Thread: Help for my homework with big numbers and ArrayList

Hello everybody,

I need help for a project in Java. I am a fresh beginner and I do not understand all the whys and hows.
We were asked to create a Java Class BigNum which should enable us to calculate with very large integers, but I am already stuck at the beginning. Here is what I have so far :

import java.util.ArrayList;
public class BigNum {
    public static void main(String[] args) { 
    ArrayList<Integer> number = new ArrayList<Integer>();
    number.add(Integer.valueOf(3));
    number.add(Integer.valueOf(2)); 
    number.add(Integer.valueOf(1)); 
    System.out.println(number);
    }
}

The next step should be adding a method that can operate usign "BigNum n = new BigNum(512);" and producing automatically the ArrayList. Also it is said to use the modulo operator % to perform this task...

I am totally lost, could someone help me with this ?
Thank you for reading this

I do not understand either why we have to use the modulo...

We are supposed on the next question to add up two BigNum and finally to multiply two BigNum.
Thank you very much for your help !

Originally Posted by Norm

You did not answer this question.

Oh, I am sorry. I did not understand your question in that way !
The constructor should only put the number in an ArrayList, but if it is an ArrayList, how could I multiply these two numbers ?
I am sorry that I am so lost...

I wanted to avoid putting my asignement online, but here it is (I have not more infos about what I should do than this precisly) :

Implement the algorithms using Java. Use test cases to verify the results on some data.
In this project we propose to make calculations with very large integers. We assume positive integers. For that we define a BigNum class. This class models an integer as an ArrayList where each of the elements corresponds to a number:

public class BigNum {
ArrayList number;
public: ...}

The first element of the "number" list of the BigNum class always corresponds to units, the second to tens, the third to hundreds, and so on. Since a list is unbounded, we can easily imagine defining very very large integers, ie having thousands of digits.
1.1 Write an operation that allows you to create a BigNum from an integer (of type int) passed as a parameter. For example, the call:

BigNum n = new BigNum (512);

will initialize the "number" list of the class to the sequence 5, 1 and 2 (the first element being 2, the second 1 and the third 5).
Note: use for this successive divisions by 10. In Java, the remainder of an integer division is given by the operator% (modulo). For example, 14% 4 => 2.
1.2 Write a "display" operation which displays the value of a BigNum on the screen.

To get the 2 from 512, I need to use %10. Is this right ?
But then how to extract the 1 and the 5 ?

I may have find the solution with your help, could it be :

int x = 512;
System.out.println(x%10);
int y = 512/10 ;
System.out.println(y%10);
int z = y/10 ;
System.out.println(z%10);

Is it the right logic ? It does give me
2
1
5
!!!

I think I have the answer, but I also think that it is not the straightest way to go :

import java.util.ArrayList;
public class BigNum {
    public static ArrayList<Integer> MakeBigNum(int a) {
        String num = Integer.toString(a);
        System.out.println(num);
        System.out.println(num.length());
        ArrayList<Integer> number = new ArrayList<Integer>();
        number.add(Integer.valueOf(a%10));
        for (int i=1; i<num.length(); i++){
            double c =(a/(Math.pow(10,i)));
            int b = (int) c;
            number.add(Integer.valueOf(b%10));
            System.out.println(b);
        }
        return number;
    }
    public static void main(String[] args) { 
    ArrayList<Integer> n = MakeBigNum(512); 
    System.out.println(n);
    ArrayList<Integer> m = MakeBigNum(1234); 
    System.out.println(m);
    ArrayList<Integer> k = MakeBigNum(54); 
    System.out.println(k);
    }
}

Thank you enormously for your help ! As this homework is due for mid-december, I will continue first with my other assignment and then come back to you if I still have problems for the rest of the homework !

But either way, thank you very much for helping me, I would never be able to do this work by myself

--- Update ---

I will of course leave out all the prints that I made and the examples

Thanks for helping me this much out !
I improved the code, and here is what I ended up with, which works :

public static ArrayList<Integer> MakeBigNum(int a) {
        ArrayList<Integer> number = new ArrayList<Integer>();
        for (int b=a; b>0;){
            number.add(Integer.valueOf(b%10));
             b =(b/10);
        }
        return number;
    }