星期二 + 3 = 星期五？ C++ 编程问题

Question

查看 main 函数，我们可以看到我已将“星期一”硬编码到我的 setDay 公共函数中。使用 c 字符串很容易从用户那里获取一周中的某一天（就像我在 setDay 中所做的那样），但是我如何要求用户将 n 添加到设置的日期“星期一”并想出“周四”？这很难，因为 typdef enum { INVALID, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY} 不会将 9 解释为 0 和/或 10 为 1。

#include <iostream>

using std::cout;
using std::endl;

class DayOfTheWeek //class is encapsulation of functions and members that manipulate the data.
{
public:

 DayOfTheWeek(); // Constructor
 virtual ~DayOfTheWeek(); // Destructor

 void setDay(const char * day); // Function to set the day
 void printDay() const;      // Function to Print the day.
 const char * getDay() const;   // Function to get the day.

 const char * plusOneDay();  // Next day function
 const char * minusOneDay(); // Previous day function
 const char * addDays(int addValue);  // function that adds days based on parameter value

private:
 char * day; // variable for the days of the week.

};

DayOfTheWeek::DayOfTheWeek() : day(0)
{
// Usually I would allocate pointer member variables
// Here in the construction of the Object
}

const char * DayOfTheWeek::getDay() const
{
 return day; // we can get the day simply by returning it.
}

const char * DayOfTheWeek::minusOneDay()
{
 if ( strcmp( day, "Monday" ) == 0)
 {
  cout << "The day before " << day << " is ";
  return "Sunday";
 }
 else if ( strcmp( day, "Tuesday" ) == 0 )
 {
  cout << "The day before " << day << " is ";
  return "Monday";
 }
 else if ( strcmp( day, "Wednesday" ) == 0 )
 {
  cout << "The day before " << day << " is ";
  return "Tuesday";
 }
 else if ( strcmp( day, "Thursday" ) == 0 )
 {
  cout << "The day before " << day << " is ";
  return "Wednesday";
 }
 else if ( strcmp( day, "Friday" ) == 0 )
 {
  cout << "The day before " << day << " is ";
  return "Thursday";
 }
 else if ( strcmp( day, "Saturday" ) == 0 )
 {
  cout << "The day before " << day << " is ";
  return "Friday";
 }
 else if ( strcmp( day, "Sunday" ) == 0 )
 {
  cout << "The day before " << day << " is ";
  return "Saturday";
 }
 else
 {
  cout << "'" << day << "'";
  return "is an invalid day of the week!";
 } 
}

const char * DayOfTheWeek::plusOneDay()
{
 if ( strcmp( day, "Monday" ) == 0)
 {
  cout << "The day after " << day << " is ";
  return "Tuesday";
 }
 else if ( strcmp( day, "Tuesday" ) == 0 )
 {
  cout << "The day after " << day << " is ";
  return "Wednesday";
 }
 else if ( strcmp( day, "Wednesday" ) == 0 )
 {
  cout << "The day after " << day << " is ";
  return "Thursday";
 }
 else if ( strcmp( day, "Thursday" ) == 0 )
 {
  cout << "The day after " << day << " is ";
  return "Friday";
 }
 else if ( strcmp( day, "Friday" ) == 0 )
 {
  cout << "The day after " << day << " is ";
  return "Saturday";
 }
 else if ( strcmp( day, "Saturday" ) == 0 )
 {
  cout << "The day after " << day << " is ";
  return "Sunday";
 }
 else if ( strcmp( day, "Sunday" ) == 0 )
 {
  cout << "The day after " << day << " is ";
  return "Monday";
 }
 else
 {
  cout << "'" << day << "'";
  return " is an invalid day of the week!";
 }
}

const char * DayOfTheWeek::addDays(int addValue)
{
 if ( addValue < 0 )
 {
  if ( strcmp( day, "Monday" ) == 0)
  {
   cout << day << " - " << -addValue << " = ";
   return "Friday";
  }
  else if ( strcmp( day, "Tuesday" ) == 0 )
  { 
   cout << day << " - " << -addValue << " = ";
   return "Saturday";
  }
  else if ( strcmp( day, "Wednesday" ) == 0 )
  {
   cout << day << " - " << -addValue << " = ";
   return "Sunday";
  }
  else if ( strcmp( day, "Thursday" ) == 0 )
  {
   cout << day << " - " << -addValue << " = ";
   return "Monday";
  }
  else if ( strcmp( day, "Friday" ) == 0 )
  {
   cout << day << " - " << -addValue << " = ";
    return "Tuesday";
  }
  else if ( strcmp( day, "Saturday" ) == 0 )
  {
   cout << day << " - " << -addValue << " = ";
   return "Wednesday";
  }
  else if ( strcmp( day, "Sunday" ) == 0 )
  {
   cout << day << " - " << -addValue << " = ";
   return "Thursday";
  }
   else
  {
   cout << "'" << day << "' ";
   return "is an invalid day of the week! ";
  }
 }
 else // if our parameter is greater than 0 (positive)
 {
  if ( strcmp( day, "Monday" ) == 0)
  {
   cout << day << " + " << addValue << " = ";
   return "Thursday";
  }
  else if ( strcmp( day, "Tuesday" ) == 0 )
  {
   cout << day << " + " << addValue << " = ";
   return "Friday";
  }
  else if ( strcmp( day, "Wednesday" ) == 0 )
  { 
   cout << day << " + " << addValue << " = ";
   return "Saturday";
  }
   else if ( strcmp( day, "Thursday" ) == 0 )
  {
   cout << day << " + " << addValue << " = ";
   return "Sunday";
  }
  else if ( strcmp( day, "Friday" ) == 0 )
  {
   cout << day << " + " << addValue << " = ";
    return "Monday";
  }
  else if ( strcmp( day, "Saturday" ) == 0 )
  {
   cout << day << " + " << addValue << " = ";
   return "Tuesday";
  }
  else if ( strcmp( day, "Sunday" ) == 0 )
  {
   cout << day << " + " << addValue << " = ";
   return "Wednesday";
  }
  else
  {
   cout << "'" << day << "' ";
   return "is an invalid day of the week! ";
  }
 }
}

