Thread: Copying every 4th byte

Hello,

I want to copy every 4th byte of file_1.bin into file_2.bin
For example - if the contents of file_1.bin are :

0x0 // Byte 1
0x1 // Byte 2
0x2 // Byte 3
0xAB // Byte 4 Skip when copying.
0x3 // Byte 1
0x4 // Byte 2
0x5 // Byte 3
0xAA // Byte 4 Skip when copying.
0x6 // Byte 1
0x7 // Byte 2
0x8 // Byte 3
0xFF // Byte 4 Skip when copying.
0x9 // Byte 1
0xA // Byte 2
0xB // Byte 3
0x1 // Byte 4 Skip when copying.
0xC // Byte 1

the contents of file_2.bin will be :

0x0
0x1
0x2
0x3
0x4
0x5
0x6
0x7
0x8
0x9
0xA
0xB
0xC

file_1.bin can be any size.
This is the C++ program that I wrote - but it doesn't work.
Please help me fix it :

Code:

#include <iostream>
#include <fstream>
using namespace std ;


int main ( )
{
    ifstream file_1 ( "file_1.bin", ios::out | ios::app | ios::binary ) ;
    ofstream file_2 ( "file_2.bin", ios::out | ios::app | ios::binary ) ;
    int index ;
    char ch ;
    for ( index = 1 ; file_1.eof ( ) != 1 ; index ++ )
    {
        if ( index % 4 )
        {
            file_1.get(ch) ;
            file_2 << ch ;
        }
    }
    return 0 ;
}

I'm a bit (byte?) confused: do you want to copy every 4th byte or do you want to copy all the bytes except every 4th byte?

Either way, your for loop is wrong as you're not always reading a byte from the input file. I would suggest:

Code:

for ( int index = 0; file_1.get(ch); ++index )
{
    if ( index % 4 == 3 )
    {
        file_2 << ch ;
    }
}

The above would copy every 4th byte. If you want to copy all the bytes except every 4th byte, then you just invert the if statement condition.

Thanks!
It worked.

Can you please point out what where the problem with how I wrote the code ?

As I mentioned, the problem is that you're not always reading a byte on each iteration of the loop. That means that when you want to skip a byte, you don't: it just ends up read on the next iteration instead.