Kotlin sum of Double or BigDecimal return unexpected result

Question

I'm trying to get the total amount of double values but it returns unexpected results. Check the following screenshot:

Link for the code Code:

import java.math.BigDecimal
/**
 * You can edit, run, and share this code.
 * play.kotlinlang.org
 */
fun main() {
    val items = listOf(MyItem(BigDecimal(3.6)), MyItem(BigDecimal(2.0)), MyItem(BigDecimal(4.3)), MyItem(BigDecimal(0.1)))
    println(items.sumOf { it.amount })
}

data class MyItem(val amount: BigDecimal)

The expected result is: 10.0

The actual result is: 9.9999999999999999167332731531132594682276248931884765625

You will get `"10.0"` is you pass strings to `BigDecimal`. You lose precision before the BigDecimals are created because you pass double values to it. — marstran, Apr 04 '22 at 13:33
There must be a better dup that also includes the advice to use the BigDecimal string constructor. — President James K. Polk, Apr 04 '22 at 15:41

score 3 · Accepted Answer · answered Apr 04 '22 at 13:33

In most cases it is better to provide numbers to BigDecimal as strings, not floats:

val items = listOf(MyItem(BigDecimal("3.6")), MyItem(BigDecimal("2.0")), MyItem(BigDecimal("4.3")), MyItem(BigDecimal("0.1")))

It solves your problem.

Few words of explanation: floating-point numbers are by nature imprecise for storing decimal fractions. They only provide a close result, but not the one we would expect.

BigDecimal tries to fix this problem. It makes some tricks to represent the number as we would expect. But to do this it needs to understand how do we want to represent numbers, what is our expected precision, etc. It can deduct this from numbers provided by strings, but not from floats/doubles.

score 1 · Answer 2 · answered Apr 04 '22 at 13:32

The problem is that you instantiate your BigDecimals using Double values which leads to rounding issues.

You can instead write:

val items = listOf(MyItem(BigDecimal("3.6")), MyItem(BigDecimal("2.0")), MyItem(BigDecimal("4.3")), MyItem(BigDecimal("0.1")))

Then your result is 10.0.

Kotlin sum of Double or BigDecimal return unexpected result

2 Answers2