How do you round a double in Dart to a given degree of precision AFTER the decimal point?

问题

Given a double, I want to round it to a given number of points of precision after the decimal point, similar to PHP's round() function.

The closest thing I can find in the Dart docs is double.toStringAsPrecision(), but this is not quite what I need because it includes the digits before the decimal point in the total points of precision.

For example, using toStringAsPrecision(3):

0.123456789 rounds to 0.123  
9.123456789 rounds to 9.12  
98.123456789 rounds to 98.1  
987.123456789 rounds to 987  
9876.123456789 rounds to 9.88e+3

As the magnitude of the number increases, I correspondingly lose precision after the decimal place.

回答1:

See the docs for num.toStringAsFixed().

String toStringAsFixed(int fractionDigits)

Returns a decimal-point string-representation of this.

Converts this to a double before computing the string representation.

If the absolute value of this is greater or equal to 10^21 then this methods returns an exponential representation computed by this.toStringAsExponential().

Examples:

1000000000000000000000.toStringAsExponential(3); // 1.000e+21

Otherwise the result is the closest string representation with exactly fractionDigits digits after the decimal point. If fractionDigits equals 0 then the decimal point is omitted.

The parameter fractionDigits must be an integer satisfying: 0 <= fractionDigits <= 20.

Examples:

1.toStringAsFixed(3);  // 1.000
(4321.12345678).toStringAsFixed(3);  // 4321.123
(4321.12345678).toStringAsFixed(5);  // 4321.12346
123456789012345678901.toStringAsFixed(3);  // 123456789012345683968.000
1000000000000000000000.toStringAsFixed(3); // 1e+21
5.25.toStringAsFixed(0); // 5

回答2:

num.toStringAsFixed() rounds. This one turns you num (n) into a string with the number of decimals you want (2), and then parses it back to your num in one sweet line of code:

n = num.parse(n.toStringAsFixed(2));

回答3:

Above solutions do not appropriately round numbers. I use:

double dp(double val, int places){ 
   double mod = pow(10.0, places); 
   return ((val * mod).round().toDouble() / mod); 
}

回答4:

void main() {
  int decimals = 2;
  int fac = pow(10, decimals);
  double d = 1.234567889;
  d = (d * fac).round() / fac;
  print("d: $d");
}

Prints: 1.23

回答5:

    var price=99.012334554;
price = price.toStringAsFixed(2);
print(price); // 99.01

That is the ref of dart. ref: https://api.dartlang.org/stable/2.3.0/dart-core/num/toStringAsFixed.html

来源：https://stackoverflow.com/questions/28419255/how-do-you-round-a-double-in-dart-to-a-given-degree-of-precision-after-the-decim

标签

dart

precision

double-precision