Dart - Subtracting some double values gives wrong result

Question

I have picked two random double numbers:

double a = 7918.52;
double b = 5000.00;

I would expect to get 2918.52 from a - b.
Well, it gives me a result of 2918.5200000000004, which seems odd.

print(a - b); // -> 2918.5200000000004

But if I change double a to 7918.54, I will get the expected result of 2918.54.

Can someone explain to me why some double values result in unexpected rounding issues and others do not?

Possible duplicate of [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) — jamesdlin, Jun 16 '19 at 15:56

creativecreatorormaybenot · Accepted Answer · 2019-06-16T17:42:08.887

6

The reason for this is floating-point arithmetic and the fact that Dart uses the IEEE 754 standard as far as I am concerned.

This happens for all languages that use floating-point arithmetic. You can read through similar questions regarding other programming languages.

General question about floating-point arithmetic in modern programming languages.

edited Jun 16 '19 at 17:42

answered Jun 16 '19 at 15:53

creativecreatorormaybenot

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Thank you. That explains it. – Zoka Jun 16 '19 at 16:06

score 0 · Answer 2 · answered Sep 17 '22 at 20:13

creativecreatorormaybenot is right, this is issue comes from the Dart language itself.

The best solution I came up with was to manually round it to the precision you expect :

double substract(double a, double b, int precision) {
  return (a - b).precision(precision);
}

double precision(int fractionDigits) {
  num mod = pow(10.0, fractionDigits);
  return ((this * mod).round().toDouble() / mod);
}

score 0 · Answer 3 · answered Dec 03 '22 at 15:04

0

A simple solution would be:

depending on how many decimals you expect (lets assume 2, you can make it more generic...)
((a * 100).toInt() - (b * 100)).toInt() / 100.

answered Dec 03 '22 at 15:04

Kowboj

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Dart - Subtracting some double values gives wrong result

3 Answers3