Day of the Week in C++

Suppose we have a date (day, month and year). From this date, we have to find the day of the week of that given date. To solve this we will use Zeller’s Algorithm. The formula to find weekday using Zeller’s Algorithm is here

𝑤=$$\lgroup d+\lfloor \frac{13(m+1)}{5} \rfloor+y+\lfloor\frac{y}{4} \rfloor+\lfloor\frac{c}{4} \rfloor+5c \rgroup mod 7$$

The formula is containing some variables; They are −

d − The day of the date.

m − It is the month code. For March to December, it is 3 to 12, for January it is 13, and for February it is 14. When we consider January or February, then the given year will be decreased by 1.

y − Last two digits of the year

c − first two digits of the year

w − The weekday. When it is 0, it is Saturday, when it is 6, it means Friday

So for example, if we want to get the weekday of 4th January 1997, then the output will be “Saturday”

The algorithm is as follows −

Algorithm

zellersAlgorithm(day, month, year)

Input − The date of the day.

Output − Which day it was, (Sunday to Saturday).

Begin
   if month > 2, then
      mon := month
   else
      mon := 12 + month
      decrease year by 1
   y := last two digit of the year
   c := first two digit of the year
   w := day + floor((13*(mon+1))/5) + y + floor(y/4) + floor(c/4) + 5*c
   w := w mod 7
   return weekday[w] //weekday will hold days from Saturday to Friday
End

Example (C++)

 Live Demo

#include
#include
using namespace std;
string weekday[7] = {"Saturday","Sunday","Monday","Tuesday", "Wednesday","Thursday","Friday"};
string zellersAlgorithm(int day, int month, int year){
   int mon;
   if(month > 2)
      mon = month; //for march to december month code is same as month
   else{
      mon = (12+month); //for Jan and Feb, month code will be 13 and 14
      year--; //decrease year for month Jan and Feb
   }
   int y = year % 100; //last two digit
   int c = year / 100; //first two digit
   int w = (day + floor((13*(mon+1))/5) + y + floor(y/4) + floor(c/4) + (5*c));
   w = w % 7;
   return weekday[w];
}
int main(){
   int day, month, year;
   cout << "Enter Day: "; cin >>day;
   cout << "Enter Month: "; cin >>month;
   cout << "Enter Year: "; cin >>year;
   cout << "It was: " <<zellersAlgorithm(day, month, year);
}

Input

(4, 1, 1997)

Output

Enter Day: 4
Enter Month: 1
Enter Year: 1997
It was: Saturday