How to Read from a Text File, Character by Character in C++

Question

I was wondering if someone could help me figure out how to read from a text file in C++, character by character. That way, I could have a while loop (while there's still text left) where I store the next character in the text document in a temp variable so I could do something with it, then repeat the process with the next character. I know how to open the file and everything, but temp = textFile.getchar() doesn't seem to work. Thanks in advance.

Answer 1

You could try something like:

char ch;
fstream fin("file", fstream::in);
while (fin >> noskipws >> ch) {
    cout << ch; // Or whatever
}

Answer 2

@cnicutar and @Pete Becker have already pointed out the possibility of using noskipws/unsetting skipws to read a character at a time without skipping over white space characters in the input.

Another possibility would be to use an istreambuf_iterator to read the data. Along with this, I'd generally use a standard algorithm like std::transform to do the reading and processing.

Just for example, let's assume we wanted to do a Caesar-like cipher, copying from standard input to standard output, but adding 3 to every upper-case character, so A would become D, B could become E, etc. (and at the end, it would wrap around so XYZ converted to ABC.

If we were going to do that in C, we'd typically use a loop something like this:

int ch;
while (EOF != (ch = getchar())) {
    if (isupper(ch)) 
        ch = ((ch - 'A') +3) % 26 + 'A';
    putchar(ch);
}

To do the same thing in C++, I'd probably write the code more like this:

std::transform(std::istreambuf_iterator<char>(std::cin),
               std::istreambuf_iterator<char>(),
               std::ostreambuf_iterator<char>(std::cout),
               [](int ch) { return isupper(ch) ? ((ch - 'A') + 3) % 26 + 'A' : ch;});

Doing the job this way, you receive the consecutive characters as the values of the parameter passed to (in this case) the lambda function (though you could use an explicit functor instead of a lambda if you preferred).