FRQ Answers: CSA 2019 AP Exam

Write the static method numberOfLeapYears, which returns the number of leap years between year1 and year2, inclusive

Write the static method dayOfWeek, which returns the integer value representing the day of the week for the given date

All helper methods are included below for running.

public class years {
    private static boolean isLeapYear(int year){
        if(year % 4 == 0){ //conditional to check if year is divisible by 4. All years divisible by 4 are leap years
            if(year % 100 == 0){ //conditional to check if year is divisible by 100. All years divisible by 100 are leap years
                if(year % 400 == 0){ //conditional to check if year is divisible by 400. All years divisible by 400 are leap years
                    return true;
                }
                else{
                    return false;
                }
            }
            else{
                return true;
            }
        }
        else{ //if none of these conditions are true, the year is not a leap year.
            return false;
        }
    }
    
     /** Returns the number of leap years between year1 and year2, inclusive.
     * Precondition: 0 <= year1 <= year2
     */
    private static int numberOfLeapYears(int year1, int year2){
        int count = 0; //the count that will be returned
        for(int i = year1; i <= year2; i++){ //loops through the years provided
            if(isLeapYear(i) == true){
                count++; //if the year is a leap year, add 1 to the count
            }
        }
        return count; //return the count
    }

    //Finds the day of the week for first day of the year
    private static int firstDayOfYear(int year){
        int first1950 = 0; //day of the week in 1950
        int day = first1950;
        //find the difference between the year and 2000
        int difference = year - 1950;
        //for every integer increase in the difference, add 1 to the day.
        for(int i = 0; i < difference; i++){
            day++;
            if(isLeapYear(1950 + i) == true){
                day++;
            }
        }
        if(day > 6){
            day = day % 7;
        }
        return day;
    }
    private static int dayOfYear(int month, int day, int year){
        int dayOfYear = 0;
        for(int i = 1; i < month; i++){
            if(i == 1 || i == 3 || i == 5 || i == 7 || i == 8 || i == 10 || i == 12){ //adding 31 if month has 31 days
                dayOfYear += 31;
            }
            else if(i == 4 || i == 6 || i == 9 || i == 11){ //adding 30 if month has 30 days
                dayOfYear += 30;
            }
            else if(i == 2){ //checking if the year is a leap year during the feburary iteration
                if(isLeapYear(year) == true){
                    dayOfYear += 29;
                }
                else{
                    dayOfYear += 28;
                }
            }
        }
        dayOfYear += day;
        return dayOfYear;
    }

    private static int dayOfWeek(int month, int day, int year){
        int dayOfWeek = 0;
        int firstDay = firstDayOfYear(year);
        int dayOfYear = dayOfYear(month, day, year) - 1;
        dayOfWeek = (firstDay + dayOfYear) % 7;
        return dayOfWeek;
    }
    
}