public function swapColumns( $column1, $column2 )
	{
		if( $column1 < 0 || $column1 >= $this->getColumnNumber() )
			throw new OutOfRangeException( 'Column key "'.$column1.'" is not valid.' );
		if( $column2 < 0 || $column2 >= $this->getColumnNumber() )
			throw new OutOfRangeException( 'Column key "'.$column2.'" is not valid.' );

		for( $row = 0; $row < $this->getRowNumber(); $row++ )
		{
			$buffer	= $this->values[$row][$column1];
			$this->values[$row][$column1]	= $this->values[$row][$column2];
			$this->values[$row][$column2]	= $buffer;
		}
	}