How to properly set precision of whole numbers and decimals?

[Andre601]

I use EvalEx in one of my projects and some people report, that the returned number seems to not be accurate, as it rounds certain digits to 0s.

As an example, just providing 1234567891 results in 1234568000 being returned which shouldn't be the case.

Right now am I using the following:

Expression exp = new Expression(expression);
return exp.eval().setScale(3, RoundingMode.HALF_UP);

I assume to fix this would I use setPrecision before the eval()?

Expression exp = new Expression(expression);
return exp.setPrecision(10).eval().setScale(3, RoundingMode.HALF_UP);

And if the above is indeed the case, would there be any way to make EvalEx only round the whole number, if A) the scale is set to zero (no decimals) and/or B) the decimals result in rounding the next whole number up?

Example:
A number 123456789.1 would become 123456789 while 123456789.9 would become 123456790.

[RobertZenz]

No, precision/scale of a BigDecimal works by setting the significant number of digits, not decimal places. So if you have the value "123456798" and you set the precision to "3" you get "12300000". Same goes if you have "1.23456789" which becomes "1.23".

What you seem to want to do is to set a "high enough" precision to do your calculation, and then round the result.

[RobertZenz]

...while the numbers wanted are...

- 22.5    -> 23
- 13150.5 -> 13151
- 0.5     -> 1
- 0.55    -> 1

That means that you want to round to zero decimal places, but do the calculation with a high enough precision.

[Andre601]

This doesn't answer the question I have here, which is how to set the decimal points to use...
After all can I not set the precision to the size of the final number, since I don't have that final number yet available...

Like the way you explain this - and how I understand this - Would I need to

• Evaluate the number
• Get the length up until the dot (i.e. 123.45 would be a length of 3)
• Add the amount of decimals the user want to include to the number length.
• Evaluate the number again, but this time use the number acquired before to set the max length
• Apply MathContext to this stuff
• get String.

???

Like as much as I try to understand this is your reply here not that helpful as it keeps me still confused on how to properly do this...

[RobertZenz]

Now I see where the confusion is coming from, my mistake for linking against the wrong method, what you're looking for is the setScale method of BigDecimal.

[Andre601]

Now I see where the confusion is coming from, my mistake for linking against the wrong method, what you're looking for is the setScale method of BigDecimal.

So, just to make things clear here:
If I use setScale(0, RoundingMode.HALF_UP), it would give me the numbers I mentioned here?

[RobertZenz]

Yes.