Is Friday the 13th really an unusual event?
That is, does the 13th of the month land on a Friday less often than on any other day of the week? To answer this question, write a program that will compute the frequency that the 13th of each month lands on Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday over a given period of N years. The time period to test will be from January 1, 1900 to December 31, 1900+N-1 for a given number of years, N. N is positive and will not exceed 400.
Note that the start year is NINETEEN HUNDRED, not NINETEEN NINETY.
There are few facts you need to know before you can solve this problem:
Do not use any built-in date functions in your computer language.
Don’t just precompute the answers, either, please.
One line with the integer N.
20
Seven space separated integers on one line. These integers represent the number of times the 13th falls on Saturday, Sunday, Monday, Tuesday, …, Friday.
36 33 34 33 35 35 34
Follow the facts and compute the answer. Define some constants and helper functions to make the code more readable.
public class friday {
// months
private static final int JANUARY = 0;
private static final int FEBRUARY = 1;
private static final int MARCH = 2;
private static final int APRIL = 3;
private static final int MAY = 4;
private static final int JUNE = 5;
private static final int JULY = 6;
private static final int AUGUST = 7;
private static final int SEPTEMBER = 8;
private static final int OCTOBER = 9;
private static final int NOVEMBER = 10;
private static final int DECEMBER = 11;
// days of week
private static final int MONDAY = 0;
private static final int TUESDAY = 1;
private static final int WEDNESDAY = 2;
private static final int THURSDAY = 3;
private static final int FRIDAY = 4;
private static final int SATURDAY = 5;
private static final int SUNDAY = 6;
// number of days in a week
private static final int DAYS_IN_WEEK = 7;
private boolean isLeapYear(int year) {
// special case for century year
if (year % 100 == 0) {
return (year % 400 == 0);
}
// check if year is divisible by 4
return (year % 4 == 0);
}
private int getDaysInMonth(int month, int year) {
switch (month) {
case APRIL:
case JUNE:
case SEPTEMBER:
case NOVEMBER:
return 30;
case JANUARY:
case MARCH:
case MAY:
case JULY:
case AUGUST:
case OCTOBER:
case DECEMBER:
return 31;
case FEBRUARY: {
if (isLeapYear(year)) {
return 29;
} else {
return 28;
}
}
default:
return 0;
}
}
private int getDayOfWeek(int startingDayOfWeek, int daysToIncrement) {
// 0 = monday ... 6 = sunday
return (startingDayOfWeek + (daysToIncrement - 1)) % DAYS_IN_WEEK;
}
public static void main(String[] args) throws IOException {
try (final BufferedReader f = new BufferedReader(new FileReader("friday.in"));
final PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("friday.out")))) {
// index 0 = monday ... 6 = sunday
final int[] days = new int[7];
// parse the number of years
final int N = Integer.parseInt(f.readLine());
// instantiate app
final friday app = new friday();
// calculate
int dayOfWeekForFirst = 0;
for (int year = 1900; year < (1900 + N); year++) {
for (int month = 0; month < 12; month++) {
int daysInMonth = app.getDaysInMonth(month, year);
int dayOfWeekForThirteen = app.getDayOfWeek(dayOfWeekForFirst, 13); // advance to the 13th day
days[dayOfWeekForThirteen] += 1;
// advance to first day of next month
dayOfWeekForFirst = app.getDayOfWeek(dayOfWeekForFirst, daysInMonth + 1); // +1 for 1st day of new month
}
}
// output
out.print(days[SATURDAY] + " ");
out.print(days[SUNDAY] + " ");
out.print(days[MONDAY] + " ");
out.print(days[TUESDAY] + " ");
out.print(days[WEDNESDAY] + " ");
out.print(days[THURSDAY] + " ");
out.println(days[FRIDAY]);
}
}
}
Link To: Java Source Code