BigDecimal Rounding using power operator

[Joe Stiefel]

Hi

I am evaluating this tool for an application I am writing. I require accurate representation that BigDecimal provides. I am running into a "java.lang.ArithmeticException: Rounding necessary" exception when trying to use the Power operator for some calculations, even though I passed in a "MathContext.DECIMAL64"(I believe this denotes "Half up" rounding to 16 places) to the BigDecComponents() constructor. Here is my code:

        Jep jep = new Jep( new BigDecComponents(MathContext.DECIMAL64));
        String math = ".1 ^ .2";
        jep.parse(math);
        Object evaluate = jep.evaluate();

I am aware that I can write my own power operator function, I am just curious why this tool does not use the rounding method associated with MathContext.DECIMAL64 or even any specified rounding passed in using other Mathcontext constructors.

Thanks

Joe

[Richard Morris]

The problem here is in the java.math.BigDecimal class is really quite limited and only supports raising to integer powers

There are two methods

BigDecimal
pow(int n)

Returns a BigDecimal whose value is (this^n), The power is computed exactly, to unlimited precision.

BigDecimal
pow(int n, MathContext mc)

Returns a BigDecimal whose value is (this^n).

So there is no straightforward way to implement non-integer powers for big decimal.

There are some third party libraries that do provide implementations. See

https://stackoverflow.com/questions/16441769/javas-bigdecimal-powerbigdecimal-exponent-is-there-a-java-library-that-does

As a general rule I would advise most people against using big decimals. There are a lot of pitfalls in using the library which is more geared towards financial calculations, there are better ways to  implement high precision see suggestions at

https://stackoverflow.com/questions/277309/java-floating-point-high-precision-library

In the core Jep we want to not have dependencies on third party libraries. In the extensions package there is a Field framework that makes it easier to include third party libraries.