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Added exercise 6.34 (Print Calculator adapted)
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‎Exercise_06/Exercise_06_34/PrintCalendar.class‎

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‎Exercise_06/Exercise_06_34/PrintCalendar.java‎

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import java.util.Scanner;
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/*
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Adapted by Sleekpanther from SquirrelCoder's initial idea from 29-Jan-17.
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Uses arrays, even though they technically aren't introduced until chapter 7. But it simplifies this a lot
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6.34 (Print calendar) Programming Exercise 3.21 uses Zeller's congruence to calculate
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the day of the week. Simplify Listing 6.12, PrintCalendar.java, using Zeller's
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algorithm to get the start day of the month.
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Exercise 3.21 details
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(Science: day of the week) Zeller's congruence is an algorithm developed by
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Christian Zeller to calculate the day of the week. The formula is:
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h=(q + (26(m+1))/10 + k + k/4 + j/4 +5*j)%7
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h is the day of the week (0: Saturday, 1: Sunday, 2: Monday, 3: Tuesday, 4: Wednesday, 5: Thursday, 6: Friday).
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q is the day of the month.
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m is the month (3: March, 4: April, ..., 12: December). January and February are counted as months 13 and 14 of the previous year.
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j is the century (i.e. year/100)
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k is the year of the century (i.e., year % 100).
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Note that the division in the formula performs an integer division. Write a program
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that prompts the user to enter a year, month, and day of the month, and
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displays the name of the day of the week.
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(Hint: January and February are counted as 13 and 14 in the formula, so you need
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to convert the user input 1 to 13 and 2 to 14 for the month and change the year to the previous year.)
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*/
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public class PrintCalendar {
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public static void main(String[] args) {
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Scanner keyboard = new Scanner(System.in);
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// get & validate user year
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System.out.print("Enter full year (e.g., 2012): ");
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int year = keyboard.nextInt();
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while (!isValidYear(year)) { //validate input
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System.out.println("Invalid Year!");
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System.out.print("Enter full year (e.g., 2012): ");
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year = keyboard.nextInt();
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}
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// get & validate user month
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System.out.print("Enter month as number between 1 and 12: ");
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int month = keyboard.nextInt();
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while (!isValidMonth(month)) { //validate input
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System.out.println("Invalid Month!");
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System.out.print("Enter month as number between 1 and 12: ");
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month = keyboard.nextInt();
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}
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printCalendarHeader(month, year); // print the calendar header
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printFirstDay(month, year); // print the calendar first day
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printCalendarItself(month, year); // print the calendar itself
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}
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public static boolean isValidYear(int year) {
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return year > 0; //might want to check an upper bound, not sure if this formula works for HUGE numbers
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}
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public static boolean isValidMonth(int month) {
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return month > 0 && month <= 12;
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}
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public static void printCalendarHeader(int month, int year) {
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String[] months = {"January", "February", "March", "April", "May", "June", "July", "August", "Septemter", "October", "November", "December"};
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System.out.print("\t\t"+months[month-1]+"\t"); //access the month array with a -1 offset since arrays count from 0
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System.out.println(year);
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System.out.println("---------------------------");
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System.out.println("Sun\tMon\tTue\tWed\tThu\tFri\tSat");
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}
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public static void printFirstDay(int month, int year) {
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int firstDay = dayOfWeek(1, month, year); //calculate the 1st day
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String leadingTabs = "1"; //Holds any leading tabs to align 1st row of numbers in a calendar. This takes care of firstDay=1
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//cases for firstDay between 2 & 6 (adds a "\t" at the beginning of the string each iteration)
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//Loop starts from 1 since we want 1 less tab than the value of firstDay (firstDay=1 is 0 tabs, firstDay=2 is 1 tab, firstDay=3 is 2 tabs, firstDay=4 is 3 tabs, firstDay=5 is 4 tabs)
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for(int i = 1; i<firstDay; i++){
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leadingTabs = "\t" + leadingTabs;
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}
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if(firstDay == 0){ //reset it & ignore what the loop did if it's 0. THIS IS A SPECIAL CASE. We want 6 tabs
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leadingTabs = "\t\t\t\t\t\t1";
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}
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System.out.print(leadingTabs + "\t");
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}
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public static void printCalendarItself(int month, int year) {
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// find out the last day of that month
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// whether it's 28/29/30/31 days
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int lastDayOfMonth = lastDayOfMonth(month, year);
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// print the calendar itself
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for (int i = 2; i <= lastDayOfMonth; i++) {
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int printedDay = dayOfWeek(i, month, year);
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if (printedDay == 1) {
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System.out.println();
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}
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System.out.print(i + "\t");
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}
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}
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//Implement Zeller's Algorithm
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public static int dayOfWeek(int dayOfMonth, int month, int year) {
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if (month == 1 || month == 2) {
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month = month + 12;
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year--;
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}
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int q, m, j, k;
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q = dayOfMonth;
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m = month; //adjusted month (corrected for January & February being 13 & 14 respectively)
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j = year/100; //century
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k = year%100; //year of the century
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int dayOfTheWeek = (q + (26*(m+1) /10) + k + k/4 + j/4 + (5*j)) % 7; //performs integer division where appropriate (like the Algorithms wants)
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return dayOfTheWeek;
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}
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public static boolean isLeapYear(int year) {
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return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
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}
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public static int lastDayOfMonth(int month, int year) {
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int lastDayOfMonth;
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if (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) {
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lastDayOfMonth = 31;
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} else if (month == 2) {
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if (isLeapYear(year)) {
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lastDayOfMonth = 29;
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} else {
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lastDayOfMonth = 28;
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}
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} else {
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lastDayOfMonth = 30;
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}
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return lastDayOfMonth;
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}
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}

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