toStringAsFixed and Number format does not work correctly

[KaushikGupta007]

Steps to reproduce

1. Run code provided on any platform

Issue Description
In Flutter, the toStringAsFixed(2) method and NumberFormat.decimalPatternDigits(locale: "en_IN", decimalDigits: 2) can produce unexpected results when rounding certain numbers. Specifically, numbers with a third decimal place of 5 (e.g., 5.505, 5.515, 5.525, ..., 5.595) might not be rounded correctly, or speaking more generally any number ending with 5 in decimal places

Expected results

The correct results for rounding should be:

5.505 -> 5.51
5.515 -> 5.52
5.525 -> 5.53
5.535 -> 5.54
5.545 -> 5.55
5.555 -> 5.56
5.565 -> 5.57
5.575 -> 5.58
5.585 -> 5.59
5.595 -> 5.60

According to rule:
If the digit to the right is greater than 5, round up

Actual results

I have checked these results using

toStringAsFixed(2) method and
NumberFormat.decimalPatternDigits(locale: "en_IN", decimalDigits: 2)

• 5.505 -> 5.50 (Wrong)
• 5.515 -> 5.51 (Wrong)
• 5.525 -> 5.53 (Correct)
• 5.535 -> 5.54 (Correct)
• 5.545 -> 5.54 (Wrong)
• 5.555 -> 5.55 (Wrong)
• 5.565 -> 5.57 (Correct)
• 5.575 -> 5.58 (Correct)
• 5.585 -> 5.58 (Wrong - Given by toStringAsFixed) -> 5.59 (Correct given by NumberFormat)
• 5.595 -> 5.59 (Wrong)

Code sample

Code sample

import 'package:intl/intl.dart';

void main() {
  double number1 = 5.505;
  double number2 = 5.515;
  double number3 = 5.525;
  double number4 = 5.535;
  double number5 = 5.545;
  double number6 = 5.555;
  double number7 = 5.565;
  double number8 = 5.575;
  double number9 = 5.585;
  double number10 = 5.595;
  
  print(number1.toStringAsFixed(2));
  print(number2.toStringAsFixed(2));
  print(number3.toStringAsFixed(2));
  print(number4.toStringAsFixed(2));
  print(number5.toStringAsFixed(2));
  print(number6.toStringAsFixed(2));
  print(number7.toStringAsFixed(2));
  print(number8.toStringAsFixed(2));
  print(number9.toStringAsFixed(2));
  print(number10.toStringAsFixed(2));

  var format = NumberFormat.decimalPatternDigits(locale: "en_IN", decimalDigits: 2);
  
  print(format.format(number1));
  print(format.format(number2));
  print(format.format(number3));
  print(format.format(number4));
  print(format.format(number5));
  print(format.format(number6));
  print(format.format(number7));
  print(format.format(number8));
  print(format.format(number9));
  print(format.format(number10));
}

Screenshots or Video

Screenshots / Video demonstration

[Upload media here]

Logs

Logs

[Paste your logs here]

Flutter Doctor output

Doctor output

[√] Flutter (Channel stable, 3.22.2, on Microsoft Windows [Version 10.0.22621.3593], locale en-IN)
    • Flutter version 3.22.2 on channel stable at C:\Users\Admin\fvm\versions\3.22.2
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 761747bfc5 (12 days ago), 2024-06-05 22:15:13 +0200
    • Engine revision edd8546116
    • Dart version 3.4.3
    • DevTools version 2.34.3

[√] Windows Version (Installed version of Windows is version 10 or higher)

[√] Android toolchain - develop for Android devices (Android SDK version 33.0.1)
    • Android SDK at C:\Users\Admin\AppData\Local\Android\sdk
    • Platform android-34, build-tools 33.0.1
    • Java binary at: C:\Program Files\Android\Android Studio\jbr\bin\java
    • Java version OpenJDK Runtime Environment (build 17.0.6+0-b2043.56-9586694)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.9.1)
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
    • Visual Studio Community 2022 version 17.9.34616.47
    • Windows 10 SDK version 10.0.22621.0

[√] Android Studio (version 2022.2)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 17.0.6+0-b2043.56-9586694)

[√] VS Code (version 1.89.1)
    • VS Code at C:\Users\Admin\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.90.0

[√] Connected device (5 available)
    • CPH2467 (mobile)                   • 1a715d74      • android-arm64  • Android 14 (API 34)
    • Android SDK built for x86 (mobile) • emulator-5554 • android-x86    • Android 11 (API 30) (emulator)
    • Windows (desktop)                  • windows       • windows-x64    • Microsoft Windows [Version 10.0.22621.3593]
    • Chrome (web)                       • chrome        • web-javascript • Google Chrome 126.0.6478.61
    • Edge (web)                         • edge          • web-javascript • Microsoft Edge 125.0.2535.85

[√] Network resources
    • All expected network resources are available.

• No issues found!

[dart-github-bot]

Summary: The toStringAsFixed(2) method and NumberFormat.decimalPatternDigits in Flutter fail to round numbers ending in 5 correctly, producing inconsistent results across different inputs. For example, 5.505 rounds to 5.50 instead of 5.51.

[mraleph]

That's how floating point numbers actually work. Floating point 5.505 is not the same as decimal number 5.505, so it absolutely correctly rounds to 5.5 if you ask less decimal digits.

I recommend:

• Learning how floating point numbers work: https://floating-point-gui.de/
• Never using floating point numbers for anything that requires correct decimal rounding (e.g. storing prices and monetary amounts).