How to make this calender program simpler/optimized?

Question

I've made a working calendar program, BUT I believe it's extremely inefficient. I think I have way too many else if statements:

import java.util.*;
public class MyOwnCalendar {
 public static void main(String[] args) {
 Scanner keyboard = new Scanner(System.in);
 System.out.print("Enter the number of days in month: ");
 int days = keyboard.nextInt();
 System.out.print("Enter which day the first day is on: ");
 int firstDay = keyboard.nextInt();
 System.out.println("S M T W Th F Sa");
 int ctr = 1;
 if (days == 31 && firstDay == 1)
 {
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 8 || ctr == 15 || ctr == 22 || ctr == 29)
 {
 System.out.println();
 }
 }
 }
 else if (days == 31 && firstDay == 2)
 {
 System.out.print("\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 7 || ctr == 14 || ctr == 21 || ctr == 28)
 {
 System.out.println();
 }
 }
 }
 else if (days == 31 && firstDay == 3)
 {
 System.out.print("\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 6 || ctr == 13 || ctr == 20 || ctr == 27)
 {
 System.out.println();
 }
 }
 }
 else if (days == 31 && firstDay == 4)
 {
 System.out.print("\t\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 5 || ctr == 12 || ctr == 19 || ctr == 26)
 {
 System.out.println();
 }
 }
 }
 else if (days == 31 && firstDay == 5)
 {
 System.out.print("\t\t\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 4 || ctr == 11 || ctr == 18 || ctr == 25)
 {
 System.out.println();
 }
 }
 }
 else if (days == 31 && firstDay == 6)
 {
 System.out.print("\t\t\t\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 3 || ctr == 10 || ctr == 17 || ctr == 24 || ctr == 31)
 {
 System.out.println();
 }
 }
 }
 else if (days == 31 && firstDay == 7)
 {
 System.out.print("\t\t\t\t\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 2 || ctr == 9 || ctr == 16 || ctr == 23 || ctr == 30)
 {
 System.out.println();
 }
 }
 }
 else if (days == 30 && firstDay == 1)
 {
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 8 || ctr == 15 || ctr == 22 || ctr == 29)
 {
 System.out.println();
 }
 }
 }
 else if (days == 30 && firstDay == 2)
 {
 System.out.print("\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 7 || ctr == 14 || ctr == 21 || ctr == 28)
 {
 System.out.println();
 }
 }
 }
 else if (days == 30 && firstDay == 3)
 {
 System.out.print("\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 6 || ctr == 13 || ctr == 20 || ctr == 27)
 {
 System.out.println();
 }
 }
 }
 else if (days == 30 && firstDay == 4)
 {
 System.out.print("\t\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 5 || ctr == 12 || ctr == 19 || ctr == 26)
 {
 System.out.println();
 }
 }
 }
 else if (days == 30 && firstDay == 5)
 {
 System.out.print("\t\t\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 4 || ctr == 11 || ctr == 18 || ctr == 25)
 {
 System.out.println();
 }
 }
 }
 else if (days == 30 && firstDay == 6)
 {
 System.out.print("\t\t\t\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 3 || ctr == 10 || ctr == 17 || ctr == 24 || ctr == 31)
 {
 System.out.println();
 }
 }
 }
 else if (days == 30 && firstDay == 7)
 {
 System.out.print("\t\t\t\t\t\t");
 while (ctr <= days)
 {
 System.out.print(ctr);
 System.out.print(" ");
 ctr++;
 if (ctr == 2 || ctr == 9 || ctr == 16 || ctr == 23 || ctr == 30)
 {
 System.out.println();
 }
 }
 }
 }
}

What are some easier ways to do this? Would a for loop have made this easier?

Answer 1

This is a classic example of where the Modulo operator is useful.

The modulo allows you to determine where in a sequence you hit a repeating part of the pattern. For example, you are processing thousands of lines of data, so you want to log some output, you print a message every 1000 lines with:

if (count % 1000 == 0) {
 System.out.println("Processing line " + count);
}

We can use the modulo to our advantage with weeks having 7 days....

So, if we do something like:

for (int i = 1; i <= daysinmonth; i++) {
 System.out.print(" " + i);
}

we will just get numbers printed accross the screen... but, if we do:

for (int i = 1; i <= daysinmonth; i++) {
 System.out.print(" " + i);
 if (i % 7 == 0) {
 System.out.println();
 }
}

then the numbers will wrap neatly.

Now, if you take that example, and extend it to start at a certain day of the week, you can do:

// note the start-at-1 offset.
for (int i = 1; i < firstDay; i++) {
 System.out.print(" "); // some spaces for the starting days.
}

now we do the same loop as before, but we offset our line-break:

for (int i = 1; i <= days; i++) {
 System.out.print(" " + i);
 if ((i + firstDay) % 7 == 0) {
 System.out.println();
 }
}

Using this logic you should be able to bring your code down to just two simple loops, a padding loop, and a number loop.