Thread: Need some math help

Hidy ho everyone,

I found this code on how to break apart a number and store it in an array online. Quite frankly, I still don't understand how it works.

	int input = Integer.parseInt(JOptionPane.showInputDialog("Please enter a 4 digit number"));
 
        int[] numbers;
 
        numbers = new int[4];
 
        for (int i = 3; i > -1; i--)
        {
            numbers[i] = (input % 10);
            input = input /10;
        }

now I'm using this code again in another homework program where the user has to input a 4 digit number and I have to break that number apart. This program is about encryption.

What I have to do is replace each digit with the result of adding 7 to the digit and getting the remainder after dividing the new value by 10. Then swap the first digit with the third, and swap the second digit with the fourth. Then print the encrypted integer (copied and pasted from my ebook text).

now I can add 7 to everything in the array, but I don't understand what its asking me to do with getting the remainder after dividing by 10. That's where I'm having my difficulty.

Did you try it? What happened?

To see how to use the modulus operator, write a very small program that uses it and print out the results. Experiment some by changing the numbers and running it again. Try it several times to see what happens.

Did you write the test program I suggested so you could see the results with different numbers?
Greg has provided you with several samples of what happens.

oh, so its just a way of separating the right most number from the left most?

edit: also, i'm not quite sure what to write for that test program.

Would it be something like:

int input, remainder

scanner.in.input

remainder = input % 10

println("Input: " + input)
println("Remainder: " + remainder)

that's probably a lot of wrong code. I was just spit balling.

2nd Edit:

I think I'm getting the hang of this. I was using that % thing just to separate the right most number from the rest.

I started messing around with the program I said above, just done correctly.

        Scanner in = new Scanner(System.in);
 
            int input = in.nextInt();
 
            int remainder = input % 10;
 
            System.out.println("Input: " + input);
            System.out.println("Remainder: " + remainder);

It looks like that % operator doesn't actually affect the initial value of the number. Input stays the same, no matter what you put in there, and remainder is just the last digit of the number you enter.

Now I have another question. That homework I have pasted above said I need to encrypt a 4 digit number via that math process. I have the individual digits stored in an array. My question is, can I combine those numbers back into a single 4 digit int?

what to write for that test program.

Yes something like that with 2-4 lines. No Scanner needed.
The print out should show you want the % operator does with 2 numbers. Don't always use % 10.
Change all the numbers, compile and execute several times to see what the results are with different values.

The divisor does not have to be 10:
3 % 5 = 3
5 % 3 = 2

Making a test program would allow you to try many combinations to see what % does.

Don't print a new line ("\n") at the end of the print out will leave the cursor on the same line.
Or use the print() method instead of the println() method.

Can you make a simple list of the operations that the code is doing?

last part is a little glossed over

Then it's not worth much when trying to design the code to write.

Which part are you trying to reverse?

That's a paragraph, not a list of steps.
Something like this
declare a local integer
initialize it to the iteration of the loop plus 7
assign the iteration of the loop to the temp value
of the remainder after dividing by 10 ???? Not sure where this goes
reset the value of the local temp variable to zero.

Can you correct my attempt at a list of the steps the code is doing now so we have something to work from towards reversing the steps.

Perhaps if you made an example it would help>
For example with the steps of 1) add 4 and 2) multiply by 7
x = 2
x = x+4 >> 6
x = x*7 >> 42

then the reverse would be
From above x = 42
x = x/7 >> 6
x = x - 4 >> 2

The >> pointed to the results that would be in x. It was not meant as a java operator.

In your example what would be the values in the array at the end of each of the steps?
What math steps would need to be done to "undo" the % operator? Make a list of numbers that are possible values in this program (7-16), apply % to them and then look at what would need to be done to get back the original numbers.
For example:
7 % 10 = 7 Nothing needed to restore 7
16 % 10 = 6 add 10 to get 16