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Thread: Multiplying BigNum

  1. #1
    Junior Member
    Join Date
    Nov 2020
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    Question 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++)
        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);
            return e;
        public static void main(String[] args) { 
        BigNum a = new BigNum(123); 
        BigNum b = new BigNum(512); 
        BigNum c = a.add(b);
        BigNum d = a.multiply(b); 
    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 !!!

  2. #2
    Super Moderator Norm's Avatar
    Join Date
    May 2010
    Eastern Florida
    Thanked 2,635 Times in 2,585 Posts

    Default Re: Multiplying BigNum

    Can you explain the logic for the multiply method?

    The posted code has compiler errors. Please fix them so the code can be compiled and executed for testing.
    If you don't understand my answer, don't ignore it, ask a question.

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