在C％F说明的联合国predictable行为 [英] Unpredictable behaviour of %f specifier in c

问题描述

通过以​​下code：

With following code:

float f = 23.456;
printf("%f", f);

输出，适合的不同率F 是：

 Value       Output
---------    --------       
f = 23.456   23.455999 
f = 23.956   23.955999
f = 23.947   23.947001
f = 23.656   23.656000

为什么值获得的未predictable还是有规律可循？

Why is the value obtained unpredictable or there is a pattern?

推荐答案

这是因为 IEEE的754 32位浮点格式这是用来用C来存储浮点数。你有尾数转换为二进制，并存储在浮动变量所需的4个字节的空间低23位。有时它是完全转换为精确的二进制，在其他时间，可能无法精确转换这样的数量的截短形式被存储，这是从存储器读取时，计算结果为稍微不同的值。

It is because of IEEE 754 32-bit floating point format which is used by C to store floating point numbers. The Mantissa you have is converted in binary and stored in lower 23 bits of 4-byte space required by float variable. Sometimes it is completely convertible to exact binary, at other times it may not be exactly converted so a truncated form of the number is stored, which is when read from memory, evaluates to a slightly different value.

在mathemetics您使用值这个行为类似于 1/3 0.33或0.333取决于您的precision的要求。

This behaviour is similar as in mathemetics you use value of 1/3 as 0.33 or 0.333 depending on your requirement of precision.

请尝试使用：结果
F = 0.25; 或结果
F = -15.625; 或任何十进制值完全转换为二进制文件，你会得到正确的结果。

Try using:
f = 0.25; or,
f = -15.625; or, any decimal value exactly convertible to binary, you will get correct result.

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