如何将文件中的字符转换为字符串

Question

The problem is that I don't know how to properly change the character in the file to a string.

I tried this but...

    char c = '"';
    fstream test_file("l.txt");
    while (test_file >> c) {
        if (c == '"') {
            test_file << " \" ";
        }
    }
    system("pause");

It doesn't work properly. The example is:

...and then he said "I'm older than you!" end of quote.

The expected result for me is:

...and then he said  " I'm older than you! "  end of quote.

The actual result of the above code is:

...and then he said " "  older than you!" " d of quote.

It seems to skip 2 characters and then replace it, but not only 1 character but also next to it.

Edit. Ok, I got a better result now with:

    string str;
    string temp;
    string quote;
    int count = 0;
    fstream test_file;
    test_file.open("l.txt");
    while (test_file >> str) {
        count++;
        quote = f_Erase_from(1, str);
        if (quote == "\"") {
            temp = " \" " + f_Erase_from_to(0, 1, str);
            test_file.close();
            test_file.open("l.txt");
            for (int i = 0; i < count - 1 ; i++) {
                test_file >> str;
            }
            test_file << temp + " ";
        }
        quote = f_Erase_from_to(0, str.size() - 1, str);
        if (quote == "\"") {
            temp = f_Erase_from_to(str.size() - 1, str.size(), str) + " \" ";
            test_file.close();
            test_file.open("l.txt");
            for (int i = 0; i < count ; i++) {
                test_file >> str;
            }
            test_file << " " + temp;
        }
    }
    system("pause");

With the result:

...and then he said " I'm lder than you! " nd of quote.

But now it's eat the first character of the next string...

Answer 1

Consider how files are stored on disk. A file is a series of bytes, one after another. If you wanted to insert a character, you'd have to move all following characters by 1 byte (let's consider ASCII for simplicity).

Now, neither your hard disk nor the operating system will do that for you. Why? Given a file with 1 MB of size and you need to insert a backslash every 100 bytes, it would move 999.9 kB, then 999.8 kB, ... then 200 bytes, then 100 bytes. In total, that's 4.998 GB of data being moved, just because you wanted to insert 10000 bytes. This has such a performance impact that it isn't done.

Then, how does one replace characters in a file?

1. Read the whole file into memory, do the replacement, write the whole thing to the file. This approach is ok for smaller files, maybe up to 50% of the RAM of the machine.

2. For larger files, use streams and read in chunks. Read the first n bytes from disk, do the replacement and write it into a temporary file. Loop this until you reach the end of the file. Then delete the original and move the temporary to the original location. Note that you need double + something space on disk.

How do you replace in a string? Well, unfortunately, there isn't simply a s.replace(string find, string replacement) method like in every other language I know. But it has been solved .