Why Are My Bigdecimal Objects Initialized with Unexpected Rounding Errors

Unexpected BigDecimal result. 1 divided by 3 results in 1. Unsure why

The result of num.divide is ignored. You need to set that back into a variable

    BigDecimal num = new BigDecimal(1.00).setScale(2);
    BigDecimal divisor = new BigDecimal(3.00).setScale(2);

    BigDecimal result = num.divide(divisor, BigDecimal.ROUND_HALF_UP);

    System.out.print(result);

This prints 0.33

Weird rounding issue

This turned out to be a regression in rails. It's reproducible with 5.0.0.1??? and is gone by 5.1.0.0???.

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I bisected it and found this commit to be the one that fixes the issue. This is the related issue.

The fix seems to be to stop using the pg gem's float encoder.

BigDecimal gives unexpected results for numbers larger than 7 or 0 decimal numbers

As per the documentation, divide​(BigDecimal divisor, RoundingMode roundingMode) returns a BigDecimal whose value is (this / divisor), and whose scale is this.scale().

Why did you get the expected result for 21099000.1 / 13196000.1?

Check the result of the following code:

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Main {
    public static void main(String[] args) {
        BigDecimal first = BigDecimal.valueOf(21099000.1);
        BigDecimal second = BigDecimal.valueOf(13196000.1);
        System.out.println("First: " + first + ", Scale: " + first.scale());
        System.out.println("Second: " + second + ", Scale: " + second.scale());

        // 21099000.0 / 13196000.0 = 1.5988936041
        System.out.println(BigDecimal.valueOf(1.5988936041).setScale(first.scale(), RoundingMode.HALF_UP));
    }
}

Output:

First: 21099000.1, Scale: 1
Second: 13196000.1, Scale: 1
1.6

As you can see, JVM has chosen the scale as 1 for first and thus the result of divide (which is 1.5988936041) is also set as 1 which is equal to 1.6 with RoundingMode.HALF_UP.

Why did you not get the expected result for 21099000.0 / 13196000.0?

Check the result of the following code:

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Main {
    public static void main(String[] args) {
        BigDecimal first = BigDecimal.valueOf(21099000.0);
        BigDecimal second = BigDecimal.valueOf(13196000.0);
        System.out.println("First: " + first + ", Scale: " + first.scale());
        System.out.println("Second: " + second + ", Scale: " + second.scale());

        // 21099000.0 / 13196000.0 = 1.5988936041
        System.out.println(BigDecimal.valueOf(1.5988936041).setScale(first.scale(), RoundingMode.HALF_UP));
    }
}

Output:

First: 2.1099E+7, Scale: -3
Second: 1.3196E+7, Scale: -3
0E+3

As you can see, JVM has chosen the scale as -3 for first and thus the result of divide (which is 1.5988936041) is also set as -3 which is equal to 0 (or 0E+3) with RoundingMode.HALF_UP.

How can I change this behavior?

As mentioned in the documentation, scale of the division is set as this.scale() which means if you set the scale of first to 1, you can get the expected result.

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Main {
    public static void main(String[] args) {
        BigDecimal first = BigDecimal.valueOf(21099000.0).setScale(1);
        BigDecimal second = BigDecimal.valueOf(13196000.0);
        System.out.println("First: " + first + ", Scale: " + first.scale());
        System.out.println("Second: " + second + ", Scale: " + second.scale());
        System.out.println("Division: " + first.divide(second, RoundingMode.HALF_UP).doubleValue());
    }
}

Output:

First: 21099000.0, Scale: 1
Second: 1.3196E+7, Scale: -3
Division: 1.6

What is the most common way?

