C＃float.ToString舍入值 [英] C# float.ToString Rounding Values

问题描述

也许我错过了一些东西，但是 float.ToString（）方法将数字舍去了数字，这使我很头痛。
在以下小代码中：
当输入12345678作为输入时，调试器中的浮点数是正确的，但 ToString 方法的输出是12345680（任何格式我试过...）

$ p $ Console.WriteLine（请输入一个浮点数）;
float theFloat = float.Parse（Console.ReadLine（））;
Console.WriteLine（string.Format（You have entered {0}，theFloat.ToString（F）））;


并输出：

 请输入浮点数
 12345678 
您已输入12345680.00 
  

帮助将不胜感激！

解决方案

从float的文档：

• 范围的浮点数是-3.4×10 ^ 38到+3.4×10 ^ 38
  
• 浮点数的精确度是7位数字。
  

您的电话号码（ 12345678 ）长度超过了8位，所以默认 到7位有效数字，产生 12345680 。 （注意默认。）

然而，尽管微软的文章中提到了浮点数的精确度，但事实上， 9位精度。

Single.ToString（） 的Microsoft文档声明：

< blockquote>

默认情况下，返回值只包含精确度的7位数字，但内部最多保留9位数字。

然后继续说：

lockquote

如果你需要更高的精度，用G9格式指定格式总是返回9位数的精度或R，如果数字可以用这个精度表示，则返回7位;如果数字只能用最大精度表示，则返回9位数字。

有了这个信息，我们可以写下这个代码：

  Console.WriteLine（12345678f.ToString（G9））; 
  

确实可以打印 12345678 p>

我无法解释的是为什么微软声明一个浮点数有7个数字的精度，然后继续让我们使用9位数字...

但是，请注意，并非所有8位（或9位）十进制数字整数都将在浮点数中具有精确的表示形式，如下面的代码所示（最后一位数字不同）： b
$ b

  Console.WriteLine（16777217f.ToString（R））; //打印16777216 
  

Maybe I am missing something but the float.ToString() method rounds numbers, which causes me a lot of headache.

Take a look at the following small code: When entering 12345678 as input, the float number in the debugger is correct but the output of the ToString methods is 12345680 (in any format I've tried...)

Console.WriteLine("Please enter a float number");
float theFloat = float.Parse(Console.ReadLine());
Console.WriteLine(string.Format("You have entered {0}", theFloat.ToString("F")));

And the output:

Please enter a float number
12345678
You have entered 12345680.00

Help will be much appreciated!

解决方案

From the documentation for float:

• The range of a float is -3.4 × 10^38 to +3.4 × 10^38
• The precision of a float is 7 digits.

Your number, 12345678, at 8 digits long exceeds the precision, so it is by default being rounded to 7 significant digits, which yields 12345680. (Note the by default.)

However, despite what that Microsoft article says about the precision of a float, in reality it holds up to 9 digits of precision.

The Microsoft documentation for Single.ToString() states:

By default, the return value only contains 7 digits of precision although a maximum of 9 digits is maintained internally.

It then goes on to say:

If you require more precision, specify format with the "G9" format specification, which always returns 9 digits of precision, or "R", which returns 7 digits if the number can be represented with that precision or 9 digits if the number can only be represented with maximum precision.

Armed with this information, we can write this code:

Console.WriteLine(12345678f.ToString("G9"));

Which does indeed print 12345678.

What I can't explain is why Microsoft state that a float has 7 digits of precision, and then goes on to let us use 9 digits...

However, note that not all 8 (or 9) decimal digit integers will have an exact representation in a float, as the following code demonstrates (the last digit differs):

Console.WriteLine(16777217f.ToString("R")); // Prints 16777216

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