CPP Permuation of school lessons

#include <algorithm>
#include <iostream>
#include <vector>

struct lesson {
  std::string name;
  unsigned int length;
};

int compare_lessons(const lesson &left, const lesson &right) {
  return right.name.compare(left.name) < 0;
}

void print_vector(std::vector<lesson> lessons, int week = 1) {
  std::cout << "Week " << week << ": " << std::endl;

  std::string days[] = {
    "Monday",
    "Tuesday",
    "Wednesday",
    "Thursday",
    "Friday",
    "Saturday",
    "Sunday"
  };

  int day = 0;

  for (size_t i = 0; i < lessons.size(); i++) {
    std::cout << days[(day) % 7] << std::endl;
    std::cout << lessons[i].name << std::endl;
    i++;
    std::cout << lessons[i].name << std::endl;
    day++;
  }
  std::cout << std::endl;
}

int main() {
  std::vector<lesson> lessons;

  lesson temp;
  do {
    std::cout << "Subject name: ";
    std::cin >> temp.name;

    if (temp.name != "exit") {
      lessons.push_back(temp);
    }
  } while (temp.name != "exit");

  std::sort(lessons.begin(), lessons.end(), compare_lessons);

  int week = 1;
  do {
    print_vector(lessons, week++);
  } while (
      std::next_permutation(lessons.begin(), lessons.end(), compare_lessons));

  return 0;
}