给定一个double,我希望将它四舍五入到小数点后的给定精度点数,类似于PHP的round()函数。

我能在Dart文档中找到的最接近的东西是double.toStringAsPrecision(),但这不是我所需要的,因为它包括了精度总分中小数点前的数字。

例如,使用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

随着数字大小的增加,小数点后的精度也相应降低。


当前回答

double value = 2.8032739273;
String formattedValue = value.toStringAsFixed(3);

其他回答

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

打印: 1.23

请参阅num.toStringAsFixed()的文档。

字符串磨损

返回this的小数字符串表示形式。

在计算字符串表示形式之前,将此转换为double。

如果this的绝对值大于或等于10^21,则该方法返回由this. tostringasexponential()计算的指数表示。

例子:

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

否则,结果是与小数点后的fractionDigits位数最接近的字符串表示形式。如果fractionDigits等于0,则小数点将被省略。

参数fractionDigits必须是满足以下条件的整数:0 <= fractionDigits <= 20。

例子:

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

num.toStringAsFixed()轮。它将你的num (n)转换成一个带有你想要的小数点数的字符串(2),然后在一行漂亮的代码中将它解析回你的num:

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

如果使用动态类型的数据。你可以使用它。

 typeDecimal(data) => num.parse(data.toString()).toStringAsFixed(2);

如果你想要使用特殊的舍入。您可以尝试这个函数(舍入)。

void main(List<String> arguments) {
list.map((e) {
 log('list1');
 rounding(e, 0.05);
 rounding(e, 0.1);
 rounding(e, 0.2);
 rounding(e, 0.25);
 rounding(e, 0.5);
 rounding(e, 1);
 rounding(e, 10);
}).toList();
list2.map((e) {
 log('list2');
 rounding(e, 0.05);
 rounding(e, 0.1);
 rounding(e, 0.2);
 rounding(e, 0.25);
 rounding(e, 0.5);
 rounding(e, 1);
 rounding(e, 10);
}).toList();
}

const list = [1.11, 1.22, 1.33, 1.44, 1.55, 1.66, 1.77, 1.88, 1.99];

const list2 = [2.19, 3.28, 4.37, 5.46, 6.55, 7.64, 8.73, 9.82, 10.91];

void rounding(double price, double count) {
log('-----------------------');
log('price: $price, count: $count');
double _priceRemainder = price % count;
double _someDiff = count / _priceRemainder;
log('_price: ${_priceRemainder.toStringAsFixed(2)}');
log('_pricePlus: ${_someDiff.toStringAsFixed(2)}');
if (_someDiff.toStringAsFixed(2) == '1.00') {
 log('_someDiff = 1');
} else if (_someDiff > 1 && _someDiff <= 2 ||
   _someDiff.toStringAsFixed(2) == '2.00') {
 log('_someDiff > 1 && _someDiff <= 2 || _someDiff.toStringAsFixed(2) == 2.00');
 log('ceilToDouble: $price: ${(price + (count - _priceRemainder)).toStringAsFixed(2)}');
 log('floorToDouble: $price: ${(price - _priceRemainder).toStringAsFixed(2)}');
 log('roundToDouble: $price: ${(price + (count - _priceRemainder)).toStringAsFixed(2)}');
} else if (_someDiff > 2) {
 log('_someDiff > 2');
 log('ceilToDouble: $price: ${(price + (count - _priceRemainder)).toStringAsFixed(2)}');
 log('floorToDouble: $price: ${(price - _priceRemainder).toStringAsFixed(2)}');
 log('roundToDouble: $price: ${(price - _priceRemainder).toStringAsFixed(2)}');
}
log('-----------------------');
}

调试控制台:


[log] price: 10.91, count: 0.05
[log] _price: 0.01
[log] _pricePlus: 5.00
[log] _someDiff > 2
[log] ceilToDouble: 10.91: 10.95
[log] floorToDouble: 10.91: 10.90
[log] roundToDouble: 10.91: 10.90
2
[log] -----------------------
[log] price: 10.91, count: 0.1
[log] _price: 0.01
[log] _pricePlus: 10.00
[log] _someDiff > 2
[log] ceilToDouble: 10.91: 11.00
[log] floorToDouble: 10.91: 10.90
[log] roundToDouble: 10.91: 10.90
2
[log] -----------------------
[log] price: 10.91, count: 0.2
[log] _price: 0.11
[log] _pricePlus: 1.82
[log] _someDiff > 1 && _someDiff <= 2 || _someDiff.toStringAsFixed(2) == 2.00
[log] ceilToDouble: 10.91: 11.00
[log] floorToDouble: 10.91: 10.80
[log] roundToDouble: 10.91: 11.00
2
[log] -----------------------
[log] price: 10.91, count: 0.25
[log] _price: 0.16
[log] _pricePlus: 1.56
[log] _someDiff > 1 && _someDiff <= 2 || _someDiff.toStringAsFixed(2) == 2.00
[log] ceilToDouble: 10.91: 11.00
[log] floorToDouble: 10.91: 10.75
[log] roundToDouble: 10.91: 11.00
2
[log] -----------------------
[log] price: 10.91, count: 0.5
[log] _price: 0.41
[log] _pricePlus: 1.22
[log] _someDiff > 1 && _someDiff <= 2 || _someDiff.toStringAsFixed(2) == 2.00
[log] ceilToDouble: 10.91: 11.00
[log] floorToDouble: 10.91: 10.50
[log] roundToDouble: 10.91: 11.00
2
[log] -----------------------
[log] price: 10.91, count: 1.0
[log] _price: 0.91
[log] _pricePlus: 1.10
[log] _someDiff > 1 && _someDiff <= 2 || _someDiff.toStringAsFixed(2) == 2.00
[log] ceilToDouble: 10.91: 11.00
[log] floorToDouble: 10.91: 10.00
[log] roundToDouble: 10.91: 11.00
2
[log] -----------------------
[log] price: 10.91, count: 10.0
[log] _price: 0.91
[log] _pricePlus: 10.99
[log] _someDiff > 2
[log] ceilToDouble: 10.91: 20.00
[log] floorToDouble: 10.91: 10.00
[log] roundToDouble: 10.91: 10.00