Incrementing dates in a loop

Question

This is an exercise; I'm forbidden to use built-in date function of C++ or any readily available library.

I have this code:

#include <iostream>
#include <vector>

using namespace std;


bool IsLeap(unsigned int  year)
{
    if (year % 4 != 0) return false;
    else
        if (year % 100 != 0) return true;
        else
            if (year % 400 != 0) return false;
            else
                return true;
}

class Date
{
public:
    unsigned short int Day;
    unsigned short int Month;
    unsigned short int Year;
    unsigned short int DayOfWeek;
};

int MonthLimit(int Month, int year)
{
    switch (Month)
    {
    case 4:
    case 6:
    case 9:
    case 11:
    {
        return 30;
        break;
    }
    case 1:
    case 3:
    case 5:
    case 7:
    case 8:
    case 12:
    {
        return 31;
        break;
    }
    case 2:
        if (IsLeap(year))
        {
            return 29;
            break;
        }
        else
        {
            return 28;
            break;
        }

    }
}



const Date FirstDayEver = { 1, 1, 1900, 1 };  //January 1, 1900 was on a Monday.

int main()
{
    unsigned int years;
    cin >> years;

    vector<int> counters(7);   //whenever a 13th falls on a day X, we increment the Xth counter. It's that simple


    for (Date i = FirstDayEver; !((i.Day == 31) && (i.Month == 12) && (i.Year == 1900 + years - 1)); i.Day++)
    {
        i.DayOfWeek = (i.DayOfWeek + 1) % 7;
        if (i.Month == MonthLimit(i.Month, i.Year))
        {
            i.Month++;
            i.Day = 1;
            cout << "Proceeded to " << i.Day << "." << i.Month << "." << i.Year << "\n";
        }
        if ((i.Day == 31) && (i.Month == 12))
        {
            i.Year++;
            i.Day = 1;
            i.Month = 1;
            cout << "Proceeded to " << i.Day << "." << i.Month << "." << i.Year << "\n";
        }

        if (i.Day == 13)
        {
            counters[i.DayOfWeek]++;
            cout << i.Day << "." << i.Month << "." << i.Year << " was a " << i.DayOfWeek << "\n";
        }
    }

    cout << counters[6] << " " << counters[7] << " " << counters[1] << " " << counters[2] << " " << counters[3] << " " << counters[4] << " " << counters[5] << "\n";

    exit(0);
}

The debug info (couts) is there to see whether anything happens in the loop. So far the only thing that happens is a lot of lines saying "13.1.1900 was a " varying number. I think the desired logic of incrementing a date is rather clear from the code, but I'm trying to increment something wrong. Where's my mistake?

Answer 1

First of all,

if (i.Month == MonthLimit(i.Month, i.Year))

is comparing the month with the number of days in the month.
Since the first month is 1, and that month has 31 days, you will never increment the month.
Since you never increment the month, you will never increment the year.
(You would probably have spotted this easier if you had named the function "DaysInMonth", or something else that makes it clearer what it returns.)

You want

if (i.Day == MonthLimit(i.Month, i.Year))

Second, MonthLimit is missing a case for October.

Answer 2

You're complicating your increment procedure too much.
I'm actually still trying to work out exactly how it all fits together (edge cases etc) but it seems that you tried to do each day/month/year independantly.

You can do all your incrementing in a simple function like this:

void increment_Date(Date &d)
{
    d.DayOfWeek = (d.DayOfWeek + 1) % 7; //increase weekday
    if (++d.Day > MonthLimit(d.Month, d.Year)//increase day, if larger than month size...
    {
        d.Day = 1;                           //reset day to 1
        if (++d.Month > 12)                  //increase month, if larger than year size
        {
          d.Month = 1;                       //reset month to 1
          d.Year++;                          //increase year
        }
    }
}

The logic is simpler because I approached it in a slightly different way than you seem to have done.
In the function above, I increase the smallest unit first (day), check for overflow and then move up to month if needed (and so on).

This works in the same way that a car's mileage dial's digits only tick up when the dial reaches 10 and goes back to 0. Or, adding 1 to 9 and 'overflowing' into the 10's digit of a number.

