C++ ifstream not reading \n?

Question

I'm trying to do operations per-line of a text file, and the way I have it right now, my ifstream object isn't detecting the \n character for each line, which is required for this project. Here's how I have it right now:

std::ifstream instream;
instream >> value;
while (value != '\n')
{
    // do code and such
}

But when I have it run the loop, all I'm getting is a single line of everything in the program. While it is doing exactly what it is supposed to in the loop, I NEED the \n to be recognized. Here's my .txt file:

LXXXVII
cCxiX
MCCCLIV
CXXXLL
MMDCLXXIII
DXLCC
MCdLxxvI
XZ
X
IV

Exactly like that. I cannot change it.

Haughay

what data type is value? You might want to use value = cin.get() instead of cin >> value. – Sniffy

If you want to read lines, use a function that reads lines. By "\n", what you really mean is an empty line. – Jailbreak

value is a char type, and it cannot be an array of any kind, so what would i use instead? and @DavidSchwartz, by \n i really mean the end of the line. – Haughay

Read about getline. – Azaria

@wbAnon Based on your comment, Ben Voigt's comment should solve your problem. – Pacificism

Answer 1

The >> operator does a "formatted input operation" which means (among other things) it skips whitespace.

To read raw characters one by one without skipping whitespace you need to use an "unformatted input operation" such as istream::get(). Assuming value is of type char, you can read each char with instream.get(value)

When you reach EOF the read will fail, so you can read every character in a loop such as:

while (instream.get(value))
  // process value

However, to read line-by-line you could read into a std::string and use std::getline

std::string line;
while (getline(instream, line))
  // ...

This is an unformatted input operation which reads everything up to a \n into the string, then discards the \n character (so you'd need to manually append a \n after each erad line to reconstruct the original input)

Vinavinaceous

alright, i see where you're coming from. so will i get a \n as the value of value? – Haughay

When using instream.get(value) yes, assuming value is of type char – Vinavinaceous

ok, well the code instream.get(value) is not giving value assigning the \n. any idea as to why? – Haughay

Why don't you just read the whole line into a std::string with getline? You can parse the string at will afterwards and don't need to fuzz around with \n, as it's stripped from the std::string, as Jonathan explained. – Bureau

because i cant use a string for the assignment. at all – Haughay

For what assignment specifically? – Bureau

This code works great and should produce what you'd like to have: ideone.com/UX6RJw (C++11 due to the auto keyword.) Output: pastie.org/private/umsqfcnyh3fpgjx3obyzw – Bureau

again, i cannot use a string. only char – Haughay

@wbAnon, this works fine for me (using string for convenience): ideone.com/HkEZka If similar code doesn't work for you then either you're doing something wrong or your file has no \n characters. If you created the file on Windows watch out for \r characters in the file too, due to Windows' stupid line endings – Vinavinaceous

ya i know... i just got it to work too. not sure why or what i did tho XD – Haughay

Answer 2

You can read your file as follows:

ifstream instream("file.txt");
string line;
while (instream >> line)
{
    cout << line;
    if (instream.peek() == '\n') // detect '\n'
    {
        cout << endl;
    }
}

This way you can track where the line in file ends and detect end of file.

Ownership

@wbAnon If this is the case, please see " #12240510" for some information – Ownership

i know how to get it to read the file char by char, the problem is that it refuses to read the \n character at the end of each line. – Haughay

@wbAnon try to use fstream::get() instead? – Ownership

Answer 3

I guess you are looking for this: instream.unsetf(std::ios_base::skipws)

Meaningful