What can I do against floating point notations Errors?

Question

I need to multiply a float by 100 in order to convert from € to cents. The problem I am having is, that the value of big numbers isn't calculated correctly.

For example:

String aa = "1000009.00";
aa = aa.replaceAll(",", ".");
float bb = Float.parseFloat(aa);
bb=Math.round(bb*100);
System.out.println(bb);

What I am getting is: 1.00000896E8

So I know this is because of the way float in Java works. I have searched for an answer but people always say "use Math.round()" which doesn't work for me.

What can i do to prevent this?

`float` has [6 to 9 significant decimal digits precision](https://en.wikipedia.org/wiki/Single-precision_floating-point_format#IEEE_754_single-precision_binary_floating-point_format:_binary32), and your number `1000009.00` is 9 digits long. You've exceeded the precision of `float`. Use `double` (15–17 digits) or `BigDecimal` (unlimited precision) instead. — Andreas, Jun 02 '17 at 18:06
Generally, don't use floating point for calculations where the exact decimals in base10 are important (e.g. money), its going to bite you at some point, especially if you're not 100% certain and fluent in how floating point works in detail. — Durandal, Jun 02 '17 at 18:21

Youcef LAIDANI · Answer 1 · 2017-06-02T18:30:00.543

0

You can use BigDecimal::multiply for example :

String aa = "1000009.00";
aa = aa.replaceAll(",", ".");
BigDecimal fullValue = new BigDecimal(aa);

System.out.println("full value  = " + fullValue.multiply(new BigDecimal(100)));

Output

full value  = 100000900.00

edited Jun 02 '17 at 18:30

answered Jun 02 '17 at 17:59

Youcef LAIDANI

55,661
15
90
140

why this down-vote? – Youcef LAIDANI Jun 02 '17 at 18:13
1

If you use `BigDecimal`, you should create it from `aa`, not `bb`, then use `movePointRight(2)` to get correct answer of `100000900`. – Andreas Jun 02 '17 at 18:13
thank you @Andreas i don't read the question correctly check my answer now ? – Youcef LAIDANI Jun 02 '17 at 18:17
1

Still not correct, since `1000009.00` multiplied by 100 should be `100000900`. – Andreas Jun 02 '17 at 18:17
@Andreas what about now? – Youcef LAIDANI Jun 02 '17 at 18:30

Yahya · Accepted Answer · 2017-06-02T18:07:05.997

0

You can use double for more precision (or BigDecimal if you expect to work with very big numbers).

String aa = "1000009.00";
double bb = Double.parseDouble(aa);
bb=Math.round(bb*100);
System.out.printf("%.2f", bb); // it prints only two digits after the decimal point

Output

100000900.00

edited Jun 02 '17 at 18:07

answered Jun 02 '17 at 17:59

Yahya

13,349
6
30
42

thx that works ! – MrGatzi Jun 02 '17 at 18:12

What can I do against floating point notations Errors?

2 Answers2