Two's complement and loss of information in C

Question

I want do the two's complement of a float data.

    unsigned long Temperature ; 
    Temperature = (~(unsigned long)(564.48))+1;

But the problem is that the cast loses information, 564 instead of 564.48. Can i do the two's complement without a loss of information?

Answer 1

That is a very weird thing to do; floating-point numbers are not stored as 2s complement, so it doesn't make a lot of sense.

Anyway, you can perhaps use the good old union trick:

union {
  float real;
  unsigned long integer;
} tmp = { 564.48 };

tmp.integer = ~tmp.integer + 1;
printf("I got %f\n", tmp.real);

When I tried it (on ideone) it printed:

I got -0.007412

Note that this relies on unspecified behavior, so it's possible it might break if your compiler does not implement the access in the most straight-forward manner. This is distinct form undefined behavior (which would make the code invalid), but still not optimal. Someone did tell me that newer standards make it clearer, but I've not found an exact reference so ... consider yourself warned.

Answer 2

You can't use ~ over floats (it must be an integer type):

#include <stdio.h>

void print_binary(size_t const size, void const * const ptr)
{
    unsigned char *b = (unsigned char *) ptr;
    unsigned char byte;
    int i, j;

    for (i = size - 1; i >= 0; i--) {
        for (j = 7; j >= 0; j--) {
            byte = b[i] & (1 << j);
            byte >>= j;
            printf("%u", byte);
        }
    }
    printf("\n");
}

int main(void)
{
    float f = 564.48f;
    char *p = (char *)&f;
    size_t i;

    print_binary(sizeof(f), &f);
    for (i = 0; i < sizeof(float); i++) {
        p[i] = ~p[i];
    }
    print_binary(sizeof(f), &f);
    f += 1.f;
    return 0;
}

Output:

01000100000011010001111010111000
10111011111100101110000101000111

Of course print_binary is there for test the result, remove it, and (as pointed out by @barakmanos) print_binary assumes little endian, the rest of the code is not affected by endiannes:

#include <stdio.h>

int main(void)
{
    float f = 564.48f;
    char *p = (char *)&f;
    size_t i;

    for (i = 0; i < sizeof(float); i++) {
        p[i] = ~p[i];
    }
    f += 1.f;
    return 0;
}

Answer 3

Casting a floating-point value to an integer value changes the "bit contents" of that value.

In order to perform two's complement on the "bit contents" of a floating-point value:

float f = 564.48f;
unsigned long Temperature = ~*(unsigned long*)&f+1;

Make sure that sizeof(long) == sizeof(float), or use double instead of float.