Reverse byte order using union, How?

Hello !

Now, I am able to get reverse byte order using the following function:

Code:

---

void GetReverseHexa(int n) {
        int a, b, c, d, e;
        a = n & 0x000000FF;
        b = n & 0x0000FF00;
        c = n & 0x00FF0000;
        d = n & 0xFF000000;

        a <<= 24;
        b <<= 8;
        c >>= 8;
        d >>=24;

        e = a|b|c|d;

        printf("\nReverse byte order: %2x\n\n",e);
}

---

parameter n is the given number (user input). Now, the task it to come up with the same output but using union. Anyone who has an idea?

Code:

---

union Word
{
        DWORD dword;
        struct
        {
                BYTE lo_byte_short0;
                BYTE hi_byte_short0;
                BYTE lo_byte_short1;
                BYTE hi_byte_short1;
        } bytes;
};

Word Reverse(Word& w)
{
        Word result;
        result.bytes.lo_byte_short0 = w.bytes.hi_byte_short1;
        result.bytes.hi_byte_short0 = w.bytes.lo_byte_short1;
        result.bytes.lo_byte_short1 = w.bytes.hi_byte_short0;
        result.bytes.hi_byte_short1 = w.bytes.lo_byte_short0;

        return result;
}

int main()
{
        Word a;
        a.dword = 0x12345678;
       
        printf("\nReverse byte order: %2x\n\n", Reverse(a).dword);


        return 0;
}

---

Ok, I got the answer I want. Now, I want to share the code here

Code:

---

void GetReverseByteOrder(int n) {
  union {
      int nGiven;
      unsigned char val[4];
  } HEXA;

  HEXA.nGiven = n; /* n is input of user */
  for (int i=0; i<4; i++)
      printf("%02x",HEXA.val[i]);
}

---

What does this piece of code do?
-- user types in a number, say 123456
-- 123456 is stored to n and passed to HEXA.nGiven
-- we know that 123456 is 1E240 in base 16
-- now, we wanted to reverse 1E240 making it 40E20100
-- 40E20100 is generated by the code above

How come? Can u imagine how? :-)

That's it! A generous man from codeguru shared the above code. I just did a little modification. I forgot his username here in this forum but anyway, many many thanks to him!

Kheun,

Thank you for your reply.
Anyway, I found a shorter code though I do not know if the one I found is good to use. What do you think?

Anyway, how does your code differs to the code I got?

Well, both are doing almost the same thing except that your code only displays in reverse order without the actual reversing of the int. :D