bigdecimal

Why do I get nil can't be coerced into BigDecimal when I try to perform a calculation: here's the code: model/drink.rb class Drink < ActiveRecord::Base belongs_to :menu before_save :total_amount def total_amount self.total_amount = self.price * self.quantity end model/menu.rb class Menu < ActiveRecord::Base has_many :drinks, :dependent => :destroy accepts_nested_attributes_for :drinks, :allow_destroy => true #Validations end * Drink is the (nested)child model and Menu the parent model When I attempt to create a new drink the browser display following error message nil can't be coerced into

I wanted to know how I can generate pi to the nth digit. I have a couple of basic ideas. Use Math.PI and increase the precision (if that's possible) Use Euler's formula to generate pi but even here, I would need to increase the precision (I think) There is also Srinivasa Ramanujan's formula for generating PI which is known for it's rapid convergence. This formula seems difficult to implement. I believe, I would have to also increase deicmal precision here. So in short, either way, I would need to increase the precision of BigDecimal depending on what the nth digit is. How would I go about

I'm looking for a method that returns a boolean if the String it is passed is a valid number (e.g. "123.55e-9", "-333,556"). I don't want to just do: public boolean isANumber(String s) { try { BigDecimal a = new BigDecimal(s); return true; } catch (NumberFormatException e) { return false; } } Clearly, the function should use a state machine (DFA) to parse the string to make sure invalid examples don't fool it (e.g. "-21,22.22.2", "33-2"). Do you know if any such library exists? I don't really want to write it myself as it's such an obvious problem that I'm sure I'd be re-inventing the wheel.

关于Map.merge()这可能是Key-Value中最通用的操作。但它也相当模糊很少使用。 merge() ：它将新值置于给定键下（如果不存在）或更新具有给定值的现有键（UPSERT）。 示例 ： //计算唯一的单词出现次数 var map = new HashMap<String, Integer>(); words.forEach(word -> { var prev = map.get(word); if (prev == null) { map.put(word, 1); } else { map.put(word, prev + 1); } }); //给定输入 对应的输出结果 var words = List.of("Foo", "Bar", "Foo", "Buzz", "Foo", "Buzz", "Fizz", "Fizz"); //... {Bar=1, Fizz=2, Foo=3, Buzz=2} //进行重构以避免条件逻辑 words.forEach(word -> { map.putIfAbsent(word, 0); map.put(word, map.get(word) + 1); }); putIfAbsent() 是必要的否则代码会在第一次出现未知的单词时中断。另外map.get(word) 里面map.put() 有点尴尬。优化 words

内容： import java.math.BigDecimal; import java.text.DecimalFormat; public class TestGetInt{ public static void main(String[] args){ double i=2, j=2.1, k=2.5, m=2.9; System.out.println("舍掉小数取整:Math.floor(2)=" + (int)Math.floor(i)); System.out.println("舍掉小数取整:Math.floor(2.1)=" + (int)Math.floor(j)); System.out.println("舍掉小数取整:Math.floor(2.5)=" + (int)Math.floor(k)); System.out.println("舍掉小数取整:Math.floor(2.9)=" + (int)Math.floor(m)); /* 这段被注释的代码不能正确的实现四舍五入取整 System.out.println("四舍五入取整:Math.rint(2)=" + (int)Math.rint(i)); System.out.println("四舍五入取整:Math.rint(2.1)=" + (int)Math.rint(j)); System.out

转贴：http://zhangyinhu8680.iteye.com/blog/1536397 BigDecimal 由任意精度的整数非标度值 和 32 位的整数标度 (scale) 组成。如果为零或正数，则标度是小数点后的位数。如果为负数，则将该数的非标度值乘以 10 的负 scale 次幂。因此，BigDecimal 表示的数值是 (unscaledValue × 10-scale)。 可以处理任意长度的浮点数运算。 BigDecimal add(BigDecimal val) //加法 BigDecimal subtract (BigDecimal val) //减法 BigDecimal multiply (BigDecimal val) //乘法 BigDecimal divide (BigDecimal val,RoundingMode mode) //除法 具体使用 计算： 　 　　加: a.add(b); 　　减: a.subtract(b); 　　乘: a.multiply(b); 　　除: a.divide(b,2);//2为精度取值 除法细解： //注意以下相除会抛出异常,原因: 通过BigDecimal的divide方法进行除法时当不整除，出现无限循环小数时，就会抛异常 //BigDecimal divideBg = a.divide(b); //解决方法是

内容： import java.math.BigDecimal; import java.text.DecimalFormat; public class TestGetInt{ public static void main(String[] args){ double i=2, j=2.1, k=2.5, m=2.9; System.out.println("舍掉小数取整:Math.floor(2)=" + (int)Math.floor(i)); System.out.println("舍掉小数取整:Math.floor(2.1)=" + (int)Math.floor(j)); System.out.println("舍掉小数取整:Math.floor(2.5)=" + (int)Math.floor(k)); System.out.println("舍掉小数取整:Math.floor(2.9)=" + (int)Math.floor(m)); /* 这段被注释的代码不能正确的实现四舍五入取整 System.out.println("四舍五入取整:Math.rint(2)=" + (int)Math.rint(i)); System.out.println("四舍五入取整:Math.rint(2.1)=" + (int)Math.rint(j)); System.out