void DayOfTheWeek::printDay() const
{
 cout << "The Value of the " << day;
}

void DayOfTheWeek::setDay(const char * day)
{
 if (day)
 {// Here I am allocating the object member char day pointer
  this->day = new char[strlen(day)+1];
  size_t length = strlen(day)+1; // +1 for trailing null char
  strcpy_s(this->day , length , day); // copying c-strings
 }
 else day = NULL; // If their was a problem with the parameter 'day'
}

DayOfTheWeek::~DayOfTheWeek()  
{
 delete day; // Free the memory allocated in SetDay
}

int main()
{ 
 DayOfTheWeek MondayObject; // declare an object

 MondayObject.setDay("Monday"); // Call our public function 'setDay' to set a day of the week
 MondayObject.printDay();        // Call our public function 'printDay' to print the day we set

 cout << " object is " << MondayObject.getDay() << endl; // Print the value of the object

 cout << MondayObject.plusOneDay()  << endl;
 cout << MondayObject.minusOneDay() << endl;
 cout << MondayObject.addDays(3)    << endl;

   MondayObject.printDay();
 cout << " object is still " << MondayObject.getDay() << endl; // Print the value of the object

 cout << MondayObject.addDays(-3)  << endl;

 return 0;
}

Answer 1

您可能只想在类中使用整数来存储星期几，而不是尝试使用字符串来存储星期几。整数和模数算法最常用于存储此类数据。

如果您将星期几存储为，例如，星期日到星期一是 0 到 6，您可以使用模运算符 % 对其进行简单的操作；只要您不知道某个操作是否在该范围内，您都可以这样做

nDay = nDay % 7;

它将达到您期望的值；例如，如果它从 6 上升到 9，它将返回到 2。

Answer 2

您可以使用枚举并封装周日-周一翻转：

enum Weekday {
  INVALID,
  MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
};
Weekday operator+(Weekday w, int n) {
  if (w == INVALID) return w;
  n %= 7;
  if (n < 0) n += 7;
  return Weekday((n + (w - 1)) % 7 + 1);
}
Weekday operator+(int n, Weekday w) { return w + n; }
Weekday operator-(Weekday w, int n) { return w + -n; }
Weekday operator-(int n, Weekday w); // not defined -- doesn't make sense and not
// to be used

std::string str(Weekday d) {
  assert(INVALID <= d && d <= SUNDAY); // function precondition
  static char const* const days[] = {
    "INVALID",
    "MONDAY", "TUESDAY", "WEDNESDAY", "THURSDAY", "FRIDAY", "SATURDAY", "SUNDAY"
  };
  return days[d];
}

// you would probably not call this directly, but it shows how you simply have to
// pick what day int(0) means, and add any int to that day to get a Weekday
Weekday Weekday_from_num(int n, Weekday zero=MONDAY) {
  return zero + n;
}

Test:

int main() {
  assert(MONDAY + 1 == TUESDAY);
  assert(MONDAY + 8 == TUESDAY);
  assert(MONDAY - 1 == SUNDAY);
  assert(MONDAY - 8 == SUNDAY);

  cout << "MONDAY + 3 is " << str(MONDAY + 3) << '\n';
  return 0;
}

但是，我会放弃 INVALID（这也简化了数学运算）。你已经写了一个 Weekday_from_str 函数，你只需要修改它来返回这个枚举。

Answer 3

这种（经常遇到的）问题不需要写一个类，一个包含一个枚举和两个函数的命名空间就足够了：

#include <iostream>
namespace day {

enum e_DAY { Sun, Mon, Tue, Wed, Thu, Fri, Sat };

inline e_DAY GetDay(unsigned int i)
{
    return (e_DAY)(i % 7);
}

inline const wchar_t* GetName(unsigned int i)
{
    static wchar_t* Names[] = { L"Sunday", L"Monday", L"Tuesday", L"Wednesday", L"Thursday", L"Friday", L"Saturday" };
    return Names[GetDay(i)];
}

} // namespace day

int main(int argc, wchar_t* argv[])
{               
    std::wcout << day::Tue + 3 << "\t" << day::GetName(day::Tue + 3) << std::endl;
    return 0;
}