Bits & Bytes

Hello all ! I was just wondering if anyone migth have any ideas about how i migth fix this problem. The problem is in this testfunction. I am reading in 4 bytes into a unsigned integer, but the data is backwards and i am tring to flip it around to display the correct vaules. After reading in 4 bytes and displaying
the data the output is " 42010000 ". However i was tring to flip the bits so it would be displayed as " 00000142 ". Does anyone have any ideas that migth help me here ???

void Test::testFunction(ifstream &inFile)
{
unsigned int buffer;

inFile.seekg(6240, ios::beg);
inFile.read(reinterpret_cast<char *>(&buffer), 4);

data = (buffer<<8)|buffer; // <--- not working here

}

thanks for any help guys :)

I'm not sure this is the simplest way, but it does the job.

Code:

---

  buffer = (buffer&0x000000ff)<<24 | (buffer&0x0000ff00)<<8 | (buffer&0x00ff0000)>>8 | (buffer&0xff000000)>>24;

---

You can also use a union and bit fields.

Code:

---

unsigned int flip(unsigned int a)
{
  union int_byte
  {
      unsigned int i;
      struct
      {
        int b1: 8;
        int b2: 8;
        int b3: 8;
        int b4: 8;
      } byte;
  };

  int_byte temp, result;

  temp.i = a;
  result.byte.b1 = temp.byte.b4;
  result.byte.b2 = temp.byte.b3;
  result.byte.b3 = temp.byte.b2;
  result.byte.b4 = temp.byte.b1;

  return result.i;
}

---

And to call function:
data = flip(buffer);

That was an excellent explanation, Swoop. Good work!

Thanks Sebasti ! :D

I dunno if this is what u r looking for but it works for flippin things around I only breifly read ur post. Here it goes.

Code:

---

#include <iostream.h>
#include <stdlib.h>
#include <string.h>
int main()
{    char in[51], out[51];
      int i=0,x,h=0;
      cout<<"Input Some Text No Less Then 50 Chars\n";
      cin>>in;
      x=strlen(in);
      while (h<x)//between here
      { out[i]=in[x];
        cout<<out[i];
        x--;
        i++;
        h++;
      }//and here

      system("PAUSE");
      return 0;
}

---

I never tried to compile it so It may not work. I didn't even go over it really. It s just to give you the idea of what I mean;

Here's a flip, using only shifts.

Code:

---

  buffer = buffer<<24 | buffer>>8<<24>>8 | buffer<<8>>24<<8 | buffer>>24;

---

Hey guys ... thanks for the replies :)

Swoopy i tried your first solution, and it worked great for any variable. However one problem, i dont understand what its really doing, hehe. Guess i will have to find some good information on the net about bit shifting (since all books i have only give a brief introdution on that topic). As for the union and bit fields i havent
had a chance to try that one yet. Anyways i just seen your lastest post with fliping using only shifts, that one looks a little more easy to understand compared to the "0X0000ff00" solution.
But, i have a question ... are the 8 and 24 magic numbers used because of a 32bit OS ? will this still work on a 16bit OS ? I see why 8 would be used, just not sure why the value 24 is used here.

Thanks again for the help :)

>But, i have a question ... are the 8 and 24 magic numbers used because of a 32bit OS ?

Yes. This code assumes an int is 4 bytes (32 bits). In your case it appears each byte needs to be moved as follows:

Code:

---

int before flip
----------------
1    2    3    4
----------------

int after flip
----------------
4    3    2    1
----------------

---

So,
byte 1 -> byte 4
byte 2 -> byte 3
byte 3 -> byte 2
byte 4 -> byte 1

If you notice, to get byte 1 to byte 4, you need to shift three bytes (24 bits). byte 2 only need to be shifted one byte (8 bits). Hence the numbers 24 and 8.

If your int is 16 bits, the only thing possible is to swap each byte:

buffer = (buffer>>8) | (buffer<<8)

As a matter of fact, my computer at work uses a 16 bit int, so I used a long (32 bits) to test the shift-only method.

The idea of the shifting of course is to shift each byte to the new position.

Hope this helps