Input Redirection reading integers and char C++ - c++

Question

Thanks for taking your time to read this!
I'm having trouble parsing through a file with input redirection and I am having trouble reading through integers and characters.
Without using getline(), how do you read in the file including integers, characters, and any amount of whitespaces? (I know the >> operator can skip whitespace but fails when it hits a character)
Thanks!

Answer 1

The first thing you need to realise is that, fundamentally, there are no things like "integers" in your file. Your file does not contain typed data: it contains bytes.
Now, since C++ doesn't support any text encodings, for our purposes here we can consider bytes equivalent to "characters". (In reality, you'll probably layer something like a UTF-8 support library on top of your code, at which point "characters" takes on a whole new meaning. But we'll save that discussion for another day.)
At the most basic, then, we can just extract a bunch of bytes. Let's say 50 at a time:
std::ifstream ifs("filename.dat");
static constexpr const size_t CHUNK_SIZE = 50;
char buf[CHUNK_SIZE];
while (ifs.read(buf, CHUNK_SIZE)) {
const size_t num_extracted = ifs.gcount();
parseData(buf, num_extracted);
}
The function parseData would then examine those bytes in whatever manner you see fit.
For many text files this is unnecessarily arduous. So, as you've discovered, the IOStreams part of the C++ Standard Library provides us with some shortcuts. For example, std::getline will read bytes up to a delimiter, rather than reading a certain quantity of bytes.
Using this, we can read in things "line by line" — assuming a "line" is a sequence of bytes terminated by a \n (or \r\n if your platform performs line-ending translation, and you haven't put the stream into binary mode):
std::ifstream ifs("filename.dat");
static constexpr const size_t CHUNK_SIZE = 50;
std::string line;
while (std::getline(ifs, line)) {
parseLine(line);
}
Instead of \n you can provide, as a third argument to std::getline, some other delimiter.
The other facility it offers is operator<<, which will pick out tokens (sequences of bytes delimited by whitespace) and attempt to "lexically cast" them; that is, it'll try to interpret friendly human ASCII text into C++ data. So if your input is "123 abc", you can pull out the "123" into an int with value 123, and the "abc" into another string.
If you need more complex parsing, though, you're back to the initial offering, and to the conclusion of my answer: read everything and parse it byte-by-byte as you see fit. To help with this, there's sscanf inherited from the C standard library, or spooky incantations from Boost; or you could just write your own algorithms.
The above is true of any compatible input stream, be it a std::ifstream, a std::istringstream, or the good old ready-provided std::istream instance named std::cin (which I guess is how you're accepting the data, given your mention of input redirection: shell scripting?).