print_calendar

/*
 Given a year, print the calendar. The output is like:Year: 2013, Month: August, Day: 22, Date of week: Thursday
*/
# include <stdio.h>

int main(){
	// input
	int y;
	printf("Please input a year: ");
	scanf("%d", &y);
    
	// check input
	if(y < 1){
		printf("Invalid input year.\n");
		return -1;
	}
    
	// output
	int dom_leap[12] = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
	int dom_nonleap[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
	char * map_m[] = {"January", "February", "March", "April", "May", "June", "July","August", "September", "Octobor", "November", "December"};
	char * map_dow[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
	int i, j, count;
	count = 1; // note: start from 1
	for(i = 0; i < 12; i++){
		// leap year
		if((y%4 == 0 && y%100 != 0) || y%400 == 0){
			for(j = 0; j < dom_leap[i]; j++){
				// output: year
				printf("Year: %d, ", y);
				// output: month
				printf("Month: %s, ", map_m[i]);
				// output: day
				printf("Day: %d, ", j + 1);
				// output: dow
				printf("Date of week: %s \n", map_dow[((y-1)*365 + (y-1)/4 - (y-1)/100 + (y-1)/400 + count) % 7]); // note: formula
				count++;
			}
		}
		// nonleap year
		else{
			for(j = 0; j < dom_nonleap[i]; j++){
				// output: year
				printf("Year: %d, ", y);
				// output: month
				printf("Month: %s, ", map_m[i]);
				// output: day
				printf("Day: %d, ", j + 1);
				// output: dow
				printf("Date of week: %s \n", map_dow[((y-1)*365 + (y-1)/4 - y/100 + y/400 + count) % 7]);
				count++;
			}
		}
		printf("\n");
	}
}


REMARK:
1. The important and hard thing is to calculate what day is is today, like Monday, Tuesday ... Sunday. The key is to remember to formula: 

char * map_dow[] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};
count = 1; // note: start from 1
map_dow[((y-1)*365 + (y-1)/4 - (y-1)/100 + (y-1)/400 + count) % 7]);