Reading a text file line by line leaves vector empty

Question

I'm trying to read a textfile which looks like this:

Gate
People
Crab
Motorbike

My code is:

string line;
vector<string> v_names;
ifstream myfile("c:/temp/test.txt");
if (! myfile.is_open()) {
    cout << "Failed to open" << endl;
}
else {
    cout << "Opened OK" << endl;
}

myfile.unsetf(ios_base::skipws);
unsigned line_count = count(istreambuf_iterator<char>(myfile), istreambuf_iterator<char>(), '\n');

while (getline(myfile, line)){
        v_names.push_back(line);
    }

If I want to get the size of my vector with v_names.size() it gives back 0. If I call v_names[0] I get the error "Vector subscript out of range"

What am I doing wrong?

Try this http://stackoverflow.com/questions/7868936/read-file-line-by-line — Rushikesh Deshpande, May 03 '17 at 09:23

Lightness Races in Orbit · Accepted Answer · 2017-05-03T09:38:38.943

4

unsigned line_count = count(istreambuf_iterator<char>(myfile), istreambuf_iterator<char>(), '\n');

Here you consumed all the data in your stream. Afterwards, there is no data remaining. So, there is nothing to loop over with getline calls.

Since it is a file stream, you can "seek" back to the beginning of the file and begin consuming all the data again:

unsigned line_count = count(
   istreambuf_iterator<char>(myfile),
   istreambuf_iterator<char>(),
   '\n'
);

myfile.seek(0, std::ios_base::beg);
myfile.clear();

while (getline(myfile, line)) {
   v_names.push_back(line);
}

However, you have a problem, which is that your line_count methodology is broken. The final line won't necessarily end in a '\n', leaving you off by one.

Mind you, as n.m. points out, counting the lines ahead of time would appear to be pointless anyway, as v_names.size() will later give you the same information. Perhaps you can just remove that code and solve the problem that way.

edited May 03 '17 at 09:38

answered May 03 '17 at 09:19

Lightness Races in Orbit

378,754
76
643
1,055

It would be somewhat easier to just drop the offending line. – n. m. could be an AI May 03 '17 at 09:25
@n.m.: Hah, true. I guess I subconsciously assumed it was needed for something later on, but on closer examination it would indeed appear to be pointless. – Lightness Races in Orbit May 03 '17 at 09:27
Thanks! That's it. I wanted to get the number of lines of the text file in an earlier iteration of my code but in the end I can just call it with v_names.size(). – Grillteller May 03 '17 at 10:54

score 1 · Answer 2 · 2017-05-03T09:34:02.297

1

I don't see why you can't read the lines first, and count them afterwards.

while (getline(myfile, line)){
    v_names.push_back(line);
}

auto line_count = v_names.size();

Then again, because the .size() isn't walking away, just call it whenever you need to know how many names you have.

SIDE NOTE: counting the number of '\n's is not guaranteed to work all the time as the final line will not necessarily end with a newline.

edited May 03 '17 at 09:34

answered May 03 '17 at 09:28

_"not every line will necessarily end with a newline"_ What else will they end with? Other than the very final line that is – Lightness Races in Orbit May 03 '17 at 09:31
@BoundaryImposition _"Other than the very final line that is"_ Exactly my point. One by off errors are not fun. The memory allocator may be nice and allocate blocks of powers of 2, but certain numbers cause certain run-time errors. – May 03 '17 at 09:33
1

Sorry, what run-time errors do you expect here? What does the memory allocator have to do with it? The worst thing that can happen is one fewer line is "reported" in `line_count`, which is not going to result in overflowing access into `v_names`, if that's what you meant. – Lightness Races in Orbit May 03 '17 at 09:37
@BoundaryImposition You're right. Brain had an off-by-one error there. – May 03 '17 at 09:52

Reading a text file line by line leaves vector empty

2 Answers2