So i have small double values and i need to convert them into string in order to display in my app. But i care only about first two significant digits. It should work like this:

convert(0.000000000003214324) = '0.0000000000032';
convert(0.000003415303) = '0.0000034';

CodePudding user response：

We can convert double to string, then check every index and take up to two nonzero (also .) strings. But the issue comes on scientific notation for long double.

You can check Convert long double to string without scientific notation (Dart)

We need to find exact String value in this case. I'm taking help from this answer.

String convert(String number) {
  String result = '';

  int maxNonZeroDigit = 2;

  for (int i = 0; maxNonZeroDigit > 0 && i < number.length; i  ) {
    result  = (number[i]);
    if (number[i] != '0' && number[i] != '.') {
      maxNonZeroDigit -= 1;
    }
  }

  return result;
}

String toExact(double value) {
  var sign = "";
  if (value < 0) {
    value = -value;
    sign = "-";
  }
  var string = value.toString();
  var e = string.lastIndexOf('e');
  if (e < 0) return "$sign$string";
  assert(string.indexOf('.') == 1);
  var offset =
      int.parse(string.substring(e   (string.startsWith('-', e   1) ? 1 : 2)));
  var digits = string.substring(0, 1)   string.substring(2, e);
  if (offset < 0) {
    return "${sign}0.${"0" * ~offset}$digits";
  }
  if (offset > 0) {
    if (offset >= digits.length) return sign   digits.padRight(offset   1, "0");
    return "$sign${digits.substring(0, offset   1)}"
        ".${digits.substring(offset   1)}";
  }
  return digits;
}


void main() {
  final num1 = 0.000000000003214324;
  final num2 = 0.000003415303;

  final v1 = convert(toExact(num1));

  final v2 = convert(toExact(num2));
  print("num 1 $v1 num2 $v2");
}

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