diffeq

diffeq

#include <iostream>
#include <cstdint>

using namespace std;

int foo(int *coefficient, int coi, uint64_t n) {
  if (n == 0) {
    return 0;
  }

  coefficient[coi] = n % 10;
  return 1 + foo(coefficient, coi + 1, n / 10);
}

int diff(int *coefficient, int num_of_digits) {
  for (int i = 0; i < num_of_digits - 1; i++) {
    coefficient[i] = (i + 1) * coefficient[i + 1];
  }

  return num_of_digits - 1;
}

void print(int *coefficient, int num_of_digits) {
  for (int i = num_of_digits - 1; i >= 0; i--) {
    if (coefficient[i] > 0) {
      if (i < num_of_digits - 1) {
        cout << "+";
      }
      cout << coefficient[i];
      if (i > 0) {
        cout << "X";
      }
      if (i > 1) {
        cout << "^" << i;
      }
    }
  }

  cout << endl;
}

int main(int argc, char *argv[]) {
  uint64_t n;
  cin >> n;

  int coefficient[64];
  int coi = 0;

  int num_of_digits = foo(coefficient, 0, n);

  print(coefficient, num_of_digits);

  num_of_digits = diff(coefficient, num_of_digits);

  print(coefficient, num_of_digits);

  return 0;
}

diffeq-v2.cpp

#include <iostream>
#include <cstdint>

using namespace std;

int foo(int *coefficient, int coi, uint64_t n) {
  if (n == 0) {
    return 0;
  }

  coefficient[coi] = n % 10;
  return 1 + foo(coefficient, coi + 1, n / 10);
}

int diff(int *coefficient, int coi, int num_of_digits) {
  if (num_of_digits == 0) {
    return 0;
  }

  coefficient[coi] = (coi + 1) * coefficient[coi + 1];
  return 1 + diff(coefficient, coi + 1, num_of_digits - 1);
}

int print(int *coefficient, int coi, const char *op) {
  int printed = 0;

  if (coi < 0) {
    return 0;
  }

  if (coefficient[coi] > 0) {
    cout << op;
    if (coi > 0) {
      if (coefficient[coi] > 1) {
        cout << coefficient[coi];
      }
      cout << "X";
    } else {
      cout << coefficient[coi];
    }
    if (coi > 1) {
      cout << "^" << coi;
    }

    printed = 1;
  }

  return printed + print(coefficient, coi - 1, "+");
}

int main(int argc, char *argv[]) {
  uint64_t n;

  cout << "Please input an integer: ";
  cin >> n;

  int coefficient[100];
  int coi = 0;

  int num_of_digits = foo(coefficient, 0, n);

  cout << "The equation is ";
  print(coefficient, num_of_digits - 1, "");
  cout << endl;

  num_of_digits = diff(coefficient, 0, num_of_digits - 1);

  cout << "The equation after differential is ";
  if (print(coefficient, num_of_digits - 1, "") == 0) {
    cout << "0";
  }
  cout << endl;

  return 0;
}