C# 打印双值
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Printing double values
提问by dharmatech
On my system, the following code prints '3.6':
在我的系统上，以下代码打印“3.6”：
double a = 1.2;
int b = 3;
double c = a * b;
Console.WriteLine(c);
But in the debugger, I can see that chas a value with more than 2 digits:
但是在调试器中，我可以看到c有一个多于 2 位的值：
I know that I can display the full representation with Console.WriteLine("{0:R}", c). Is this the only and recommended way to display the actual value of a double?
我知道我可以用Console.WriteLine("{0:R}", c). 这是显示 a 的实际值的唯一且推荐的方法double吗？
update
更新
Going with the above example, I'd like to print csuch that if the user were to take the printed value and insert that back into the code in a test using ==, the comparison would be true. In this case c == 3.5999999999999996returns true.
按照上面的例子，我想打印c，如果用户要使用打印的值并将其插入到测试中的代码中==，则比较结果为真。在这种情况下c == 3.5999999999999996返回 true。
回答by John
double a = 1.2;
int b = 3;
double c = a * b;
string formatted = c.ToString("N5");
Console.WriteLine(formatted);
回答by Tim Schmelter
Console.WriteLinecalls Double.ToStringwhich uses the the "G" format specifier. This uses the current culture to determine the number of decimal places (1 for "en-US").
Console.WriteLine呼叫Double.ToString其使用的“G”格式说明。这使用当前区域性来确定小数位数（“en-US”为 1）。
If you want to display 8 decimal places you can use the numeric format specifier:
如果要显示 8 位小数，可以使用数字格式说明符：
Console.WriteLine(c.ToString("N8"));
Standard Numeric Format Strings
Edit: The debugger uses this method to convert a double to a string:
编辑：调试器使用此方法将双精度转换为字符串：
I assume it's the cheapest way to convert it.
我认为这是转换它的最便宜的方式。
Where i have found it: How does Visual Studio display a System.Double during debugging?
回答by MethodMan
You could also use a different approach for example if you want to return 2 decimal places you could try something like this
您也可以使用不同的方法，例如，如果您想返回 2 个小数位，您可以尝试这样的操作
double a = 1.2;
int b = 3;
double c = a * b;
var s = string.Format("{0:0.00}", c);
Console.WriteLine(s);
Output = 3.60
输出 = 3.60
if you want to suppress the last 0 where out put is 3.6 you could do
如果您想抑制输出为 3.6 的最后一个 0，您可以这样做
var s = string.Format("{0:0.##}", c);
Output = 3.6 feel free to play around with it
输出 = 3.6 随意使用它
回答by Eric Lippert
3.999999999999996 is not the actual value of the double either; that's just the value rounded off to fifteen places or whatever. There is no built-in way to display the actual exactvalue that the double is representing. This is really too bad, because every normal double can be represented exactly as a decimal string, and it would be nice to be able to see that.
3.999999999999996 也不是 double 的实际值；这只是四舍五入到十五位之类的值。没有内置方法来显示double 所代表的实际确切值。这真的太糟糕了，因为每个普通的 double 都可以精确地表示为一个十进制字符串，如果能够看到这一点就好了。
As a public service, I've put source code for a device which does that on my blog:
作为一项公共服务，我在我的博客上放置了一个设备的源代码：
http://ericlippert.com/2011/02/17/looking-inside-a-double/
http://ericlippert.com/2011/02/17/looking-inside-a-double/
Note that it uses the Rational class from Microsoft Solver Foundation. If you don't have that then you can either download it for free, or write your own Rational class; it's character-building to do so.
请注意，它使用来自 Microsoft Solver Foundation 的 Rational 类。如果您没有，那么您可以免费下载它，或者编写您自己的 Rational 类；这样做是为了塑造性格。
If the subject of how doubles work internally interests you, consider checking out my archive of handy articles explaining all that. It's at:
如果您对双打内部如何工作的主题感兴趣，请考虑查看我解释所有这些的方便文章档案。它在：
http://blogs.msdn.com/b/ericlippert/archive/tags/floating+point+arithmetic/
http://blogs.msdn.com/b/ericlippert/archive/tags/floating+point+arithmetic/
Start from the bottom; those are in reverse-chronological order.
从底部开始；这些是按时间倒序排列的。
