Read a string line by line using c++ - 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 string h;
h will be:
Hello there.
How are you today?
I am fine, thank you.
I need to extract Hello there., How are you today?, and I am fine, thank you. somehow.

Answer 1

#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 2

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 3

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

Answer 4

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;
}
}

Answer 5

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!