getting results from different constructors

I'm having a problem with the thirs case of my program...I'm trying to get a value for y only from two sets of coordinates. This works fine in the first two cases since I'm calling findY from slopeAndIntercept...but when I get into the third case my constructors get mixed up...any help?

Code:

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// THIS IS THE .H FILE
#include <iostream>
using namespace std;

//File: Line.h

class Line
{
public:
        //constructors
        Line( double newSlope = 0, double newIntercept = 0);
        Line( double newx1, double newy1, double newx2, double newy2 );

        void output1(); //output for slope and y-intercept
        void output2(); //output for the coordinates
       
        double findY(double x); // find y-coord from given x-coord

private:
        double slope;
        double intercept;
        double x1, y1, x2, y2;
};

#include "line.cpp"

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Code:

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//LINE .CPP
Line::Line( double newSlope, double newIntercept )
                : slope(newSlope), intercept(newIntercept){}

Line::Line( double newx1, double newy1, double newx2, double newy2 )
                : x1(newx1), y1(newy1), x2(newx2), y2(newy2){}

void Line::output1(){

        cout << "Initializing slope of line to: " << slope << endl;
        cout << "Initializing y-intercept to:  " << intercept << endl;
}

void Line::output2(){
        cout << "\n( x1, y1 ) = ( " << x1 << ", " << y1 << " )" << endl;
        cout << "( x2, y2 ) = ( " << x2 << ", " << y2 << " )" << endl;
}

double Line::findY(double x)
{
        if ( (slope != 0) && (intercept != 0) ){
                        return (slope*x + intercept);
        } else {
                slope = (y2 - y1)/(x2 - x1);
                intercept = (y1 - slope*(x1));
                return (slope*x + intercept);
        }
 
}

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Code:

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//MAIN.CPP
#include "Line.h"

int main()
{
        double x = 0;
        double y = 0;
   
                cout << "\n----------<Case1>---------\n" << endl;

                Line slopeAndIntercept(1, 2), coordinates(2, 4, 3, -1);
               
                slopeAndIntercept.output1();
                coordinates.output2();

                x = 12;
                cout << "\nCalculating y-coordinate when x = " << x << endl;
                y = slopeAndIntercept.findY(x);
                cout << "When x = " << x << " , y = " << y << endl;

                cout << "\n----------<Case2>---------\n" << endl;

                Line slopeAndIntercept1(4, -1), coordinates1(0, 0, 0, 0);
               
                slopeAndIntercept1.output1();

                x = 7;
                cout << "\nCalculating y-coordinate when x = " << x << endl;
                y = slopeAndIntercept1.findY(x);
                cout << "When x = " << x << " , y = " << y << endl;

                cout << "\n----------<Case3>---------\n" << endl;

                Line slopeAndIntercept2(0, 0),coordinates2(0, 0, 1, 1);
                coordinates2.output2();               

                x = 2;
                cout << "\nCalculating y-coordinate when x = " << x << endl;
                y = coordinates2.findY(x);
                cout << "When x = " << x << " , y = " << y << endl;

        return 0;

}

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thanks, axon

Do you ever actually initialize your slope and intercept in your "third case" ? Perhaps you want a function to calculate the slop and intercept based on the two points, initialize them within that function, and then call findY...or you could initialize them to zero by default (in your second constructor) and then check in findY whether they are 0 or not and calculate them if they are...

thats the whole point...I want to get the slope and the intercept from the two sets of coordinates, without initilizing the first constructor to a value different than 0....

axon

>>without initilizing the first constructor to a value different than 0....

Eh? I'm not sure what you mean...it should be pretty simple if you know the math you need...or is that the problem?

...mmmm...that definately is not the problem! if you don't have much to say except comments like that you should refrain from saying anything. Look in the code and you'll see that the math is fine

axon

>> slope = (y2 - y1)/(x2 - x1);

If x2-x1 evaluates to zero, your program will crash (a divide by zero exception will occur).

Second, the solution to your problem is to calculate slope & intercept *within* the 4 parameter constructor, thus simplifying further calculation:

Code:

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Line::Line( double newx1, double newy1, double newx2, double newy2 )
                : x1(newx1), y1(newy1), x2(newx2), y2(newy2)
{
 // prevent divide by zero!
 double valid_denominator = x2 - x1;
 slope = valid_denominator ? (y2 - y1)/(valid_denominator) : 0;
 intercept = (y1 - slope*(x1));
}

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Will allow you to do:

Code:

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double Line::findY(double x)
{
 return (slope*x + intercept);       
}

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thanks sebastiani...that worked great! I got rid of the extra constructors that initialized the variables to 0. I think I wanted it to be more complicated then it really is...Thanks !

axon