Polynomial Regression (Quadratic Fit) in C++

PolynomialRegression.h

#ifndef _POLYNOMIAL_REGRESSION_H
#define _POLYNOMIAL_REGRESSION_H  __POLYNOMIAL_REGRESSION_H
/**
 * PURPOSE:
 *
 *  Polynomial Regression aims to fit a non-linear relationship to a set of
 *  points. It approximates this by solving a series of linear equations using 
 *  a least-squares approach.
 *
 *  We can model the expected value y as an nth degree polynomial, yielding
 *  the general polynomial regression model:
 *
 *  y = a0 + a1 * x + a2 * x^2 + ... + an * x^n
 *
 * LICENSE:
 *
 * MIT License
 * 
 * Copyright (c) 2020 Chris Engelsma
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *
 * @author Chris Engelsma
 */
#include <vector>
#include <stdlib.h>
#include <cmath>
#include <stdexcept>

template <class TYPE>
class PolynomialRegression {
  public:

    PolynomialRegression();
    virtual ~PolynomialRegression(){};

    bool fitIt(
      const std::vector<TYPE> & x,
      const std::vector<TYPE> & y,
      const int &             order,
      std::vector<TYPE> &     coeffs);
};

template <class TYPE>
PolynomialRegression<TYPE>::PolynomialRegression() {};

template <class TYPE>
bool PolynomialRegression<TYPE>::fitIt( 
  const std::vector<TYPE> & x,
  const std::vector<TYPE> & y,
  const int &               order,
  std::vector<TYPE> &       coeffs)
{
  // The size of xValues and yValues should be same
  if (x.size() != y.size()) {
    throw std::runtime_error( "The size of x & y arrays are different" );
    return false;
  }
  // The size of xValues and yValues cannot be 0, should not happen
  if (x.size() == 0 || y.size() == 0) {
    throw std::runtime_error( "The size of x or y arrays is 0" );
    return false;
  }
  
  size_t N = x.size();
  int n = order;
  int np1 = n + 1;
  int np2 = n + 2;
  int tnp1 = 2 * n + 1;
  TYPE tmp;

  // X = vector that stores values of sigma(xi^2n)
  std::vector<TYPE> X(tnp1);
  for (int i = 0; i < tnp1; ++i) {
    X[i] = 0;
    for (int j = 0; j < N; ++j)
      X[i] += (TYPE)pow(x[j], i);
  }

  // a = vector to store final coefficients.
  std::vector<TYPE> a(np1);

  // B = normal augmented matrix that stores the equations.
  std::vector<std::vector<TYPE> > B(np1, std::vector<TYPE> (np2, 0));

  for (int i = 0; i <= n; ++i) 
    for (int j = 0; j <= n; ++j) 
      B[i][j] = X[i + j];

  // Y = vector to store values of sigma(xi^n * yi)
  std::vector<TYPE> Y(np1);
  for (int i = 0; i < np1; ++i) {
    Y[i] = (TYPE)0;
    for (int j = 0; j < N; ++j) {
      Y[i] += (TYPE)pow(x[j], i)*y[j];
    }
  }

  // Load values of Y as last column of B
  for (int i = 0; i <= n; ++i) 
    B[i][np1] = Y[i];

  n += 1;
  int nm1 = n-1;

  // Pivotisation of the B matrix.
  for (int i = 0; i < n; ++i) 
    for (int k = i+1; k < n; ++k) 
      if (B[i][i] < B[k][i]) 
        for (int j = 0; j <= n; ++j) {
          tmp = B[i][j];
          B[i][j] = B[k][j];
          B[k][j] = tmp;
        }

  // Performs the Gaussian elimination.
  // (1) Make all elements below the pivot equals to zero
  //     or eliminate the variable.
  for (int i=0; i<nm1; ++i)
    for (int k =i+1; k<n; ++k) {
      TYPE t = B[k][i] / B[i][i];
      for (int j=0; j<=n; ++j)
        B[k][j] -= t*B[i][j];         // (1)
    }

