CODEWITHSUNDEEP
c++
Michail Dnischenko
Michail Dnischenko

Reputation: 29

How to increse numbers in file?

I need to read file in parts ( for example by 4 bytes) and then increment numbers in files by one and then write back; this part only fills in file by 1; How to increase this number on 1?

void Prepare()
{

    //ifstream fileRead("\FILE", ios::in | ios::binary);
    ofstream fileOut("\FILE.bin", ios::out | ios::binary);
  
   int count = 10485760;
    
        for (int i = 0; i < count-1; i++)
        {
            fileOut << 1;
        }
    
    fileOut.close();

}

Upvotes: 0

Views: 49

Answers (1)

Joseph Larson
Joseph Larson

Reputation: 9058

If I understand your question, you need to read the file then write it out, changing the data. You can't really do it the way you've started.

There are two basic ways to do this. You can read the entire file into memory, then manipulate the memory, close the file, open it again for output this time (truncating it) and write it back out. This is easiest, but I don't think it's the approach you're looking for.

The other choice is to manipulate the file in place. That's trickier, but not that hard. You need to read about random access I/O (input/output). If you google for c++ random access file you'll get some good hits, but I'll show you a little bit.

// Open the file.
std::ifstream file{"file.dat"};

// Jump to a particular location in the file. Beginning is 0.
file.seekg(128);

// Read 4 bytes
char bytes[4];
file.read(bytes, 4);

// Manipulate it (more below)
int number = bytesToInt(bytes);
++number;
intToBytes(number, bytes);

// Seek again
file.seekg(128);
file.write(bytes, 4);

So the only remaining trick is that you have to convert the bytes to a number and then back into bytes. Due to endianness, it's not safe to read directly into the number. You also need to know the endianness of the data in the file. That's a separate topic you can look up if you're not already familiar with it.

(Specifically, you need to implement those two methods after verifying how the data is stored in your file.)

There may be other ways to do this, but the key to this method is the random access file.

Upvotes: 3

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