问加速C++，练习3-5:学生成绩计算器

EN

#include <iostream>
#include <iomanip>
#include <string>
#include <vector>

using std::cin;
using std::cout;
using std::endl;
using std::setprecision;
using std::streamsize;
using std::string;
using std::vector;

/*****
 * This program is designed to take a list of students, then ask for their
 * midterm, final and homework grades. It then calculates the final grade
 * for each student, with the following weights:
 *   Midterm:  20%
 *   Final:    40%
 *   Homework: 40% of average grade
 */
int main()
{
  // How many homework grades each student requires
  int assignments = 3;

  // Get all student names and store them in the first vector
  vector<string> students;
  {
    string student;
    cout << "Please enter all students forenames, or an empty line when done."
         << endl
         << "Student name: ";

    while (getline(cin, student) && !student.empty()) {
      students.push_back(student);
      cout << "Student name: ";
    }

    cout << endl
         << students.size() << " students entered."
         << endl << endl;
  }

  // Ask for the midterm, final and `assignments` homework grades for students
  vector<double> midterms;
  vector<double> finals;
  vector<double> homework;
  {
    double grade;

    for (int s = 0; s < students.size(); s++) {
      // invariant: we've received grades for `s` students
      cout << "Grades for " << students[s] << endl;

      cout << " - Midterm: ";
      cin >> grade;
      midterms.push_back(grade);

      cout << " - Final: ";
      cin >> grade;
      finals.push_back(grade);

      for (int a = 0; a < assignments; a++) {
        // invariant: we've received the grade for `a` assignments
        cout << " - Assignment " << a + 1 << ": ";
        cin >> grade;
        homework.push_back(grade);
      }
      cout << endl;
    }
  }

  // Calculate and print the overall grades
  cout << "Overall Grades:" << endl;
  streamsize precision = cout.precision();
  {
    typedef vector<string>::size_type vec_size;
    double overallGrade,
           midtermGrade,
           finalGrade,
           homeworkGrades;

    for (vec_size s = 0; s < students.size(); s++) {
      midtermGrade   = midterms[s];
      finalGrade     = finals[s];
      homeworkGrades = 0;

      // Pull out the homework grades
      int start = s * assignments,
          end   = (s + 1) * assignments - 1;

      for (vec_size h = 0; h < assignments; h++) {
        // invariant: we've summed the grades for
        homeworkGrades += homework[start + h];
      }

      // Calculate the overall grade
      overallGrade = 0.2 * midtermGrade
                     + 0.4 * finalGrade
                     + 0.4 * homeworkGrades / assignments;

      cout << " - " << students[s] << ": "
           << setprecision(3) << overallGrade << setprecision(precision)
           << endl;
    }
  }

  return 0;
}