How to fit the 2D scatter data with a line with C++

This page describes the algorithm easier than Wikipedia, without extra steps to calculate the means etc. : http://faculty.cs.niu.edu/~hutchins/csci230/best-fit.htm . Almost quoted from there, in C++ it's:

#include <vector>
#include <cmath>

struct Point {
  double _x, _y;
};
struct Line {
  double _slope, _yInt;
  double getYforX(double x) {
    return _slope*x + _yInt;
  }
  // Construct line from points
  bool fitPoints(const std::vector<Point> &pts) {
    int nPoints = pts.size();
    if( nPoints < 2 ) {
      // Fail: infinitely many lines passing through this single point
      return false;
    }
    double sumX=0, sumY=0, sumXY=0, sumX2=0;
    for(int i=0; i<nPoints; i++) {
      sumX += pts[i]._x;
      sumY += pts[i]._y;
      sumXY += pts[i]._x * pts[i]._y;
      sumX2 += pts[i]._x * pts[i]._x;
    }
    double xMean = sumX / nPoints;
    double yMean = sumY / nPoints;
    double denominator = sumX2 - sumX * xMean;
    // You can tune the eps (1e-7) below for your specific task
    if( std::fabs(denominator) < 1e-7 ) {
      // Fail: it seems a vertical line
      return false;
    }
    _slope = (sumXY - sumX * yMean) / denominator;
    _yInt = yMean - _slope * xMean;
    return true;
  }
};

Please, be aware that both this algorithm and the algorithm from Wikipedia ( http://en.wikipedia.org/wiki/Simple_linear_regression#Fitting_the_regression_line ) fail in case the "best" description of points is a vertical line. They fail because they use

y = k*x + b 

line equation which intrinsically is not capable to describe vertical lines. If you want to cover also the cases when data points are "best" described by vertical lines, you need a line fitting algorithm which uses

A*x + B*y + C = 0

line equation. You can still modify the current algorithm to produce that equation:

y = k*x + b <=>
y - k*x - b = 0 <=>
B=1, A=-k, C=-b

In terms of the above code:

B=1, A=-_slope, C=-_yInt

And in "then" block of the if checking for denominator equal to 0, instead of // Fail: it seems a vertical line, produce the following line equation:

x = xMean <=>
x - xMean = 0 <=>
A=1, B=0, C=-xMean

I've just noticed that the original article I was referring to has been deleted. And this web page proposes a little different formula for line fitting: http://hotmath.com/hotmath_help/topics/line-of-best-fit.html

double denominator = sumX2 - 2 * sumX * xMean + nPoints * xMean * xMean;
...
_slope = (sumXY - sumY*xMean - sumX * yMean + nPoints * xMean * yMean) / denominator;

The formulas are identical because nPoints*xMean == sumX and nPoints*xMean*yMean == sumX * yMean == sumY * xMean.

I would suggest coding it from scratch. It is a very simple implementation in C++. You can code up both the intercept and gradient for least-squares fit (the same method as polyfit) from your data directly from the formulas here

http://en.wikipedia.org/wiki/Simple_linear_regression#Fitting_the_regression_line

These are closed form formulas that you can easily evaluate yourself using loops. If you were using higher degree fits then I would suggest a matrix library or more sophisticated algorithms but for simple linear regression as you describe above this is all you need. Matrices and linear algebra routines would be overkill for such a problem (in my opinion).

Equation of line is Ax + By + C=0.

So it can be easily( when B is not so close to zero ) convert to y = (-A/B)*x + (-C/B)

typedef double scalar_type;
typedef std::array< scalar_type, 2 > point_type;
typedef std::vector< point_type > cloud_type;

bool fit( scalar_type & A, scalar_type & B, scalar_type & C, cloud_type const& cloud )
{
    if( cloud.size() < 2 ){ return false; }

    scalar_type X=0, Y=0, XY=0, X2=0, Y2=0;

    for( auto const& point: cloud )
    { // Do all calculation symmetric regarding X and Y
        X  += point[0];
        Y  += point[1];
        XY += point[0] * point[1];
        X2 += point[0] * point[0];
        Y2 += point[1] * point[1];
    }

    X  /= cloud.size();
    Y  /= cloud.size();
    XY /= cloud.size();
    X2 /= cloud.size();
    Y2 /= cloud.size();

    A = - ( XY - X * Y ); //!< Common for both solution

    scalar_type Bx = X2 - X * X;
    scalar_type By = Y2 - Y * Y;

    if( fabs( Bx ) < fabs( By ) ) //!< Test verticality/horizontality
    { // Line is more Vertical.
        B = By;
        std::swap(A,B);
    }
    else
    {   // Line is more Horizontal.
        // Classical solution, when we expect more horizontal-like line
        B = Bx;
    }
    C = - ( A * X + B * Y );

    //Optional normalization:
    // scalar_type  D = sqrt( A*A + B*B );
    // A /= D;
    // B /= D;
    // C /= D;
    return true;
}