task_sem2_3.1

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#define e 1e-10

double *el(int n, int i, int j, double *a) {
    return a + i * n + j;
}

int maxLine(int n, int i, int j, double *a) {
    double max = *el(n, i, j, a);
    int maxi = i;

    for (int k = i; k < n; k++)
        if (fabs(*el(n, k, j, a)) > fabs(max)) {
            max = *el(n, k, j, a);
            maxi = k;
        }

    if (max == 0.0)
        return -1;
    else
        return maxi;
}

void swapLines(int n, double *a, double *b, int l, int m) {
    for (int j = 0; j < n; j++) {
        double t = *el(n, l, j, a);
        *el(n, l, j, a) = *el(n, m, j, a);
        *el(n, m, j, a) = t;
    }

    double t = *(b + l);
    *(b + l) = *(b + m);
    *(b + m) = t;
}

int linsys(int n, double *a, double *b, double *x) {
    for (int i = 0; i < n - 1; i++) { // for all lines excluding the last...
        int maxl = maxLine(n, i, i, a); // ...we find main item in column and then...
        if (maxl == -1)
            return 0;
        if (maxl > *el(n, i, i, a))
            swapLines(n, a, b, i, maxl);

        for (int k = i + 1; k < n; k++) { // ...we will set all items from current column to zero excluding current line
            double coeff = -(*el(n, k, i, a)) / *el(n, i, i, a);
            for (int l = i; l < n; l++)
                *el(n, k, l, a) += *el(n, i, l, a) * coeff;

            *(b + k) += *(b + i) * coeff;
        }
    }

    if (*el(n, n - 1, n - 1, a) == 0.0)
        return 0;
    *(x + n - 1) = *(b + n - 1) / *el(n, n - 1, n - 1, a);
    for (int i = n - 2; i >= 0; i--) {
        double y = 0.0;
        for (int j = i + 1; j < n; j++)
            y += *el(n, i, j, a) * *(x + j);

        *(x + i) = (*(b + i) - y) / *el(n, i, i, a);
    }

    return 1;
}

int main(int argc, char *argv[]) {
    double df[4], f[2], mods[2];
    double x0, y0; // approximation

    mods[0] = 0.0; // set mods to zero
    mods[1] = 0.0;

    printf("Начальное приближение x = "); scanf("%lf", &x0);
    printf("Начальное приближение y = "); scanf("%lf", &y0);

    getchar();
    do {
        printf("поправка к x = %.16e\nпоправка к y = %.16e\n", mods[0], mods[1]);
        getchar();

        // calculate df matrix
        *(df + 0) = log(x0) + 1;
        *(df + 1) = log(y0) + 1;
        *(df + 2) = pow(x0, -(2.0 / 3.0)) / 3;
        *(df + 3) = pow(y0, -(2.0 / 3.0)) / 3;

        // calculate f matrix
        *(f + 0) = -(x0 * log(x0) + y0 * log(y0) - 1);
        *(f + 1) = -(cbrt(x0) + cbrt(y0) - 2);

        if (!linsys(2, df, f, mods)) {
            printf("Несовместная система уравнений\n");
            return 1;
        }

        x0 += mods[0];
        y0 += mods[1];
    } while (sqrt(pow(mods[0], 2) + pow(mods[1], 2)) > e);

    printf("x = %.16e\ny = %.16e\n", x0, y0);

    return 0;
}