BigInteger not increasing?

Question

For some reason my val variable isn't increasing within the while-loop. Any reason as to why it's not doing so? val.add(BigInteger.ONE);

import java.math.*;

public static void main(String[] args) {

    /* Variables */
    BigInteger PQ = new BigInteger("17");
    BigInteger E = new BigInteger("7");

    BigInteger val = new BigInteger("2");

    /* Find the PRIME factors of PQ */
    while (PQ.compareTo(BigInteger.ONE) == 1) { // while the first value is greater
        if (PQ.mod(val).equals(BigInteger.ZERO)) {
            System.out.print(val + "  ");
        } else {
            val.add(BigInteger.ONE);
        }
    }
}

Although [`BigInteger.compareTo(BigInteger val)`](https://docs.oracle.com/javase/8/docs/api/java/math/BigInteger.html#compareTo-java.math.BigInteger-) explicitly lists return value as *"-1, 0 or 1"*, it is better to code to the more general contract inherited from `Comparable` which says *"a negative integer, zero, or a positive integer"*, so you should write `a.compareTo(b) > 0` instead of `a.compareTo(b) == 1`. It also reads better, since it means `a > b`, i.e. `a.compareTo(b) OP 0` means `a OP b`, where `OP` can be `<`, `<=`, `==`, `>=`, `>`, or `!=`. — Andreas, Sep 11 '16 at 04:31

Mitch Wheat · Accepted Answer · 2016-09-11T04:17:37.190

7

val.add(BigInteger.ONE) returns a new value. You do not assign it to anything:

val = val.add(BigInteger.ONE);

Class BigInteger

edited Sep 11 '16 at 04:17

answered Sep 11 '16 at 04:15

Mitch Wheat

295,962
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Wow. Didn't even realize that. Thanks! – Dandy Sep 11 '16 at 04:17
1

also note: that won't be an efficient method to factor large numbers. – Mitch Wheat Sep 11 '16 at 04:22
"Won't be efficient" could be restated as "is impractically inefficient" ..... – Stephen C Sep 11 '16 at 04:33
@Dandy BigInteger like String is immutable so you can't alter it. You can only return a new value (and just like using Strings) – Peter Lawrey Sep 11 '16 at 06:42

score 0 · Answer 2 · answered Sep 11 '16 at 07:28

0

You can initialize the sum again.

BigInteger val=BigInteger.valueOf(10);
BigInteger val2=BigInteger.valueOf(20);
BigInteger val3=va1.add(val2);

answered Sep 11 '16 at 07:28

kratostoical

84
6

BigInteger not increasing?

2 Answers2