如何使用可变精度的 Java String.format？
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How to Java String.format with a variable precision?
提问by Willi Ballenthin
I'd like to vary the precision of a double representation in a string I'm formatting based on user input. Right now I'm trying something like:
我想根据用户输入改变我正在格式化的字符串中双重表示的精度。现在我正在尝试类似的东西：
String foo = String.format("%.*f\n", precision, my_double);
however I receive a java.util.UnknownFormatConversionException. My inspiration for this approach was C printf and this resource(section 1.3.1).
但是我收到了一个java.util.UnknownFormatConversionException. 我对这种方法的灵感来自 C printf 和这个资源（第 1.3.1 节）。
Do I have a simple syntax error somewhere, does Java support this case, or is there a better approach?
我在某处有一个简单的语法错误，Java 支持这种情况，还是有更好的方法？
Edit:
编辑：
I suppose I could do something like:
我想我可以这样做：
String foo = String.format("%." + precision + "f\n", my_double);
but I'd still be interested in native support for such an operation.
但我仍然对这种操作的本机支持感兴趣。
采纳答案by vicatcu
You sort of answered your own question - build your format string dynamically... valid format strings follow the conventions outlined here: http://java.sun.com/j2se/1.5.0/docs/api/java/util/Formatter.html#syntax.
你有点回答你自己的问题 - 动态构建你的格式字符串......有效的格式字符串遵循此处概述的约定：http: //java.sun.com/j2se/1.5.0/docs/api/java/util/Formatter .html#syntax。
If you want a formatted decimal that occupies 8 total characters (including the decimal point) and you wanted 4 digits after the decimal point, your format string should look like "%8.4f"...
如果您想要一个占 8 个字符（包括小数点）的格式化小数，并且您想要小数点后 4 个数字，那么您的格式字符串应该类似于“%8.4f”...
To my knowledge there is no "native support" in Java beyond format strings being flexible.
据我所知，除了灵活的格式字符串之外，Java 中没有“本机支持”。
回答by Gerdi
Why not :
为什么不 ：
String form = "%."+precision+"f\n";
String foo = String.format(form, my_double);
or :
或者 ：
public static String myFormat(String src, int precision, Object args...)
{
String form = "%."+precision+"f\n";
return String.format(form, args);
}
回答by Willi Mentzel
You can use the DecimalFormatclass.
您可以使用DecimalFormat类。
double d1 = 3.14159;
double d2 = 1.235;
DecimalFormat df = new DecimalFormat("#.##");
double roundedD1 = df.format(d); // 3.14
double roundedD2 = df.format(d); // 1.24
If you want to set the precision at run time call:
如果要在运行时设置精度调用：
df.setMaximumFractionDigits(precision)
