iterate through lines in a string c++

Question

here's what I need to do. I have a string in C++. For every line in the string, I need to append a few characters (like ">> ") to the beginning of the line. What I am struggling with is a good way to split the string around newlines, iterate through the elements appending the characters, and then rejoin the string together. I've seen a few ideas, such as strtok(), but I was hoping c++ strings would have something a little more elegant.

Hope this http://stackoverflow.com/questions/236129/splitting-a-string-in-c could help — halfelf, Sep 20 '12 at 14:15

score 29 · Accepted Answer · answered Sep 20 '12 at 14:19

Here's a straight-forward solution. Maybe not the most efficient, but unless this is hot code or the string is huge, it should do fine. We suppose that your input string is called input:

#include <string>
#include <sstream>

std::string result;

std::istringstream iss(input);

for (std::string line; std::getline(iss, line); )
{
    result += ">> " + line + "\n";
}

// now use "result"

score 5 · Answer 2 · edited May 23 '17 at 10:31

A more functional approach would be to use a getline-based iterator as shown in this answer and then use that with std::transform for transforming all input lines, like this:

std::string transmogrify( const std::string &s ) {
    struct Local {
        static std::string indentLine( const std::string &s ) {
            return ">> " + s;
        }
    };

    std::istringstream input( s );
    std::ostringstream output;
    std::transform( std::istream_iterator<line>( input ), 
                    std::istream_iterator<line>(),
                    std::ostream_iterator<std::string>( output, "\n" ),
                    Local::indentLine );
    return output.str();
}

The indentLine helper actually indents the line, the newlines are inserted by the ostream_iterator.

@Potatoswatter: Right, I adjusted the wording. – Frerich Raabe Sep 20 '12 at 14:39 — Frerich Raabe, Sep 20 '12 at 14:39

score 3 · Answer 3 · answered Sep 20 '12 at 14:16

If the data in your string is basically like a file, try using std::stringstream.

std::istringstream lines( string_of_lines );
std::ostringstream indented_lines;
std::string one_line;
while ( getline( lines, one_line ) ) {
    indented_lines << ">> " << one_line << '\n';
}
std::cout << indented_lines.str();

score 1 · Answer 4 · answered Sep 20 '12 at 14:22

You can wrap it in a stringstream and use std::getline to extract a line at a time:

std::string transmogrify(std::string const & in) {
    std::istringstream ss(in);
    std::string line, out;
    while (getline(ss, line)) {
        out += ">> ";
        out += line;
        out += '\n';
    }
    return out;
}

iterate through lines in a string c++

4 Answers4