Get all decimal places in a number after division

问题

I am currently using the BigDecimal and it is giving me more decimals but not nearly enough for what I am trying to do. I need to be able to get all the way to the 10^6 digit. This is my current code

BigDecimal num = new BigDecimal(103993/33102.0);
    pw.println(num.toString());

and it outputs 3.14159265301190249175533608649857342243194580078125

where the number actually has a lot more decimals: http://www.wolframalpha.com/input/?i=103993%2F33102

回答1:

You are loosing the precision when evaluating this:

103993/33102.0

as a double division. Actually, the following:

BigDecimal num = new BigDecimal(103993/33102.0);

is equivlent to:

double d = 103993/33102.0;
BigDecimal num = new BigDecimal(d);

instead, use:

int scale = 100;
BigDecimal num1 = new BigDecimal(103993);
BigDecimal num2 = new BigDecimal(33102);
System.out.println(num1.divide(num2, scale, RoundingMode.HALF_UP).toString());

OUTPUT:

3.1415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737296840070086399613316415926530119026040722614947737

回答2:

The problem is how you are making your number. The 103993/33102.0 is evaluated as a double precision floating point expression (double) before the BigDecimal class ever gets involved. You should make separate BigDecimal objects and use BigDecimal.divide to get the number you want.

Since you want exact results, I'd probably pass both of your numbers in as integers when starting.

Even after doing that, I doubt you'll be able to get out to the 10^6 digit. You might need to implement your own division algorithm to get out to that level as any sane implementation of arbitrary precision math is going to stop long before that point (at least by default).

来源：https://stackoverflow.com/questions/15164636/get-all-decimal-places-in-a-number-after-division