如何比较C或C++中的日/月/年？

Question

I have a program that asks the user to input to dates then it displays which one is more recent I've done it like this

if (year1>year2 || month1>month2 || day1>day2)
    return -1;

if (year1<year2 || month1<month2 || day1<day2)
    return +1;

but the output is not quite correct.

Answer 1

Here's a clean way to do it:

#include <tuple> // for std::tie

auto date1 = std::tie(year1, month1, day1);
auto date2 = std::tie(year2, month2, day2);

if (date1 == date2)
  return 0;

return (date1 < date2) ? -1 : 1; 

The comparisons of std::tie objects are lexicographical, so this returns -1 if date1 is less than date2 , 0 if they are the same, and 1 if date1 is greater than date2 .

You might be better off defining your own date type (or use boost::datetime ).

struct Date
{
  unsigned year;
  unsigned month;
  unsigned day;
};

bool operator<(const Date& lhs, const Date& rhs)
{
  return std::tie(lhs.year, lhs.month, lhs.day) < 
         std::tie(rhs.year, rhs.month, rhs.day);
}

bool operator>(const Date& lhs, const Date& rhs) { .... }

bool operator==(const Date& lhs, const Date& rhs) { .... }

int date_cmp(const Date& lhs, const Date& rhs) 
{
  // use operators above to return -1, 0, 1 accordingly
}

Answer 2

You need a much more complicated check than that:

if (year1 > year2)
    return -1;
else if (year1 < year2)
    return +1;

if (month1 > month2)
    return -1;
else if (month1 < month2)
    return +1;

if (day1 > day2)
    return -1;
else if (day1 < day2)
    return +1;

return 0;

NOTE: Returning -1 for first is greater than second seems counter-intuititive to me, however I have followed the semantics provided by the OP.

Answer 3

This statement

if (year1>year2 || month1>month2 || day1>day2)
    return -1;

tests if any one of the three conditions is true. So, if year1 is higher than year 2, or month1 is higher than month2. Lets stop there. Consider

year1 = 2013, month1 = 12, day1 = 31;
year2 = 2014, month2 = 1, day1 = 1;

We know that, infact, year2 is a higher value, but what happens is

is year1 > year2? no
ok, but is month1 > month2? yes

This makes it look like the first year is a higher value, but it's not, it just a higher month value.

As you get further into C++ you'll find that it's a good idea to try and adopt a convention of making all your comparisons use a single operator (< or >), when you reach a point where you are working with operators you'll understand why.

if (year2 < year1)
    return 1;
// we reach this line when year1 <= year2
if (year1 < year2) // elimnate the < case
    return -1;

// having eliminated both non-matches,
// we know that by reaching point that both
// dates have the same year. Now repeat for
// the month value.
if (month2 < month1)
    return 1;
if (month1 < month2)
    return -1;

// year and month must be the same, repeat for day.
if (day2 < day1)
    return 1;
if (day1 < day2)
    return -1;
return 0; // exact match

Answer 4

//You can try this

int lday,lmonth,lyear;
int nday,nmonth,nyear;
int lhour,lminute;
int nhour,nminute;
sscanf(New_Time,"%d-%d",&nhour,&nminute); //reads the numbers
sscanf(Last_Time,"%d-%d",&lhour,&lminute); //from the string
sscanf(New_Date,"%d-%d-%d",&nday,&nmonth,&nyear);
sscanf(Last_Date,"%d-%d-%d",&lday,&lmonth,&lyear);
//cout << "Last date: " << lday << "-" << lmonth << "-" << lyear  <<endl;
//cout << "New date: " << nday << "-" << nmonth << "-" << nyear  <<endl;

if(nyear>lyear)
    return 0;
if(nyear==lyear) {
    if(nmonth > lmonth)
        return 0;
    if (nmonth == lmonth) {
        if(nday > lday)
            return 0;
        if (nday == lday) {
            if( nhour > lhour)
                return 0;
            if( nhour == lhour) {
                if(nminute>lminute) {
                    //cout << "new time >= last time" << endl <<endl;
                    return 0;
                }
                else return 1;
            }
            else return 1;
        }
        else return 1;
    }
    else return 1;
}
else return 1;

Answer 5

struct Day
{
    int value;

    explicit Day(int value)
    {
        this->value = value;
    }
};

struct Month
{
    int value;

    explicit Month(int value)
    {
        this->value = value;
    }
};

struct Year
{
    int value;

    explicit Year(int value)
    {
        this->value = value;
    }
};

class Date {
public:
    Date(Day newDay, Month newMonth, Year newYear)
    {
        _day = newDay.value;
        _month = newMonth.value;
        _year = newYear.value;
    }

    int GetYear() const {
        return _year;
    };
    int GetMonth() const {
        return _month;
    };
    int GetDay() const {
        return _day;
    };

private:
    int _year;
    int _month;
    int _day;
};    

bool operator < (const Date& lhs, const Date& rhs)
    {
        if (lhs.GetYear() == rhs.GetYear()) {
            if (lhs.GetMonth() == rhs.GetMonth()) {
                if (lhs.GetDay() == rhs.GetDay()) {
                    return false;
                }
    
                return lhs.GetDay() < rhs.GetDay();
            }
    
            return lhs.GetMonth() < rhs.GetMonth();
        }
    
        return lhs.GetYear() < rhs.GetYear();
    };