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将浮点数转换为 4 uint8_t

[英]Convert a float to 4 uint8_t

 原文  2014-08-19 14:48:34  2   4    c++/ casting/ type-conversion/ floating-point-conversion/ uint8t

问题描述

I have got a float variable that I need to send through a CAN protocol.我有一个float变量，需要通过 CAN 协议发送。 To do so, this float of 32 bits must be cut in 4 uint8_t variables .为此，必须将这个 32 位浮点数切成4 个uint8_t变量。

I have absolutely no idea of how to do.我完全不知道该怎么做。 I was first thinking of convert the float to an int but some answers that I found on the internet, which use cast or union doesn't seems to work.我首先考虑将浮点数转换为 int 但我在互联网上找到的一些答案，使用cast或union似乎不起作用。

Here's a simple example of what I am trying to do :这是我正在尝试做的一个简单示例：

float f;
uint8_t ut1,ut2,ut3,ut4;

//8 first bits of f into ut1
//8 second bits of f in ut2
...

// Then I can send the uint8_t through CAN
...

4 个解决方案

解决方案1
 8 已采纳  2014-08-19 14:54:09

You normally do this by casting the float to an array of uint8_t.您通常通过将浮点数转换为 uint8_t 数组来完成此操作。

In C you can do it like this:在 C 中，你可以这样做：

uint8_t *array;
array = (unit8_t*)(&f);

in C++ use the reinterpret_cast在 C++ 中使用 reinterpret_cast

uint8_t *array;
array = reinterpret_cast<uint8_t*>(&f);

Then array[0], ..., array[3] are your bytes.然后 array[0], ..., array[3] 是你的字节。

解决方案2
 2  2014-08-19 16:18:18

First you should note that the standard imposes no specific size restrictions on float .首先，您应该注意标准对float没有特定的大小限制。 It's possible that a float wouldn't fit into four bytes on some imaginable architecture (although I'm not aware of any).在某些可以想象的架构上， float可能不适合四个字节（尽管我不知道）。 You should at least (static_)assert that it will fit before attempting anything.在尝试任何事情之前，您至少应该 (static_) 断言它适合。

Then I think the simplest way is to assert that CHAR_BIT is 8 , and use the legal aliasing to unsigned char* with reinterpret_cast :然后我认为最简单的方法是断言CHAR_BIT是8 ，并使用reinterpret_cast对unsigned char*的合法别名：

static_assert(sizeof(float) == 4);
float f = 0; // whatever value
unsigned char* float_as_char = reinterpret_cast<unsigned char*>(&f);

This totally ignores the endian issue though, so maybe what you really want is to make a copy of the bytes so you can fix that up:不过，这完全忽略了字节序问题，所以也许您真正想要的是制作字节的副本，以便您可以修复它：

static_assert(sizeof(float) == 4);
float f = 0; // whatever value
uint8_t bytes[4];
std::memcpy(bytes, &f);
// Fix up the order of the bytes in "bytes" now.

解决方案3
 1  2014-08-19 14:58:53

Here's a union approach that gives separate names for integer parts instead of one array: 这是一个联合方法，它为整数部分而不是一个数组提供单独的名称： 

union {
    float f;
    struct {
        uint8_t ut1, ut2, ut3, ut4;
    } bytes;
} value;
value.f = 1.f;
uint8_t first = value.bytes.ut1;

I was initially concerned that this use of union is not strictly legal according to the standard: C++ Undefined behaviour with unions but ComicSansMS's argument in a comment to rashmatash's answer is compelling. 我最初担心的是，根据标准，这种union使用并不严格合法： C ++ Undefined对工会的行为，但ComicSansMS在对rashmatash的答案的评论中的论点是引人注目的。

解决方案4
 1  2014-08-19 15:00:02

You can do this illegal operation:你可以做这个非法操作：

float f = someFloatValue;
uint8_t* i = reinterpret_cast<uint8_t*>(&f);

Although this works most of the time, it is not supported by c++ standard and compilers might generate code with undefined behaviour.尽管这在大多数情况下都有效，但 C++ 标准不支持它，并且编译器可能会生成具有未定义行为的代码。

Another solution is using unions:另一种解决方案是使用联合：

union{
    float f;
    uint8_t i[4];
}
f = someFloatValue;
// now i's contain the bit pattern of f

It's unclear if all compilers yield consistent results, but it seems safer than the first aproach.尚不清楚是否所有编译器都会产生一致的结果，但它似乎比第一种方法更安全。

You can also pack the value of f in a 32-bit integer.您还可以将f的值打包为 32 位整数。 This, however can result in losing a bit of precision, but depending on how accurately you want to keep f , would be the best solution.然而，这可能会导致精度下降，但取决于您想要保持f准确程度，这将是最佳解决方案。

解决方案5
 0  2021-02-04 22:06:11

Here is a table based implementation based off this paper这是基于本文的基于表格的实现

It converts a 32 bit floating point number to a 16 bit floating point number which it stores in an unsigned 16 bit value.它将 32 位浮点数转换为 16 位浮点数，并将其存储在一个无符号的 16 位值中。 The two LUT's are in static memory so arrays of these can be created, and it handles all the corner cases along with being fast.(see the paper for details)这两个 LUT 位于静态内存中，因此可以创建这些数组，并且它可以快速处理所有极端情况。（有关详细信息，请参阅论文）

using flt32 = float;
class flt16 {
public: // clucnky but functional and fast
flt16(flt32 in) { 
    ftoi_t fi{ in };
    half = basetable[(fi.i >> 23) & 0x1ff] + ((fi.i & 0x007fffff) >> shifttable[(fi.i >> 23) & 0x1ff]);
}

union ftoi_t{
    flt32 f;
    uint32 i;
};

uint16 half;

static constexpr uint16 basetable[512] = {
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 3072, 4096, 5120, 6144, 7168, 8192,
    9216, 10240, 11264, 12288, 13312, 14336, 15360, 16384, 17408, 18432, 19456, 20480, 21504, 22528,
    23552, 24576, 25600, 26624, 27648, 28672, 29696, 30720, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744,
    31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 31744, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768, 32768,
    32769, 32770, 32772, 32776, 32784, 32800, 32832, 32896, 33024, 33280, 33792, 34816, 35840, 36864,
    37888, 38912, 39936, 40960, 41984, 43008, 44032, 45056, 46080, 47104, 48128, 49152, 50176, 51200,
    52224, 53248, 54272, 55296, 56320, 57344, 58368, 59392, 60416, 61440, 62464, 63488, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512,
    64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512, 64512
};

static constexpr uint8 shifttable[512] = {
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 
    14, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 
    13, 13, 13, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 13, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 23, 22, 21, 20, 19, 
    18, 17, 16, 15, 14, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 
    13, 13, 13, 13, 13, 13, 13, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 
    24, 24, 24, 24, 24, 24, 24, 13
};

}; };

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