How can I know the end of line of a text file?

[Nikko YL]

I have a text file "RPN.txt".

2 3 4 + *
7 4 3 + - 2 +
12 46 + 23 -
1 5 + 8 4 1 - - *
6 3 - 5 * 9 +
12 23 11 + 45 * +
2 11 + 4 7 + *
231 111 +

I have to process line by line. Each line has a answer, which is stored in the vector result.
Here is my algorithm:

std::ifstream infile("RPN.txt");
std::vector<int> result;
int answer;
// evaluate the answer of line 1 ...
result.push_back(answer);
// repeat for the next line until end of file (no next line) 

I attempted to use infile >> in order to extract the data in the text file. But fail to stop when the end of a line is reached.

What is the correct approach I should use? (without storing the whole line by getline and then processing the string)

[Volatile Pulse]

#include <string>
// ...
// That should get you the entire line as a string
std::getline(infile, answer);

getline() will read infile until it reaches the first carriage return/new line character and store it in answer.

[Nikko YL]

I want to process in the text file, but not to store the whole line first.
How can I do this without std::getline?

[Volatile Pulse]

If you need to evaluate everything, one part at a time, do what you were doing, but then add: if (infile.peek() == '\n' || infile.peek() == '\r')

That should check to see if the next character is an end of line character, but it's not as surefire, and requires a little more effort. My suggestion is to use getline to store the string and parse it as if your file was only one line long. Then grab the next line.

[Nikko YL]

After detecting the next character is an end of line character, how to jump into the next line?

[Volatile Pulse]

All of the functions you need, can be found here: http://www.cplusplus.com/reference/fstream/fstream/

I don't know what would be best for your specific situation, but there are tons of different options out there, and I still believe getline is your best option.

[Nikko YL]

What is the reason that C++ does not offer the convenient function eoln()?

[cire]

What is the reason that C++ does not offer the convenient function eoln()?

C++ offers the convenient function getline, and the convenient stringstream family of types to turn that line into a stream.

#include <iostream>
#include <fstream>
#include <sstream>
#include <stack>
#include <string>
#include <cassert>
#include <cctype>

int evaluate(int lhs, int rhs, const std::string& op)
{
    bool valid_operator_match = true;
    int result;

    if (op == "+")
        result = lhs + rhs;
    else if (op == "-")
        result = lhs - rhs;
    else if (op == "*")
        result = lhs * rhs;
    else if (op == "/")
        result = lhs / rhs;
    else
        valid_operator_match = false;

    assert(valid_operator_match);

    std::cout << '\t' << lhs << ' ' << op << ' ' << rhs << " = " << result << '\n';
    return result;
}

bool is_operator(const std::string& token)
{
    return token.size() == 1 && !std::isdigit(token[0]);
}


int main()
{
    std::stack<int> values;
    std::ifstream infile("RPN.txt");

    std::string line;
    while (std::getline(infile, line))
    {
        assert(values.empty());

        std::istringstream stream(line); 
        std::cout << "Evaluating \"" << line << "\"\n";

        while (stream)
        {
            std::string token;

            while (stream >> token && !is_operator(token))
                values.push(std::stoi(token));

            if (stream)
            {
                assert(values.size() > 1);
                int rhs = values.top();
                values.pop();

                int lhs = values.top();
                values.pop();

                values.push(evaluate(lhs, rhs, token));
            }
        }

        assert(values.size() == 1);
        std::cout << "Result is " << values.top() << "\n\n" ;
        values.pop();
    }
}

[Nikko YL]

Thanks all!
Finally I have used the getline.