Read a file line by line with specific data C++

Question

I have a file with this format:

The first line is the number of lines, so I can make a loop to read the file with the number of lines. For the other lines, I would like to read the file line by line and store the data until I get a "0" on the line that's why there is a 0 at the end of each line. The first column is the task name. The others columns are the constraints name.

I tried to code something but It doesn't seem to work

printf("Constraints :\n");
for (int t = 1; t <= numberofTasks; t++) 
{
    F >> currentTask;
    printf("%c\t", currentTask);
    F >> currentConstraint;
    while (currentConstraint != '0') 
    {
        printf("%c", currentConstraint);
        F >> currentConstraint;
    };
    printf("\n");
};

The "0" represents the end of the constraints for a task.

I think my code doesn't work properly because the constraint 10 for the task 4 contains a "0" too.

Thanks in advance for your help

Regards

Why are you mixing the streams? Either stick with C++ I/O (`operator>>`) or use C I/O streams (`printf`). — Thomas Matthews, May 08 '18 at 18:04
What type `currentConstraint` is? Without complete code it is everyone's guess but yours. This is what Slava was trying to tell you, post Minimal Complete and Verifiable Example — Killzone Kid, May 08 '18 at 18:04
I recommend using `std::getline` and `std::string` to read in a line of text. You can use `std::istringstream` to read in the numbers from the string. — Thomas Matthews, May 08 '18 at 18:05
You may find Option 2 of the first answer to [Read file line by line](https://stackoverflow.com/questions/7868936/read-file-line-by-line) to be extremely helpful. — user4581301, May 08 '18 at 18:16

score 0 · Accepted Answer · answered May 08 '18 at 21:02

The problem is that you are reading individual characters from the file, not reading whole integers, or even line-by-line. Change your currentTask and currentConstraint variables to int instead of char, and use std::getline() to read lines that you then read integers from.

Try this:

F >> numberofTasks;
F.ignore();

std::cout << "Constraints :" << std::endl;
for (int t = 1; t <= numberofTasks; ++t) 
{
    std::string line;
    if (!std::getline(F, line)) break;

    std::istringstream iss(line);

    iss >> currentTask;
    std::cout << currentTask << "\t";

    while ((iss >> currentConstraint) && (currentConstraint != 0))
    {
        std::cout << currentConstraint << " ";
    }

    std::cout << std::endl;
}

Live Demo

That being said, the terminating 0 on each line is unnecessary. std::getline() will stop reading when it reaches the end of a line, and operator>> will stop reading when it reaches the end of the stream.

Live Demo

I see what you did, you just read until the end of the line, instead of reading until a 0. And we only take constraints different from 0. Thanks for your help — Toms lns, May 08 '18 at 21:34

Read a file line by line with specific data C++

1 Answers1