RMS approximation. Least squares method

#include <iostream>
#include <cmath>

#define N_FUNC 9

using namespace std;

const string L_FUNC[] = {
    "b+a*x",
    "b+a/x",
    "b+a*lg x",
    "b+a^x",
    "b+x^a",
    "e^(b+a/x)",
    "1/(b+a*x)",
    "1/(b+a*lg x)",
    "x/(b+a*x)"
};

double calcAvgArithm(double* arr, int n) {
    double avg = 0;
    for (int i = 0; i < n; i++) avg += arr[i] / n;
    return avg;
}

double calcAvgGeom(double* arr, int n) {
    double avg = 1;
    for (int i = 0; i < n; i++) avg *= pow(arr[i], 1. / n);
    return avg;
}

double calcAvgHarm(double* arr, int n) {
    double avg = 0;
    for (int i = 0; i < n; i++) avg += 1 / arr[i];
    return n / avg;
}

double calcYtilda(double xAvg, double* x, double* y, int n) {
    for (int i = 0; i < n - 1; i++)
        if (xAvg >= x[i] && x[i + 1] >= xAvg)
            return y[i] + (y[i + 1] - y[i]) * (xAvg - x[i]) / (x[i + 1] - x[i]);
}

void calcCoef(double* x, double* y, int n, double &a, double &b) {
    double xSum = 0, ySum = 0, xySum = 0, xSqrSum = 0;

    for (int i = 0; i < n; i++) {
        xSum += x[i];
        ySum += y[i];
        xySum += x[i] * y[i];
        xSqrSum += pow(x[i], 2);
    }
    a = (xySum - (1. / n) * xSum * ySum) / (xSqrSum - (1. / n) * pow(xSum, 2));
    b = (1. / n) * ySum - (a / n) * xSum;
}

int getTable(double* x, double* y, int n) {
    double table[N_FUNC][4];
    int min = 0;

    table[0][0] = calcAvgArithm(x, n);
    table[1][0] = calcAvgGeom(x, n);
    table[2][0] = calcAvgHarm(x, n);

    table[0][2] = calcYtilda(table[0][0], x, y, n);
    table[1][2] = calcYtilda(table[1][0], x, y, n);
    table[2][2] = calcYtilda(table[2][0], x, y, n);

    for (int i = 3; i < N_FUNC; i += 3)
        for (int j = 0; j < 3; j++) {
            table[i + j][0] = table[j][0];
            table[i + j][2] = table[j][2];
        }

    table[0][1] = calcAvgArithm(y, n);
    table[3][1] = calcAvgGeom(y, n);
    table[6][1] = calcAvgHarm(y, n);

    for (int i = 1; i < N_FUNC / 3; i++)
        for (int j = 0; j < N_FUNC; j += 3)
            table[j + i][1] = table[j][1];

    for (int i = 0; i < N_FUNC; i++) {
        table[i][3] = fabs(table[i][1] - table[i][2]) / table[i][2];
        if (table[min][3] > table[i][3]) min = i;
    }

    cout << "#| xAvg  | yAvg  |  ~y   |     eps      | func" << endl;
    cout << "----------------------------------------------" << endl;

    for (int i = 0; i < N_FUNC; i++) {
        cout << i + 1;
        for (int j = 0; j < 4; j++) {
            cout << "|" << table[i][j];
        }
        cout << "\t|" << L_FUNC[i];
        if (i == min) {
            cout << "\t <----";
        }
        cout << endl;
    }
    cout << endl;
    return min;
}

void getFunc(int iFunc, double* x, double* y, int n) {
    double a, b;

    cout << "Func: " << L_FUNC[iFunc] << endl << endl;

    switch(iFunc) {
        case 0:
            calcCoef(x, y, n, a, b);
            break;
        case 1:
            for (int i = 0; i < n; i++)
                x[i] = 1 / x[i];
            calcCoef(x, y, n, a, b);
            break;
        case 2:
            for (int i = 0; i < n; i++)
                x[i] = log(x[i]);
            calcCoef(x, y, n, a, b);
            break;
        case 3:
            for (int i = 0; i < n; i++)
                y[i] = log(y[i]);
            calcCoef(x, y, n, a, b);
            a = exp(a);
            b = exp(b);
            break;
        case 4:
            for (int i = 0; i < n; i++) {
                x[i] = log(x[i]);
                y[i] = log(y[i]);
            }
            calcCoef(x, y, n, a, b);
            b = exp(b);
            break;
        case 5:
            for (int i = 0; i < n; i++) {
                x[i] = 1 / x[i];
                y[i] = log(y[i]);
            }
            calcCoef(x, y, n, a, b);
            break;
        case 6:
            for (int i = 0; i < n; i++)
                y[i] = 1 / y[i];
            calcCoef(x, y, n, a, b);
            break;
        case 7:
            for (int i = 0; i < n; i++) {
                y[i] = 1 / y[i];
                x[i] = log(x[i]);
            }
            calcCoef(x, y, n, a, b);
            break;
        case 8:
            for (int i = 0; i < n; i++) {
                y[i] = 1 / y[i];
                x[i] = 1 / x[i];
            }
            calcCoef(x, y, n, a, b);
            break;
    }
    cout << "a = " << a << endl;
    cout << "b = " << b << endl;
}

int main() {
    int n;

    cout << "Enter count of poins: ";
    cin >> n;

    double x[n], y[n], a, b;

    cout << "Enter x array: ";
    for (int i = 0; i < n; i++) {
        cin >> x[i];
    }
    cout << "Enter y array: ";
    for (int i = 0; i < n; i++) {
        cin >> y[i];
    }
    cout << endl;

    getFunc(getTable(x, y, n), x, y, n);

    return 0;
}