  // Back substitution.
  // (1) Set the variable as the rhs of last equation
  // (2) Subtract all lhs values except the target coefficient.
  // (3) Divide rhs by coefficient of variable being calculated.
  for (int i=nm1; i >= 0; --i) {
    a[i] = B[i][n];                   // (1)
    for (int j = 0; j<n; ++j)
      if (j != i)
        a[i] -= B[i][j] * a[j];       // (2)
    a[i] /= B[i][i];                  // (3)
  }

  coeffs.resize(a.size());
  for (size_t i = 0; i < a.size(); ++i) 
    coeffs[i] = a[i];

  return true;
}
#endif

There is either a flaw in the code or in my understanding of how to use the code. With the following code and data, the coefficients seems to be increasingly erroneous. With the data, a 6-order polynomial should be a near exact fit, but it is very off.

The code is:

`#include
#include

#include "PolynomialRegression.h"

int main( int argc, const char* argv[] ) {
const int order = atoi(argv[1]);
std::cout << "order: " << order << '\n';
int samples;
std::cin >> samples;
std::cout << "samples: " << samples << '\n';
std::vector x;
for ( int i = 0; i < samples; ++i ) {
double tmp;
std::cin >> tmp;
x.push_back(tmp);
}
std::vector y;
for ( int i = 0; i < samples; ++i ) {
double tmp;
std::cin >> tmp;
y.push_back(tmp);
}
PolynomialRegression polyreg;
std::vector coeffs;
polyreg.fitIt( x, y, order, coeffs );
std::cout << "coefficients:";
for ( int j = 0; j <= order; ++j ) {
std::cout << ' ' << coeffs[j];
}
std::cout << '\n';
std::cout << "entered :";
for ( int k = 0; k < samples; ++k ) {
std::cout << ' ' << y[k];
}
std::cout << '\n';
std::cout << "computed:";
for ( int l = 0; l < samples; ++l ) {
std::cout << ' ' << coeffs[0] + coeffs[1]*x[l] + coeffs[2]*x[l]*x[l];
}
std::cout << '\n';
}
`

The data is:

7
0.365616 1.854900 3.709790 7.419590 14.839200 29.678300 59.356700
0.246729 0.204710 0.174115 0.355591 0.878094 1.728551 2.897817

The problem above is actually in the test program that I provided. The evaluation polynomial wasn't growing with the order. The updated program follows.
`#include
#include

#include "PolynomialRegression.h"

int main( int argc, const char* argv[] ) {

// get the order from the command line.
const int order = atoi(argv[1]);
std::cout << "order: " << order << '\n';

// Read the number of samples.
int samples;
std::cin >> samples;
std::cout << "samples: " << samples << '\n';

// Read the x coordinates.
std::vector x;
for ( int i = 0; i < samples; ++i ) {
double tmp;
std::cin >> tmp;
x.push_back(tmp);
}

// Read the y coordinates.
std::vector y;
for ( int i = 0; i < samples; ++i ) {
double tmp;
std::cin >> tmp;
y.push_back(tmp);
}

// Do the regression.
PolynomialRegression polyreg;
std::vector coeffs;
polyreg.fitIt( x, y, order, coeffs );

// Print the coefficients.
std::cout << "coefficients:";
for ( int j = 0; j <= order; ++j ) {
std::cout << ' ' << coeffs[j];
}
std::cout << '\n';

// Print the entered y coordinates.
std::cout << "entered :";
for ( int k = 0; k < samples; ++k ) {
std::cout << ' ' << y[k];
}
std::cout << '\n';

// Print the computed y coordinates.
std::cout << "computed:";
for ( int l = 0; l < samples; ++l ) {
double value = coeffs[0];
for ( int m = 1; m <= order; ++m ) {
value += coeffs[m]*pow(x[l],m);
}
std::cout << ' ' << value;
}
std::cout << '\n';
}
`