CODEWITHSUNDEEP
javabiginteger
Prabhash kothi
Prabhash kothi

Reputation: 45

intValue() method for conversion of bigInteger to integer is giving wrong results

static BigInteger []fact = new BigInteger[1000003];

this array contains the factorial of integers from 0 to 1000003

I have X as a BigInteger variable which is coming from a modulo operation

 while(m.compareTo(BigInteger.valueOf(0)) == 1 ||  n.compareTo(BigInteger.valueOf(0)) == 1){

        if(m.compareTo(BigInteger.valueOf(0)) == 1 || m.compareTo(BigInteger.valueOf(0)) == 0)
        {
            x = m.mod(mod);
            m = m.divide(mod);

        }

Now when I'm trying "fact[X]" it is giving this error at "fact[X]":

 BigInteger cannot be converted to int
        temp = (fact[x].multiply(fact[(x.subtract(y)).intValue()])).mod(mod);

after changing X to X.intValue() the value of X is getting Changed.

How can i access fact[X]? help!!

Upvotes: 0

Views: 7452

Answers (2)

T.Gounelle
T.Gounelle

Reputation: 6033

You have to make the difference between BigInteger and int or Integer. There is no automatic conversion (autoboxing/unboxing) between BigInteger and int. Also, the dimension of an array of objects is always an int. Then each time you want to access an element in your BigInteger array, you have to explicitly convert the index value into an int, this is done by calling BigInteger#intValue().
I tried to comment your code, assuming there is no problems in the x and y computing.
Note that an array of BigInteger contains only null references when you create it. Meaning you need to have create and set the array element before trying to do something on it. 

final int max = 1000003;
BigInteger [] fact = new BigInteger[max];
BigInteger mod = BigInteger.valueOf(max);
BigInteger x = ...; // computed somewhere
BigInteger y = ...; // computed somewhere

BigInteger temp = 
   fact[x.intValue()]          // x is a BI, take intValue() to access array element
     .multiply(                // operands are BI
        fact[x.subtract(y)     // operands are BI
                 .intValue()]) // take intValue() to access array
                    .mod(mod); // operands are BI, result is BI

Upvotes: 1

Prashant
Prashant

Reputation: 2614

array index should be integer. If you are using fact[X] then X must be integer not BigInteger

change your this logic if x and y is BigInteger

 temp = (fact[x].multiply(fact[(x.subtract(y)).intValue()])).mod(mod);

to 

 temp = (fact[x.intValue()].multiply(fact[(x.subtract(y)).intValue()])).mod(mod);

Upvotes: 1

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