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Replace value in QByteArray & QDataStream

Posted on 2011-03-10
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Last Modified: 2012-05-11
Hi experts,

I'm trying without success to replace some values in a byte array or in a data stream in QT 4.7

Here's my situation:
I have a binary file with data. Each two bytes of this file represent a signed integer value. I need to replace all the integer values which are 0 with 22 and save the file with the new values.

Any ideas how to proceed ?

Thanks,
Peter
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Question by:PeterVarain
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LVL 16

Expert Comment

by:HooKooDooKu
ID: 35098671
Unless there is an issue with byte order, you should be able to open the file in read/write mode, and call the Read function passing the pointer to a DWORD variable and reading two bytes.  If the variable has a value of 0, seek backwards 2 bytes and write the value 22.

CFile f;
f.Open( strFileName, CFile::modeRead | CFile::modeWrite );
DWORD i;
const DWORD v22 = 22;
while( f.Read( &i, 2 ) == 2 )
{
  if( i == 0 )
  {
    f.Seek( -2, CFile::current );
    f.Write( &v22 );
  }
}

If there is an issue with byte order, then you just need to replace v22=22 with v22=0x1600;
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LVL 16

Expert Comment

by:HooKooDooKu
ID: 35098683
Opps... that should have been something like
f.Write( &v22, 2 )

and you might have to type case the pointers to get something like this to properly compile.
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Author Comment

by:PeterVarain
ID: 35098977
Sorry, I haven't explained very clear what I want to do here...

The file which is saved is not the same file (the source file). I need to move all the data from the source file to a QByteArray and in the process change the 0 values to 22. Then, I need to compress this byte array with qCompress and only then write it to a newly created file.

So, my problem is that I have to move the data from the source file to the byte array and change to zeros...

All the best,
Peter
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LVL 33

Accepted Solution

by:
sarabande earned 500 total points
ID: 35107498
you could read the file using std::ifstream in binary mode:

   std::ifstream infile("xxx.dat", std::ios_base::in | std::ios_base::binary);
   qint16 q16;
   while (infile.read((char*)(&q16), sizeof(q16))
   {
   }

note the qint16 should be 2 bytes what i have taken for granted.

the in loop you could check on 0 and replace by 22. then add to QByteArray

  QByteArray qarr;
  ...
       
        if (q16 == 0)
            q16 = 22;
       qarr.append((char*)(&q16), 2);  

note, the QByteArray would get binary data that way, same as in the file. if you need hexstring conversion you mast do a conversion first.

Sara

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LVL 33

Expert Comment

by:sarabande
ID: 35107504
there is a ) missing at right of while statement.

Sara
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Author Closing Comment

by:PeterVarain
ID: 35114953
Thanks Sara!
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