CODEWITHSUNDEEP
javamatrixbiginteger
user10449716
user10449716

Reputation: 13

How can I implement Matrix multiplication with BigInteger?

I'm trying to implement the matrix multiplication algorithm, but I have an issue dealing with BigInteger.

public static void main(String[] args) {
   Scanner scan = new Scanner(System.in);

    System.out.println("Size of Matrix A");
   int tamA1 = scan.nextInt();
   int tamA2 = scan.nextInt();

    System.out.println("Size of Matrix B");
   int tamB1 = scan.nextInt();
   int tamB2 = scan.nextInt();

   BigInteger[][] A = new BigInteger[tamA1][tamA2];
   BigInteger[][] B = new BigInteger[tamB1] [tamB2];
   BigInteger[][] C = new BigInteger[A.length][B[0].length];



    System.out.println("Values of Matrix A");
    for (int i = 0; i < tamA1; i++) {
        for (int j = 0; j < tamA2; j++) {
            A[i][j] = scan.nextBigInteger();
        }
    }

    System.out.println("Values of Matrix B");
    for (int i = 0; i < tamB1; i++) {
        for (int j = 0; j < tamB2; j++) {
            B[i][j] = scan.nextBigInteger();
        }
    }


   if (A[0].length == B.length) {
    for (int i = 0; i < A.length; i++) {
        for (int j = 0; j < B[0].length; j++) {
            for (int k = 0; k < A[0].length; k++) {
                C[i][j] =C[i][j].add(A[i][k].multiply(B[k][j])); // Result
            }
        }
    }

   }

    System.out.println(" C is equal to: ");
    for (int i = 0; i < A.length; i++) {
        for (int j = 0; j < B.length; j++) {
            System.out.print(C[i][j]+" ");
        }
        System.out.println("");
    }


   }

Take a look at this output:

Matrix A
2 2
Matrix B
2 2
Values of A
2 2
1 0
Values of B
3 4
5 6
Exception in thread "main" java.lang.NullPointerException
    at matrixmultiplication.MatrixMultiplication.main(MatrixMultiplication.java:56)
C:\Users\Luis Miguel\AppData\Local\NetBeans\Cache\8.2\executor-snippets\run.xml:53: Java returned: 1

I don't know why I can do this operation with int and not with BigInteger.

Thanks in advance.

Upvotes: 1

Views: 420

Answers (1)

user555045
user555045

Reputation: 64903

The "difference" (or really a similarity, depending on how you think about it) between an array of int and an array of BigInteger is that when you make a new array of integers, all the integers exist (and are zero) while a new array of BigInteger is filled with null. Of course add cannot be called on null.

There are different fixes, for example you could fill the matrix C with zeros, or you could modify the multiplication so that it does not read from C at all but instead sums into a local variable which you initialize to zero.

Upvotes: 1

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