BigInteger mathematical functions not returning expected values

Question

I have a BigInteger method which takes a string[] array input of 4 numbers, converts the numbers into an int[], and then applies numerous mathematical operations to it.

public BigInteger convert32Bit(String[] array)
{
    System.out.println("Array being converted is "+Arrays.toString(array)+"\n");
    int[] tempArray = new int[array.length];
    ArrayList<BigInteger> tempBigIntList = new ArrayList<BigInteger>();
    int i = 0;
    for(String s:array)
    {
        int power = 4-i;
        tempArray[i]= Integer.parseInt(s);
        String string = Integer.toString(tempArray[0]);
        BigInteger myBigInt = new BigInteger(string);
        BigInteger num2 = myBigInt.multiply(new BigInteger("256").pow(power));
        System.out.println(tempArray[i]+" is being multiplied by 256^"+power+" which equals "+num2);
        tempBigIntList.add(num2);
        i++;
    }

    BigInteger bigInt32Bit = new BigInteger("0");
    for(BigInteger bI:tempBigIntList)
    {
        bigInt32Bit.add(bI);
    }

    System.out.println("\nThe final value is "+bigInt32Bit);

    return bigInt32Bit;
}

However there is a problem. If I take the array "123", "0", "245", "23" as the input. I get the following output.

The output I am expecting is

Array being converted is [123, 0, 245, 23]

123 is being multiplied by 256^4 which equals 528280977408
0 is being multiplied by 256^3 which equals 0
245 is being multiplied by 256^2 which equals 16056320
23 is being multiplied by 256^1 which equals 5888

The final value is 528297039616

Can someone please help fix this?

In your loop your `String string = Integer.toString(tempArray[0]);` should be `tempArray[i]` — justhalf, Dec 23 '15 at 13:50
@justhalf Thank you. That fixed the working out the values problem — Dan, Dec 23 '15 at 13:51
If it's just a typo, this question should be closed as it will likely not help future readers. But since you have the confusion on the immutability of `BigInteger`, I think this question is quite ok. — justhalf, Dec 23 '15 at 13:54
@Dan, because this question is unlikely to be helpful for further visitors. You should not worry about this as your problem is actually solved. — Tagir Valeev, Dec 23 '15 at 13:54
This seems to be a duplicate of: http://stackoverflow.com/questions/1783912/java-how-to-use-biginteger — justhalf, Dec 23 '15 at 14:01

score 3 · Accepted Answer · edited May 23 '17 at 12:15

3

Replace this line

bigInt32Bit.add(bI);

with

bigInt32Bit = bigInt32Bit.add(bI);

You do this because BigInteger is immutable. This means that you have to create a new value for bigInt32Bit instead of just adjusting an old one. Also (as @justhalf says) replace the line

String string = Integer.toString(tempArray[0]);

with

String string = Integer.toString(tempArray[i]);

so that you use the correct value when applying the mathematical operators.

edited May 23 '17 at 12:15

Community

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answered Dec 23 '15 at 13:50

Tagir Valeev

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Also there is a typo in his code, where `String string = Integer.toString(tempArray[0]);` should be `tempArray[i]` instead of `tempArray[0]` – justhalf Dec 23 '15 at 13:51
Thank you both of you. Fixed the problems :) – Dan Dec 23 '15 at 13:52
1

@TagirValeev: Actually you don't need to change it to community wiki, it's just a small typo =) – justhalf Dec 23 '15 at 13:52

score 0 · Answer 2 · answered Dec 23 '15 at 13:51

BigInteger is immutable and hence bigInt32Bit.add(bI); would lead the value of whatever you have first element. Inorder to add all the values you could do something like:

 bigInt32Bit = bigInt32Bit.add(bI);//assign it

Also you are just passing very first element of your array as input to bigInteger like String string = Integer.toString(tempArray[0]);, it should be String string = Integer.toString(tempArray[i]);. I would not use array if its not used anywhere insteadi would just use an integer variable.

BigInteger mathematical functions not returning expected values

2 Answers2