On what days does the thirteenth occur? USACO

Question

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.

Here's what I have:

#include <iostream>
#include <fstream>
#include <string>
#include <vector>

using namespace std;
int main(){
    ifstream fin("fridayin.txt");
    ofstream fout("fridayout.txt");
    int N;
    fin >> N;
    int current_year, end_year = 1900 + N - 1, current_day = 1; //set current_year, end_year, and current_day to 1(Monday)
    int daycounter[7] = { 0 };  //this will record how many times a day occurs on the 13th
    int current_month = 1;
    int day;
    for (current_month = 1; current_month <= 12; current_month++){
        for (current_year = 1900; current_year <= end_year; current_year++){    //jan 13=saturday
            int yp = current_year - 1900;
            if (current_year < 2000){   //2000 is a leap year 
                day = (6 + yp + yp / 4 - yp / 100) % 7;
                daycounter[day]++;  //increment the day counter
            }
            else if (current_year > 2000){  //check if it's after 2000, if it is add 1 to 6 to get 0 (mod 7)
                day = (yp + yp / 4 - yp / 100) % 7;
                daycounter[day]++;  //increment the day counter
            }
        }
    }
    int i;
    for (i = 0; i < 7; i++){
        fout << daycounter[i] << ' ';
    }


    return 0;
}

I'm computing the January 13ths then the February 13ths,... December 13ths.

Here's input:

Correct output:

36 33 34 33 35 35 34

My output:

48 36 36 24 24 36 36

I think I know what's wrong, since January 13th, 1900 is a Saturday I made it 6 mod 7 but that's not true for February 13th, 1900 and the other months. I'd have to change the equations and create an if statement but that'd be extremely long.

Don't use handcrafted C++ for this for the love of god. Time is one of the most PITA things to handle without solid library. [See this](http://stackoverflow.com/questions/6841333/why-is-subtracting-these-two-times-in-1927-giving-a-strange-result) to know what I'm talking about. — rr-, Jul 05 '15 at 14:30
Are you properly accounting for all leap years? Do you know how to define a leap year? Where is the function to check for one? — duffymo, Jul 05 '15 at 14:31
@rr- This is a competition problem, weird (but very valid in the real world) DateTime issues are ignored. — Aryaman, Jul 19 '18 at 20:21

duffymo · Answer 1 · 2015-07-05T16:32:51.440

Here's a Java implementation:

package time;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.LinkedHashMap;
import java.util.Map;

/**
 * Superstition calculates how frequently the 13th falls on each day of the week
 * @author Michael
 * @link https://stackoverflow.com/questions/31231343/on-what-days-does-the-thirteenth-occur-usaco
 * @since 7/5/2015 10:31 AM
 */
public class Superstition {

    public static final DateFormat DEFAULT_FORMAT = new SimpleDateFormat("yyyy-MMM-dd");
    public static final int DEFAULT_MAX_YEARS = 400;
    public static final String START_DATE = "1900-Jan-13";
    public static final int MONTHS_PER_YEAR = 12;


    public static void main(String[] args) {
        Map<Integer, Integer> frequencies = new LinkedHashMap<Integer, Integer>() {{
            put(Calendar.SUNDAY, 0);
            put(Calendar.MONDAY, 0);
            put(Calendar.TUESDAY, 0);
            put(Calendar.WEDNESDAY, 0);
            put(Calendar.THURSDAY, 0);
            put(Calendar.FRIDAY, 0);
            put(Calendar.SATURDAY, 0);
        }};
        try {
            int maxYears = args.length > 0 ? Integer.parseInt(args[0]) : DEFAULT_MAX_YEARS;
            Calendar calendar = Calendar.getInstance();
            calendar.setTime(DEFAULT_FORMAT.parse(START_DATE));
            for (int i = 0; i < maxYears; ++i) {
                for (int j = 0; j < MONTHS_PER_YEAR; ++j) {
                    int dayOfWeek = calendar.get(Calendar.DAY_OF_WEEK);
                    frequencies.put(dayOfWeek, (frequencies.get(dayOfWeek) + 1));
                    calendar.add(Calendar.MONTH, 1);
                }
            }
        } catch (ParseException e) {
            e.printStackTrace();
         } finally {
            System.out.println(frequencies);
        }
    }
}

Here's the output for the years from 1900 to 2300:

com.intellij.rt.execution.application.AppMain time.Superstition
{1=687, 2=685, 3=685, 4=687, 5=684, 6=688, 7=684}

Process finished with exit code 0

As expected, the frequencies with which the 13th falls on each day of the week are roughly the same. Sum of values equals (# years)*(12 months per year), as it should be.

On what days does the thirteenth occur? USACO

1 Answers1

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