C Program to find day of week given date

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As reported also by Wikipedia, in 1990 Michael Keith and Tom Craver published an expression to minimise the number of keystrokes needed to enter a self-contained function for converting a Gregorian date into a numerical day of the week.

The expression does preserve neither y nor d, and returns a zero-based index representing the day, starting with Sunday, i.e. if the day is Monday the expression returns 1.

A code example which uses the expression follows:

int d    = 15   ; //Day     1-31
int m    = 5    ; //Month   1-12`
int y    = 2013 ; //Year    2013` 

int weekday  = (d += m < 3 ? y-- : y - 2, 23*m/9 + d + 4 + y/4- y/100 + y/400)%7;

The expression uses the comma operator, as discussed in this answer.

Enjoy! 😉

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