What is special about this math?

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Java floating point arithmetic

What is special about this double math in Java? I would expect the answer of .9 - 1 to be -0.1, however the response is -0.09999999999999998

double a = 0.9;
double b = 1.0;
double c = a - b;
System.out.println(c);
>>-0.09999999999999998

Mandatory read : http://download.oracle.com/docs/cd/E19957-01/806-3568/ncg_goldberg.html — Mike Kwan, Aug 04 '11 at 14:01
The float questions will never end!!!!! http://stackoverflow.com/questions/61872/use-float-or-decimal-for-accounting-application-dollar-amount — Joe, Aug 04 '11 at 14:01

score 2 · Answer 1 · answered Aug 04 '11 at 14:01

2

Yet another question about precision of doubles. Doubles have limited precision. Not all numbers can be represented precisely!

answered Aug 04 '11 at 14:01

Armen Tsirunyan

score 2 · Accepted Answer · edited May 23 '17 at 12:10

2

Floating point precision is difficult for computers. See this article and Retain precision with double in Java for details

For accurate floating point arithmetic use BigDecimal

edited May 23 '17 at 12:10

Community

answered Aug 04 '11 at 14:02

Nivas

score 1 · Answer 3 · answered Aug 04 '11 at 14:01

1

The value 0.9 cannot be represented exactly in binary form.

answered Aug 04 '11 at 14:01

Atreys

score 1 · Answer 4 · answered Aug 04 '11 at 14:01

1

double is not exact. this is the nearest number that can be represented by double.

answered Aug 04 '11 at 14:01

amit

score 1 · Answer 5 · answered Aug 04 '11 at 14:03

1

There are an infinite number of numbers between any two values.

There are a finite number of memory bits on any computer. So any way we chose to represent floating point numbers will never be exact.

answered Aug 04 '11 at 14:03

Mark Bolusmjak

actually, even if there were infinite but countable memory, it still won't be enough, since there are uncountable number of elements between each two numbers. [so any turing machine will never be able to precisely represent a real number] – amit Aug 04 '11 at 14:06

5 Answers5