How to replace characters inplace fast using C++

Question

I have a text file (66GB) where I would like to replace some characters. I can't load the whole thing into memory.

This is the basic idea of what I was hoping to do:

std::ifstream i(infile.c_str()); // ifsteam 
while(i.good()) {
    getline(i, line); 
    for(int c=0;c<line.length();c++) {
        if(line[c]=='Q')
            // *** REPLACE Q WITH X HERE
    }
}

My question is: how do I put the new character so that is actually replaces Q?

Subquestion: is there a better / faster way to do this?

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I am working on a virtual ubuntu server: 2 Cores, 4GB of memory, OS is ubuntu.

Answer 1

You could use something like this which I would think would be faster.

std::ifstream ifs("input_file_name", std::ios::binary);
std::ofstream ofs("output_file_name", std::ios::binary);

char buf[4096]; // larger = faster (within limits)

while(ifs.read(buf, sizeof(buf)) || ifs.gcount())
{
    // replace the characters
    std::replace(buf, buf + ifs.gcount(), 'Q', 'X');

    // write to a new file
    ofs.write(buf, ifs.gcount());
}

If you don't want to produce a separate file (more dangerous) then you could modify the original file something a bit like this (untested code):

std::fstream fs("input_file_name", std::ios::in|std::ios::out|std::ios::binary);

char buf[4096]; // larger = faster (within limits)

auto beg = fs.tellg();

while(fs.read(buf, sizeof(buf)) || fs.gcount())
{
    auto end = fs.tellg();

    // replace the characters
    std::replace(buf, buf + fs.gcount(), 'Q', 'X');

    // return to start of block
    fs.seekp(beg);

    // overwrite this block
    fs.write(buf, fs.gcount());

    // shift old beginning to the end
    beg = end;

    // go to new beginning to start reading the next block
    fs.seekg(beg);
}