Test if BigDecimal can change integral part of other BigDecimal

Question

I need a way to express that into valid java code: I have 2 BigDecimals, and I want to know if the smaller BigDecimal can (when added to the bigger BigDecimal once) change the integral part of the bigger BigDecimal. Example:

0.6; 0.4 ->true

0.6; 0.39 ->false

Is there any efficient way of doing that, or do I have to test?

Why is one true and the other false? Can you express your requirement precisely? — user207421, Sep 04 '15 at 11:45
Maybe you meant the non fractional part? In your example both examples change the fractional part of the bigger decimal, but only one changes the non fractional part. `0.6 + 0.4 = 1.0 , 0 -> 1` and `0.6 + 0.39 = 0.99 , 0 -> 0` — Alkis Kalogeris, Sep 04 '15 at 11:46
`Math.floor(numbA) != Math.floor(numbA + numB)` should be ok — Hacketo, Sep 04 '15 at 11:47
And can we please get out of the habit of referring to fractional parts as decimals? Every digit in both these numbers is a decimal. There is an integral part and a fractional part, and a decimal point. Come this question, I have now seen 'decimal' used to refer to all three. It's meaningless. — user207421, Sep 04 '15 at 11:47
Sorry for my english, with decimal part I mean the part before the '.'. I thought thats how its called, correct me if I'm wrong. — Distjubo, Sep 04 '15 at 11:48
If you only test one set of numbers, I would guess your best bet is to simply add them and check (as suggested by @Hacketo). If you have to test *lots* of smaller numbers with the same big number, you could calculate the threshold (`Math.floor(big)+1-big`) once and then compare each small number to that. Perhaps that would increase performance a bit. — icke, Sep 04 '15 at 11:50

Titus · Accepted Answer · 2015-09-04T12:24:00.033

2

You can do something like this:

BigDecimal b1 = new BigDecimal("0.90");
BigDecimal b2 = new BigDecimal("0.20");
if(b1.add(b2).intValue() > Math.max(b1.intValue(), b2.intValue())){
    System.out.println("The integral has changed");
}else {
    System.out.println("The integral is the same");
}

How the intValue() method works:

Converts this BigDecimal to an int. This conversion is analogous to the narrowing primitive conversion from double to short ... Specification: any fractional part of this BigDecimal will be discarded ...

edited Sep 04 '15 at 12:24

answered Sep 04 '15 at 12:02

Titus

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This doesn't work for very big numbers. Since the OP is using `BigDecimal` you have to account the possibility of very big non fractional part. An example would be `BigDecimal bigA = new BigDecimal(Integer.MAX_VALUE + ".90");` – Alkis Kalogeris Sep 04 '15 at 16:42

score 0 · Answer 2 · edited Sep 04 '15 at 16:45

0

I've edited my answer after realizing Titus' answer was simpler. Credit goes to him/her.

 public static boolean integral(BigDecimal a, BigDecimal b) {

    BigDecimal greater;

    if (a.compareTo(b) == 1) {
        greater = a;
    } else {
        greater = b;
    }

    BigDecimal sum = a.add(b);

    return (sum.intValue() != greater.intValue());
}

edited Sep 04 '15 at 16:45

Alkis Kalogeris

17,044
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answered Sep 04 '15 at 11:55

ninesalt

4,054
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`charAt(0)` well .., `integral(new BigDecimal("13.39"), new BigDecimal("0.7"))` return false – Hacketo Sep 04 '15 at 11:58
when comparing strings, you have to use `equals` and not `==` or `!=` – Hacketo Sep 04 '15 at 12:06
No this do not work .. try with `integral(new BigDecimal("13.39"), new BigDecimal("0.1"))` should return false . BTW see [this post](http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java) – Hacketo Sep 04 '15 at 12:12
Oh. I've always been hesitant to use == but most of the time it worked fine. Thanks for that. Edited the post. – ninesalt Sep 04 '15 at 12:16

Alkis Kalogeris · Answer 3 · 2015-09-04T16:43:10.477

private static boolean isNonFractionalChanged(BigDecimal a, BigDecimal b) {
    BigInteger aInt = a.toBigInteger();
    BigInteger aNewInt = a.add(b).toBigInteger();

    return aInt.compareTo(aNewInt) != 0;
}

The above works even for very big numbers.

BigDecimal a = new BigDecimal("0.90");
BigDecimal b = new BigDecimal("0.20");
System.out.println(a + " + " + b + " = " + a.add(b) + " : " + isNonFractionalChanged(a, b));

BigDecimal bigA = new BigDecimal(Integer.MAX_VALUE + ".90");
System.out.println(bigA + " + " + b + " = " + bigA.add(b) + " : " + isNonFractionalChanged(bigA, b));

Test if BigDecimal can change integral part of other BigDecimal

3 Answers3