day_of_week.txt

/*
 22. get a date(mm/dd/yyyy) from user. Print exact the week of dates(Sun to Sat). 
EXAMPLE:
INPUT: Please input a date (dd/mm/yyyy): 03/04/2012
OUTPUT: 
1/4/2012/Sunday
2/4/2012/Monday
3/4/2012/Tuesday
4/4/2012/Wednesday
5/4/2012/Thursday
6/4/2012/Friday
7/4/2012/Saturday 
*/

#include <iostream>
using namespace std;

/*getndays() get the number of days in a month depending on if it's leap year or nonleap year*/
int getndays(int month ,int year){
    int ndaysOfMonth_leap[12] = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    int ndaysOfMonth_nonleap[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
	
    // leap year
    if((year%4 == 0 && year%100 != 0) || year%400 == 0)
        return ndaysOfMonth_leap[month - 1];
    // nonleap year
    else
        return ndaysOfMonth_nonleap[month - 1];
}

/*getndays(int month, int day, int year): gets the number of days since 01/01/year.*/
int getndays(int month, int day, int year){
    int ndays=0;
    for (int i=1; i<month; ++i){
        ndays += getndays(i,year);//it's not "ndays += getndays(i-1,year)"
    }
    ndays += day;
    return ndays;
}

int main(){
    int month, day, year;
    cout<<"Input month, day and year: "<<endl;
    cin>>month>>day>>year;
    if (year<0){
        cout<<"wrong year"<<endl;
        return -1;
    }
    if (month<1 || month>12){
        cout<<"wrong month"<<endl;
        return -1;
    }
    if (day<1 || day>getndays(month,year)){
        cout<<"wrong day"<<endl;
        return -1;
    }
    
    /*dow = days of week = {Sunday, Monday, Tuesday, ... , Saturday}, we need to find what day is today. i.e. dow*/
    string downame[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
    int dow = ((year-1)*365 + (year-1)/4 - (year-1)/100 + (year-1)/400 + getndays(month, day, year)) % 7;
    cout<<"The day of week is: "<<dow<<endl;
    
    /*Now we need to find the starting day of the week, i.e. the first sunday.*/
    for (int i=dow-1; i>=0; --i){
        if (day==1){
            if (month==1){
                year--;
                month=12;
                day = getndays(month,year);
            }
            else{
                month--;
                day = getndays(month,year);
            }
        }
        day--;
    }
    
    /*Now print the whole week*/
    for (int i=0; i<7; ++i){
        cout<<"mmddyyyy:"<<month<<day<<year<<" "<<downame[i]<<endl;
        if (day==getndays(month,year)){
            if (month==12){
                year++;
                month=1;
                day=1;
            }
            else{
                month++;
                day=1;
            }
        }
        day++;
    }
    return 0;
}



REMARK:
1. Understand the meaning of the problem. The corner cases should be paid much attention.
2. Remember how to calulate what day it is today(i.e. the formula), given mmddyyyy.