result=elements[k];
        return j<=i ? result : 0;
    }
    
    public double setElementAt(int i,int j,double value) {
    	int k=f(i,j);
        double oldValue=elements[k];
        if(j<=i) elements[k]=value;
        else if(value!=0)
        	throw new RuntimeException("Incorrect value!");
     	return oldValue;
    }
    
    protected Matrix create(int rows,int cols) {
    	return new TriangularMatrix(rows,cols);
    }

 	//Override
    public Matrix add(Matrix B) {
    	if(B instanceof TriangularMatrix) return plus((TriangularMatrix)B);
    	Matrix A=new OrdinaryMatrix(this);
        return A.add(B);
    } 
    
    //Private method    
    private TriangularMatrix plus(TriangularMatrix B) {
        Matrix A = this;
        if (B.n != A.n)
        	throw new RuntimeException("Illegal matrix dimensions!");
        TriangularMatrix C = new TriangularMatrix(n,n);
        for (int i = 1; i <= n; i++)
            for (int j = 1; j <= i; j++)
                C.setElementAt(i,j,A.elementAt(i,j) + B.elementAt(i,j));
        return C;
    }

    //Other public methods - inherited from Matrix
}