Read a string line by line using c++

Question

I have a std::string with multiple lines and I need to read it line by line. Please show me how to do it with a small example.

Ex: I have a string

Answer 1

this would help you : http://www.cplusplus.com/reference/iostream/istream/getline/

Answer 2

#include <sstream>
#include <iostream>

int main() {
    std::istringstream f("line1\nline2\nline3");
    std::string line;    
    while (std::getline(f, line)) {
        std::cout << line << std::endl;
    }
}

Answer 3

There are several ways to do that.

You can use std::string::find in a loop for '\n' characters and substr() between the positions.

You can use std::istringstream and std::getline( istr, line ) (Probably the easiest)

You can use boost::tokenize

Answer 4

I was looking for some standard implementation for a function which can return a particular line from a string. I came across this question and the accepted answer is very useful. I also have my own implementation which I would like to share:

// CODE: A
std::string getLine(const std::string& str, int line)
{
    size_t pos = 0;
    if (line < 0)
        return std::string();

    while ((line-- > 0) and (pos < str.length()))
        pos = str.find("\n", pos) + 1;
    if (pos >= str.length())
        return std::string();
    size_t end = str.find("\n", pos);
    return str.substr(pos, (end == std::string::npos ? std::string::npos : (end - pos + 1)));
}

But I have replaced my own implementation with the one shown in the accepted answer as it uses standard function and would be less bug-prone..

// CODE: B
std::string getLine(const std::string& str, int lineNo)
{
    std::string line;
    std::istringstream stream(str);
    while (lineNo-- >= 0)
        std::getline(stream, line);
    return line;
}

There is behavioral difference between the two implementations. CODE: B removes the newline from each line it returns. CODE: A doesn't remove newline.

My intention of posting my answer to this not-active question is to make others see possible implementations.

NOTE:

I didn't want any kind of optimization and wanted to perform a task given to me in a Hackathon!

Answer 5

If you'd rather not use streams:

int main() {
  string out = "line1\nline2\nline3";
  size_t start = 0;
  size_t end;
  while (1) {
    string this_line;
    if ((end = out.find("\n", start)) == string::npos) {
      if (!(this_line = out.substr(start)).empty()) {
        printf("%s\n", this_line.c_str());
      }

      break;
    }

    this_line = out.substr(start, end - start);
    printf("%s\n", this_line.c_str());
    start = end + 1;
  }
}