The last example worked well and there is no problem using it. However, the most common way is to use divide​(BigDecimal divisor, int scale, RoundingMode roundingMode).

import java.math.BigDecimal;
import java.math.RoundingMode;

public class Main {
    public static void main(String[] args) {
        BigDecimal first = BigDecimal.valueOf(21099000.0);
        BigDecimal second = BigDecimal.valueOf(13196000.0);
        System.out.println("First: " + first + ", Scale: " + first.scale());
        System.out.println("Second: " + second + ", Scale: " + second.scale());
        System.out.println("Division: " + first.divide(second, 1, RoundingMode.HALF_UP).doubleValue());
    }
}

Output:

First: 2.1099E+7, Scale: -3
Second: 1.3196E+7, Scale: -3
Division: 1.6

Problem with Java BigDecimal.remainder() - irrational and inaccurate returns

Even though you're working with BigDecimal objects to obtain precise results, you're introducing an imprecision when declaring the objects that represent the bills and coins:

BigDecimal fifty = new BigDecimal(50.0);
BigDecimal twenty = new BigDecimal(20.0);
// ...

The values passed to the constructor are interpreted as doubles, but some of them cannot be accurately captured in a 64 bit double-precision format.

You should use the String-based constructor instead:

BigDecimal fifty = new BigDecimal("50.0");
BigDecimal twenty = new BigDecimal("20.0");
// ...

This will give you the correct output:

Cost: 70.70
$ Received: 100
Total change returned: 29.30
0.30
0.10
0.00
$50: 0 
$20: 1 
$10: 0 
$5: 1 
$2: 2 
$1: 0 
$0.50: 0 
$0.20: 1 
$0.10: 1 
$0.05: 0 

Stange values after BigDecimal initialization

Use BigDecimal.valueOf(double) static method: this will initialize the BigDecimal from string representation of the double number which is what you need.

Deprecated constructor for BigDecimal.setScale(int, int) and RoundingMode enums

According to the docs, setScale(int, int), has not been recommended since Java 1.5, when enums were first introduced:

The new setScale(int, RoundingMode) method should be used in preference to this legacy method.

It was finally deprecated in Java 9.

You should call setScale(2, RoundingMode.HALF_EVEN) instead. It makes error checking much simpler, since you can't pass in an undefined enum, but you can certainly pass in an integer mode which is undefined.

BigDecimal - to use new or valueOf

Those are two separate questions: "What should I use for BigDecimal?" and "What do I do in general?"

For BigDecimal: this is a bit tricky, because they don't do the same thing. BigDecimal.valueOf(double) will use the canonical String representation of the double value passed in to instantiate the BigDecimal object. In other words: The value of the BigDecimal object will be what you see when you do System.out.println(d).

If you use new BigDecimal(d) however, then the BigDecimal will try to represent the double value as accurately as possible. This will usually result in a lot more digits being stored than you want. Strictly speaking, it's more correct than valueOf(), but it's a lot less intuitive.

There's a nice explanation of this in the JavaDoc:

The results of this constructor can be somewhat unpredictable. One might assume that writing new BigDecimal(0.1) in Java creates a BigDecimal which is exactly equal to 0.1 (an unscaled value of 1, with a scale of 1), but it is actually equal to 0.1000000000000000055511151231257827021181583404541015625. This is because 0.1 cannot be represented exactly as a double (or, for that matter, as a binary fraction of any finite length). Thus, the value that is being passed in to the constructor is not exactly equal to 0.1, appearances notwithstanding.

In general, if the result is the same (i.e. not in the case of BigDecimal, but in most other cases), then valueOf() should be preferred: it can do caching of common values (as seen on Integer.valueOf()) and it can even change the caching behaviour without the caller having to be changed. new will always instantiate a new value, even if not necessary (best example: new Boolean(true) vs. Boolean.valueOf(true)).

Android - Issue with conversion of double to Big Decimal

You're not using the result of round. You should at least be doing:

totalPrice = round(totalPrice, 2, BigDecimal.ROUND_HALF_UP);

However, you shouldn't be using double for currency values to start with. Use BigDecimal throughout - otherwise you may well find you get unexpected results due to the way binary floating point works.

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