Scala addition of double gives different answer

Question

i am trying to financial app, where sum of credit and debit is 0

Why this is like this

scala> 7595.00-8544.38+949.38
res7: Double = 7.958078640513122E-13

but when sorted and added

scala> 7595.00+949.38-8544.38
res8: Double = 0.0

well the problem is numbers are at random but they always sum upto to zero

i did same with node and found same results. what is the problem.

i wrote this function it works

def sum(xs: List[Double]): Double = {
    xs match {
      case x :: tail => BigDecimal(x +  sum(tail)).setScale(2,BigDecimal.RoundingMode.HALF_UP).toDouble 
      case Nil => 0
    } 
}

it's strange though..

dhg · Accepted Answer · 2014-08-01T07:35:16.763

4

Floating point numbers are not exact representations of the underlying values. This is just a fact of how floating point numbers are encoded in binary.

For more information, check out the wikipedia article.

If you want exactness, use BigDecimal:

 scala> BigDecimal(759500,2) - BigDecimal(854438,2) + BigDecimal(94938,2)
 res1: scala.math.BigDecimal = 0.00

edited Aug 01 '14 at 07:35

answered Aug 01 '14 at 07:29

dhg

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Thanks ,i prefer scala> BigDecimal(7595.00) - BigDecimal(8544.38) + BigDecimal(949.38) as the amount can be in one or two decimal – AmeyChavan Aug 01 '14 at 07:45
@ameychavan, don't do that because if you create a BigDouble out of a floating point number, then you might just be creating a BigDouble that already has the rounding error in it. E.g. `BigDecimal(1.1+2.2)`. – dhg Aug 01 '14 at 07:47
,i'm not using double, but transforming right from the string i get from the inputstream – AmeyChavan Aug 01 '14 at 07:52
Uhh, The `8544.38` in `BigDecimal(8544.38)` is a double... But yeah, going from a string is fine. – dhg Aug 01 '14 at 08:02

score 4 · Answer 2 · answered Aug 01 '14 at 07:29

4

This is why you should never use floating-point math for money. What you have there is a rounding error.

answered Aug 01 '14 at 07:29

chrylis -cautiouslyoptimistic-

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thanks,it would be helpful if you add alternatives – AmeyChavan Aug 02 '14 at 06:32

Scala addition of double gives different answer

2 Answers2