如何在 C 中使用按位运算符翻转浮点数的符号

Question

This is the code I have so far

// given the 32 bits of a float return it with its sign flipped
uint32_t sign_flip(uint32_t f) {
    int mask = 1;    
    f = ~f; // 1's Complement 
    while (mask > 0 && (mask & f)) 
    { 
        f ^= mask; 
        mask <<= 1; 
    } 
    f ^= mask; // 2's complment 
    return f; // REPLACE ME WITH YOUR CODE
}

Expected output:

./sign_flip -42
sign_flip(-42) returned 42

output:

./sign_flip -42
sign_flip(-42) returned 0.10546875

How would I go about fixing this? I am not sure what the issue is

Answer 1

IEEE 754 floating-point format is not 2's complement. Just flip most-significant bit:

float f = 42;
*(uint32_t*)&f ^= (1 << 31);
printf("%f\n", f);

Answer 2

It is reasonable to expect a float to have a single isolated sign bit regardless of encoding, IEEE 754 or not.

Find that bit and flip it. No need to assume common endian-ness of float/uint32_t . Use a union to avoid strict aliasing violations.

uint32_t sign_flip(uint32_t f) {
  assert(sizeof(float) == sizeof(uint32_t));
  static const union {
    uint32_t u32;
    float f;
  } plus1 = { .f = 1.0f }, minus1 = { .f = -1.0f };
  return f ^ (plus1.u32 ^ minus1.u32);
}