c++ getline() doesn't seem to operate correctly, Doesn't read the line till the end

Question

I have tried using cin.ignore(); instead of the dummy variable, didn't help. content of the file looks like this:

D Name Surname

A Name2 Surname2

...

amount of the students is unknown.

...

output should look like this:

Name Surname class: D

Name2 Surname2 class: A

but mine output loos like this:

Name Class A

urname Class S

Name2 Class E

urname2 Class S ...

here is the code:

#include <iostream>
#include <fstream>
using namespace std;
 
int main() 
{
    ifstream file("students.txt");
    
    if (!file.is_open()) 
    {
        cout << "file can't be opened!" << endl;
        exit(-1);
    }

        char classs;
        string name, dummy;

    while(file >> classs >> name) 
    {
        
      cin.get(classs);
      getline(cin, dummy);
      getline(cin, name);
      
      cout << name << " " << "calass " << classs << endl;
    }
}

There are multiple input operations into the same variables. Please reduce this to a [mcve]. Provide the minimal input data (or use a `stringstream`), the actual output and the expected output. As a new user here, also take the [tour] and read [ask]. — Ulrich Eckhardt, Nov 03 '20 at 07:21
Are the blank lines really in your input? Why are you reading from `cin`, overwriting what is read from the file? — MikeCAT, Nov 03 '20 at 07:21
It's not a good idea to mix two styles of input the way you're doing. Just make your loop read each line as a string: `while(getline(file, line))` and then put that into a `std::istringstream` and read the values out. — paddy, Nov 03 '20 at 07:21

paddy · Accepted Answer · 2020-11-03T07:37:50.250

It looks like you've tried all kinds of things to make this work, including reading from cin in the middle of your loop that reads from file.. Very strange. I suggest you throw out the entire loop and try something simple as follows:

You just read one line at a time. For each line you read, you put that into a string stream and read the values out of that. To keep it even simpler, just read a string for everything, even if it's supposed to be just one character.

// Note: requires <sstream>

string line;
while(getline(file, line))
{
    istringstream iss(line);
    string cls, name, surname;
    if (iss >> cls >> name >> surname)
    {
        cout << name << " " << surname << " class: " << cls << endl;
    }
}

This does assume that there are always exactly two names, and it seems like that might not be the case for your input.

As an alternative, you could try reading the single character, then ignoring whitespace, and then read the entire rest of line:

char cls;
string fullname;
while (getline(file >> cls >> std::ws, fullname))
{
    cout << fullname << " class: " << cls << endl;
}

Note that the above is being a bit lazy, and as a result this might break if you encounter an empty line in your file. So, combining with the istringstream approach would be the way to go:

string line;
while (getline(file, line))
{
    istringstream iss(line);
    char cls;
    string fullname;
    if (iss >> cls >> std::ws >> fullname)
    {
        // Note you may also want to test cls is valid (e.g. isalpha(cls))
        cout << fullname << " class: " << cls << endl;
    }
}

In the end, if your input is line-based, then as a general preference you should read lines and then decide what to actually do with them.

Lyubomir Vasilev · Answer 2 · 2020-11-03T08:53:43.477

0

You are using both getline() and >> for reading from the file stream, and you shouldn't be. Here is an example using just >>:

std::string cls, name, surname;
while(file >> cls >> name >> surname) 
{
    std::cout << name << " " << surname << " class: " << cls << std::endl;
}

I am assuming that each line of the file has exactly three elements - class, name and surname, as that is the example in the question. If that is not the case, then you'll really need to read the file's contents line by line using getline() and then parse each line using for example istringstream.

Also, you are using both file and cin and your data is only in the file, so you don't need to be using cin at all.

edited Nov 03 '20 at 08:53

answered Nov 03 '20 at 07:26

Lyubomir Vasilev

2,757
14
21

it doesnt work like that using a variable surname. The name in the file can be f.e. Name SecondName Surname. – Corroshi Nov 03 '20 at 07:34
To be fair, you never suggested that and your example input didn't either. – paddy Nov 03 '20 at 07:35
Please read [Why is iostream::eof inside a loop condition considered wrong](https://stackoverflow.com/q/5605125/1553090) – paddy Nov 03 '20 at 07:42

c++ getline() doesn't seem to operate correctly, Doesn't read the line till the end

2 Answers2