Solved

Changing a bit value in a binary file

Posted on 1999-01-05
9
203 Views
Last Modified: 2010-04-02
Hi,

I have pulled out my hair trying to figure this out.  I've got the following code which finds a unique number within a bitmap file, and lets the calling method know if the bit representation is turned on or off:

byte valByte;
ifstream readMap (BMP_FILE, ios::in|ios::binary);

if (readMap) {
  // param is a 6-digit number string
  uint16 bitNum = atoi ((char *) &param);
  uint32 byteOffset = bitNum / 8;
  uint16 bitOffset  = bitNum % 8;
  readMap.seekg (byteOffset, ios::beg);
  // Error checking deleted
  readMap.seekg (0, ios::end);
  // Error checking deleted
  readMap.read (&valByte, 1);
  if (valByte & (int)pow (2, bitOffset) && readMap)
    // This bit is turned on
  else
    // This bit is turned off

I now need to be able to flip the bit value.  I was thinking I could try something like this:

if ((valByte & (int)pow (2, bitOffset)) == 0) {
  // If the bit is off, turn it on
  (valByte & (int)pow (2, bitOffset)) = 1;
} else {
  // The card must be on, turn it off
  (valByte & (int)pow (2, bitOffset)) = 0;
};
// Now save the byte back to the file

But for obvious reasons (no Lvalue), this won't work.

For information, this code will only be run on PC-based (8-bit byte based) machines.  Any help would be immensely appreciated!!
0
Comment
Question by:cgoldfarb
  • 3
  • 3
  • 2
  • +1
9 Comments
 
LVL 84

Expert Comment

by:ozo
ID: 1181605
valByte ^= (1<<bitOffset);
0
 
LVL 7

Expert Comment

by:faster
ID: 1181606
ozo means to replace all pow (2, bitOffset) with (1<<bitOffset) and for flip the bit value, use
valByte ^= (1<<bitOffset); this single line does the job of your if/else block.
0
 

Author Comment

by:cgoldfarb
ID: 1181607
Thank you both for your help.  It's exactly what I was looking for.  Will hopefully return the favor...
0
 

Accepted Solution

by:
erick1217 earned 200 total points
ID: 1181608
try to turning on
valByte |= (int)pow (2, bitOffset);

and to turning off

valByte &= 256 - (int)pow (2, bitOffset);



0
Top 6 Sources for Identifying Threat Actor TTPs

Understanding your enemy is essential. These six sources will help you identify the most popular threat actor tactics, techniques, and procedures (TTPs).

 
LVL 84

Expert Comment

by:ozo
ID: 1181609
255-pow may work better than 256-pow
but it's still pretty silly to use transcendental functions to approximate the << operator
0
 

Expert Comment

by:erick1217
ID: 1181610
sorry ozo but the power of bit 4 (by example) is 8
and 256 - 8 = 248
valByte &= 248 will set the bit 4 to zero
0
 
LVL 84

Expert Comment

by:ozo
ID: 1181611
valByte &= 248 will set bits 0, 1, and 2 to zero
0
 

Expert Comment

by:erick1217
ID: 1181612
yes.. you're right... excuse me...
0
 

Author Comment

by:cgoldfarb
ID: 1181613
erick1217's answer was good, although ozo and faster hit the nail on the head.  They really deserve the points, as they were the first to respond.
0

Featured Post

Highfive + Dolby Voice = No More Audio Complaints!

Poor audio quality is one of the top reasons people don’t use video conferencing. Get the crispest, clearest audio powered by Dolby Voice in every meeting. Highfive and Dolby Voice deliver the best video conferencing and audio experience for every meeting and every room.

Join & Write a Comment

When writing generic code, using template meta-programming techniques, it is sometimes useful to know if a type is convertible to another type. A good example of when this might be is if you are writing diagnostic instrumentation for code to generat…
C++ Properties One feature missing from standard C++ that you will find in many other Object Oriented Programming languages is something called a Property (http://www.experts-exchange.com/Programming/Languages/CPP/A_3912-Object-Properties-in-C.ht…
The goal of the tutorial is to teach the user how to use functions in C++. The video will cover how to define functions, how to call functions and how to create functions prototypes. Microsoft Visual C++ 2010 Express will be used as a text editor an…
The viewer will be introduced to the member functions push_back and pop_back of the vector class. The video will teach the difference between the two as well as how to use each one along with its functionality.

707 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

18 Experts available now in Live!

Get 1:1 Help Now