lab4

const int n = 10;

float y(float x) {
    return std::sin(x)/3 + std::sin(2*x)/3 + std::cos(2*x);
}
float x(int i) {
    return a + (float)i * h;
}

// y'' + 4y = sin(x), y(0) = y0 = 1, y'(0) = y'0 = 1
int main() {
    {
        const float a = y(0.0f);
        const float b = y(1.0f);
        std::vector<std::vector<float>> mat(n+1);
        for (int i = 0; i < n+1; i++) {
            mat[i] = std::vector<float>(n+1);
            std::fill(mat[i].begin(), mat[i].end(), 0);
        }

        const float l  = 1.0f / n;
        const float hs = h * h;
        const float k  = 4 * hs - 2;

        mat[0][0]   = 1; mat[0][n] = a;
        mat[n][n-1] = 1; mat[n][n] = b;
        for (int i = 1; i < n; i++) {
            mat[i][i-1] = 1;
            mat[i][i]   = k;
            mat[i][i+1] = 1;
            mat[i][n]   = hs * std::sin((float)i * l);
        }

        std::vector<float>* solution = solve(mat); //from lab2

        for (int i = 0; i <= n; i++) {
            std::cout << (std::fabs(y(x(i)) - (*solution)[i])) << std::endl;
        }
    }

    return 0;
}