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.