Question

I wrote this code to do the IEEE 754 floating point arithmetic on a 4byte string.

It takes in the bytes, converts them to binary and with the binary I get the sign, exponent, and mantissa and then do the calculation.

It all works just about perfectl, 0xDEADBEEF gives me 6259853398707798016 and the true answer is 6.259853398707798016E18, now these are same values and I wont have anything this large in the project I’m working with, all other smaller values put the decimal in the correct place.

Here is my code:
float calcByteValue(uint8_t data[]) {
int i;
int j = 0;
int index;
int sign, exp;
float mant;

   char bits[8] = {0};
   int *binary = malloc(32*sizeof *binary);
   for (index = 0;index < 4;index++) {
      for (i = 0;i < 8;i++,j++) {
         bits[i] = (data[index] >> 7-i) & 0x01;
         if (bits[i] == 1) {
            binary[j] = 1;
         } else {
            binary[j] = 0;
         }
      }
      printf("\nindex(%d)\n", index);
   }

   sign = getSign(&(binary[0]));
   mant = getMant(&(binary[0]));
   exp = getExp(&(binary[0]));

   printf("\nBinary: ");
   for (i = 0;i < 32;i++)
      printf("%d", binary[i]);
   printf("\nsign:%d, exp:%d, mant:%f\n",sign, exp, mant);

   float f = pow(-1.0, sign) * mant * pow(2,exp);
   printf("\n%f\n", f);
   return f;
}

//-------------------------------------------------------------------

int getSign(int *bin) {
   return bin[0];
}

int getExp (int *bin) {
     int expInt, i, b, sum;
     int exp = 0;

     for (i = 0;i < 8;i++) {
        b = 1;
        b = b<<(7-i);
        if (bin[i+1] == 1)
           exp += bin[i+1] * b;
     }

     return exp-127;

}

float getMant(int *bin) {
   int i,j;
   float b;
   float m;
   int manBin[24] = {0};
   manBin[0] = 1;
   for (i = 1,j=9;j < 32;i++,j++) {
       manBin[i] = bin[j];
       printf("%d",manBin[i]);
   }
   for (i = 0;i < 24;i++) {
      m += manBin[i] * pow(2,-i);;
   }
   return m;
}

Now, my teacher told me that there is a much easier way where I can just take in the stream of bytes, and turn it into a float and it should work. I tried doing it that way but could not figure it out if my life depended on it.

I’m not asking you to do my homework for me, I have it done and working, but I just need to know if I could of done it differently/easier/more efficiently.

EDIT: there are a couple special cases I need to handle, but it’s just things like if the exponent is all zeros blah blah blah. Easy to implement.

Answer 1

Here is my working code:

    unsigned char val[4] = {0, 0, 0xc8, 0x41};
    cout << val << endl;

    cout << "--------------------------------------------" << endl;
    float f = *(float*)&val;

    cout << f << endl;
    return 0;