## LinearRegression Class Definition

### ( C++ For Scientists and Engineers )

The following listing of header file linreg.h is the class definition of the LinearRegression class.

```/*  linreg.h
Linear Regression calculation class

by: David C. Swaim II, Ph.D.

This class implements a standard linear regression on
experimental data using a least squares fit to a straight
line graph.  Calculates coefficients a and b of the equation:

y = a + b * x

for data points of x and y.  Also calculates the coefficient of
determination, the coefficient of correlation, and standard
error of estimate.

The value n (number of points) must be greater than 2 to
calculate the regression.  This is primarily because the
standard error has a (N-2) in the denominator.

Check haveData() to see if there is enough data in
LinearRegression to get values.

You can think of the x,y pairs as 2 dimensional points.
The class Point2D is included to allow pairing x and y
values together to represent a point on a plane.

*/
#ifndef _LINREG_H_
#define _LINREG_H_
#include <iostream.h>

class Point2D
{
public:
Point2D(double X = 0.0, double Y = 0.0) : x(X), y(Y) { }

void setPoint(double X, double Y) { x = X; y = Y; }
void setX(double X) { x = X; }
void setY(double Y) { y = Y; }

double getX() const { return x; }
double getY() const { return y; }

private:
double x, y;
};

class LinearRegression
{
friend ostream& operator<<(ostream& out, LinearRegression& lr);

public:
// Constructor using an array of Point2D objects
// This is also the default constructor
LinearRegression(Point2D *p = 0, long size = 0);

// Constructor using arrays of x values and y values
LinearRegression(double *x, double *y, long size = 0);

virtual void addXY(const double& x, const double& y);
void addPoint(const Point2D& p) { addXY(p.getX(), p.getY()); }

// Must have at least 3 points to calculate
// standard error of estimate.  Do we have enough data?
int haveData() const { return (n > 2 ? 1 : 0); }
long items() const { return n; }

virtual double getA() const { return a; }
virtual double getB() const { return b; }

double getCoefDeterm() const  { return coefD; }
double getCoefCorrel() const { return coefC; }
double getStdErrorEst() const { return stdError; }
virtual double estimateY(double x) const { return (a + b * x); }

protected:
long n;             // number of data points input so far
double sumX, sumY;  // sums of x and y
double sumXsquared, // sum of x squares
sumYsquared; // sum y squares
double sumXY;       // sum of x*y

double a, b;        // coefficients of f(x) = a + b*x
double coefD,       // coefficient of determination
coefC,       // coefficient of correlation
stdError;    // standard error of estimate

void Calculate();   // calculate coefficients
};

#endif                      // end of linreg.h
```

Copyright © 1998 by David C. Swaim II, all rights reserved.