Otherwise your code is good. The Switch statement for MonthLimit was a good choice. I'd add a switch statement in a function to return a string for day of week:

std::String printable_DOW(int DOW)
{
    switch (DOW)
    case 0:
        return "Sunday";
    case 1:
        return "Monday";
    case 2:
        return "Tuesday";
    case 3:
        return "Wednesday";
    case 4:
        return "Thursday";
    case 5:
        return "Friday";
    case 6:
        return "Saturday";
}

and a print_date function too:

void print_date(Date d)
{
    std::cout<< printable_DOW(d.DayOfWeek) 
             << ", " << d.Day << "." << d.Month << "." d.Year;
}

and it's pretty easy to put it all together and use too:

int main
{

    Date my_date;
    my_date.Day = 1;
    my_date.Month = 1;
    my_date.Year = 2000;
    my_date.DayOfWeek = 0;  //assuming 0 == Sunday, 1 = Monday...
    for (int daycount = 0; daycount < 5114 /*days since 1/1/2000 to today*/; daycount++)
    {
        increment_date(my_date);
        //debug:
        print_date(my_date);
    }
    //print "today's" date:
    print_date(my_date);
}

As a class: It's mostly here and (mostly) works. Feel free to edit people!

class date
{
private:
    unsigned short int day;
    unsigned short int month;
    unsigned short int year;
    unsigned short int day_of_week;
    int month_limit();
    bool is_leap_year();
public:
    date();
    date(int in_day, int in_month, int in_year, int in_day_of_week);
    date& date::operator=(date rhs)
    date& operator++();    //pre-increment (++date)
    date operator++(int);  //post-increment (date++)

    //these are "special" they don't work exactly as you might think and are slightly broken right now,
    //but I'm putting them in regardless
    //they also don't work on DOW right now.
    date& operator+=(const date& rhs)               
    date& operator-=(const date& rhs)
    inline date operator+(date lhs, const date& rhs)
    inline date operator-(date lhs, const date& rhs)
}
//phwew

date::date(int in_day, int in_month, int in_year, int in_day_of_week)
{
    this->day = in_day;
    this->month = in_month;
    this->year = in_year;
    this->day_of_week = in_doay_of_week;
}

date::month_limit()
{
    switch (this->month)
    {
    case 4:
    case 6:
    case 9:
    case 11:
    {
        return 30;
        break;
    }
    case 1:
    case 3:
    case 5:
    case 7:
    case 8:
    case 12:
    {
        return 31;
        break;
    }
    case 2:
        if (is_leap_year(this->year))
        {
            return 29;
            break;
        }
        else
        {
            return 28;
            break;
        }

    }
}

bool is_leap_year()
{
    if (this->year % 4 != 0) return false;
    else
        if (this->year % 100 != 0) return true;
        else
            if (this->year % 400 != 0) return false;
            else
                return true;
}

date& date::operator=(date rhs)
{
  swap(rhs);
  return *this;
}

date& date::operator++()
{
    this->day_of_week = (this->day_of_week + 1) % 7; //increase weekday
    this->day++;
    if (++(this->day) > month_limit())           //increase day, if larger than month size...
        {
            this->day = 1;                           //reset day to 1
            if (++this->month > 12)                  //increase month, if larger than year size
            {
              this->month = 1;                       //reset month to 1
              this->year++;                          //increase year
            }
        }
    return *this;
}

date date::operator++(int)
{
    date tmp(*this);
    operator++();
    return tmp;
}

//adds years on, then months, then days
date& date::operator+=(const date& rhs)
{
    this->year  += rhs.year;
    this->month += rhs.month;
    this->day   += rhs.day;
    if (this->month > 12)  //get to the right month
    {
        this->year  = this->month / 12;
        this->month = this->month % 12;
    }
    if (this->day > month_limit())
    {
        this->month = this->day / month_limit();
        this->day   = this->day % month_limit();
        if (this->month > 12)  //recalculate  **yes, I know this is currently wrong if more than one month is added on in days**
        {
            this->year  = this->month / 12;
            this->month = this->month % 12;
        }
    }
    return *this;
}

inline date date::operator+(date lhs, const date& rhs)
{
  lhs += rhs;
  return lhs;
}

//subtracts years, then months, then days
date& date::operator-=(const date& rhs)
{
    if ((rhs.year < this->year) || ((rhs.year == this->year) && (rhs.month < this->month))
       || (((rhs.year == this->year) && (rhs.month == this->month)) && (rhs.day < this->day)
    {
        swap(rhs);
    }
    this->year -= rhs.year;
    this->month -= rhs.month;
    this->day -= rhs.day;

    return *this;
}

inline date date::operator+(date lhs, const date& rhs)
{
  lhs += rhs;
  return lhs;
}