Class problem

This is a discussion on Class problem within the C++ Programming forums, part of the General Programming Boards category; I have a program that should take some points that make a straight line and calculate a slope. I am ...

  1. #1
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    Oct 2005
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    36

    Class problem

    I have a program that should take some points that make a straight line and calculate a slope. I am getting some random garbage and I don't know why.

    I have a class "Line":
    Code:
    class Line  
    {
          private:
    	Point p1;
    	Point p2;
    	double slope;
    	double yInt;
    
          public:
                    Point readPts(Point p1, Point p2);
                    void slopePts(Point p1, Point p2);
    }
    and a class "Point":
    Code:
    class Point
    {
          private:
                  double dX;
                  double dY;
    
          public:
                  friend class Line;
    };
    In my Line.cpp file, I have:
    Code:
    Point Line::readPts(Point p1, Point p2)
    {
           cout <<"Enter the point's X coordinate: ";
           cin >> p1.dX;
    
           cout <<"And its Y coordinate: ";
           cin >> p1.dY;
    
           cout <<"Enter the point's X coordinate: ";
           cin >> p2.dX;
    
           cout <<"And its Y coordinate: ";
           cin >> p2.dY;
    }
    
    
    void Line::slopePts(Point p1, Point p2)
    {
        slope = (p2.dY - p1.dY) / (p2.dX - p1.dX);
    }
    In my main.cpp, here is where I call the function to read in points:
    Code:
    Line AB, BC;
    
    AB.readPts(p1, p2); //reading in points for line AB
    CD.readPts(p1, p2); //reading in points for line CD

    If I print "p1.dX" or "p1.dY" etc. in the "readPts" function, obviously the correctly entered numbers will come out. However, when I print these in the "slopePts" function, they will be garbage. This is why my slope is comming out as garbage as well.

  2. #2
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    You are passing the points by value to the readPts function, so that function is getting a copy and reading data into that copy. The original variables p1 and p2 in main.cpp never change and are probably just un-initialized.

    You could pass the points by reference to the function, or you could make the read function a member of the Point class, or what probably makes the most sense is to remove the function parameters completely and read into the member variables of the Line class.

  3. #3
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    IL
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    dX and dY are private, don't they have to be protected for other classes to reference them via the friend statement?

    Point Line::readPts(Point p1, Point p2)

    Also notice that function doesn't return a point.


    These two variables could be replaced with functions that retrieve the slope and yInt instead of storing them in memory:

    double slope;
    double yInt;
    Last edited by theJ89; 03-26-2006 at 06:54 PM.

  4. #4
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    No, a friend is allowed access to a class's privates.

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