//
// File "input.txt" contains a polynomial:
// first the number of its coefficients, then
// the coefficients in descending order.
// For example, for the polynomial
//     3*x^3 + 5^x^2 - 7^x + 1
// the file "input.txt" contains:
//     4
//     3 5 -7 1
//
// The program must write the coefficients of
// the derivative of this polynomial in descending order
// into the file "output.txt".
// In this example, the derivative of the polynomial is
//     9*x^2 + 10*x - 7,
// so the file "output.txt" should contain
//     9 10 -7
//
// The program should use the same array for the
// coefficients of polynomial (input data) and
// the resulting coefficient of derivative (output data).
//
#include <iostream>
#include <fstream>
#include <cstring>      /* for strerror(errno) */
#include <cmath>        /* Not necessary in this example */

using namespace std;

void computeDerivative(double *a, int n);

int main() {
    // Open the input file
    ifstream f("input.txt");
    if (!f.is_open()) {
        cerr << "Cannot open the input file: " 
             << strerror(errno) << endl;
        return -1;
    }

    // Read the size of array
    int n;
    if (!(f >> n) || n <= 0) {
        cerr << "Illegal input file." << endl;
        f.close();
        return -1;
    }

    // Allocate an array in dynamic memory
    double *a = new double[n];

    // Read coefficients of a polynomial
    for (int i = 0; i < n; ++i) {
        if (!(f >> a[i])) {
            cerr << "Incorrect format of input file." << endl;
            f.close();
            delete[] a;
            return -1;
        }
    }
    f.close();

    computeDerivative(a, n);

    // Open the output file
    ofstream g("output.txt");
    if (!g.is_open()) {
        cerr << "Cannot open the output file: " 
             << strerror(errno) << endl;
        delete[] a;
        return -1;
    }

    // Write results to both the output file and the console
    if (n <= 1) {
        // Zero derivative
        g << 0 << endl;
        cout << 0 << endl;
    } else {
        for (int i = 0; i < n-1; ++i) {
            if (i > 0) {
                g << ' ';   // Write a separator
                cout << ' ';
            }
            g << a[i];
            cout << a[i];
        }
        g << endl;
        cout << endl;
    }
    g.close();
    delete[] a;

    return 0;   // Success!
}

void computeDerivative(double *a, int n) {
    int deg = n - 1;    // Degree of a term
    int i = 0;          // Index of the term coefficient in array
    while (deg > 0) {
        a[i] *= deg;
        --deg;
        ++i;
    }
}