double to hex string & hex string to double

Question

What I\'m trying to do is to convert a double to hex string and then back to double.

The following code does conversion double-to-hex string.

char *          


        

Answer 1

Almost the same procedure should do

void hex2double(const char* buf, double& a)
{
   char tmpbuf[3]={0};
   char *d2c;
   unsigned int tmp;
   d2c = (char *) &a;
   char *n = buf;
   int i;
   for(i = 0; i < 8; i++)
   {
      tmpbuf[0]=*buf++;
      tmpbuf[1]=*buf++;
      sscanf(tmpbuf, "%X", &tmp);
      *d2c++=tmp;
   }
}

Quick & dirty.

Note, however, that this is playing with fire. First, your hex strings are only usable on machines with the same double format, and the same endianness. Second, the conversion functions are short on strict aliasing rule.

Answer 2

I am using the below function, to convert double to hexadecimal.

char * double2HexString(double a) 
{ 
   char *buf = new char[17]; // double is 8-byte long, so we have 2*8 + terminating \0 
   char *d2c; 
   d2c = (char *) &a; 
   char *n = buf; 
   int i; 
   for(i = 0; i < 8; i++) 
   { 
      sprintf(n, "%02X", *d2c++); 
      n += 2; 
   }  
   *(n) = '\0'; 
}  

passing 1.0 & 16.0 to this function, the return values observed are 000000000000FF37 & 0000000000003040 respectively. I think we should get 1 & 10.

Answer 3

Using sprintf is slow, to be honest, but you can revert it with sscanf, doing almost exactly the same thing.

Well, actually, you'd have to copy each two characters to a buffer string, to decode each individually. My first try, below is incorrect:

double hexString2Double(char *buf)
{
  char *buf2 = new char[3];
  double a;
  char* c2d;
  c2d = (char *) &a;
  int i;

  buf2[2] = '\0'

  for(i = 0; i < 16; i++)
  {
    buf2[0] = *buf++;
    buf2[1] = *buf++;
    sscanf(buf2, "%X", c2d++);
  }

  return a;
}

You see, %X is decoded as an int, not as a byte. It might even work, depending on low-ending/high-endian issues, but it's basically broken. So, let's try to get around that:

double hexString2Double(char *buf)
{
  char *buf2 = new char[3];
  double a;
  char* c2d;
  c2d = (char *) &a;
  int i;
  int decoder;

  buf2[2] = '\0'

  for(i = 0; i < 16; i++)
  {
    buf2[0] = *buf++;
    buf2[1] = *buf++;
    sscanf(buf2, "%X", &decoder);
    c2d++ = (char) decoder;
  }

  return a;
}

Barring syntax errors and such, I think this should work.