Thread: Multiplying BigNum

Hello everybody !

I am writing code to create a BigNum class and being able to add it with others and I also have to write it so I can multiply it with others, but I can't seem to get that working... Here is my code :

public class BigNum {
    public static boolean showZero = true; // I am not sure if that is relevant
    public static final int MAX_DIGITS = 40;
    private int[] digits = new int [MAX_DIGITS];
    public BigNum(){
        for (int i=0; i < MAX_DIGITS; i++)
            digits[i]=0;
    }
    public BigNum(int a) {
        for (int i=0; i<MAX_DIGITS; i++){
            int stock = (int) (a %10);
            digits[i] = stock;
            a = a/10;
        }   
    }
    public String show(){
        String s ="";
        for (int i = MAX_DIGITS-1; i>=0; i--){
            if (digits[i] != 0) {
                s = s + digits[i];
            }     
        }
        System.out.println("BigNum = "+s);
        return s;
    }
    public BigNum add(BigNum t){
        BigNum sum = new BigNum ();
        int a = 0;
        for (int i = 0; i< MAX_DIGITS; i++){
            sum.digits[i] = (this.digits[i]+ t.digits[i]+a)%10;
            a = (this.digits[i] + t.digits[i] + a)/10;
        }
        return sum;
    }
    public BigNum multiply(BigNum u){
        BigNum e = new BigNum ();
        for (int i = 0; i< 2; i++){             // the condition i<2 is wrong, but I don't know what to put in else
            BigNum mult = new BigNum ();
            int a = 0;         
            for (int k = 0; k<MAX_DIGITS; k++){
                mult.digits[k] = (this.digits[i] * u.digits[k] + a)%10;
                a = (this.digits[i] * u.digits[k] + a)/10;
            }
            e = e.somme(prod);
            System.out.println("e="+e.affiche());
        }
        return e;
    }
 
    public static void main(String[] args) { 
    BigNum a = new BigNum(123); 
    BigNum b = new BigNum(512); 
    a.show();
    b.show();
    BigNum c = a.add(b);
    c.show(); 
    BigNum d = a.multiply(b); 
    d.show(); 
    }
}

My results for the add method works good, but I can't seem to get the multiply method going... Somehow in the second cycle for the multiply I get 256 and not 1024... Also I don't know how to get the program to add a 0 at the right whenever a new cycle begins...

Thank you for helping me in advance !!!