C++ stringstream value extraction

Question

I am trying to extract values from myString1 using std::stringstream like shown below:

// Example program
#include <iostream>
#include <string>
#include <sstream>

using namespace std;

int main()
{
  string myString1 = "+50years";
  string myString2 = "+50years-4months+3weeks+5minutes";

  stringstream ss (myString1);

  char mathOperator;
  int value;
  string timeUnit;

  ss >> mathOperator >> value >> timeUnit;

  cout << "mathOperator: " << mathOperator << endl;
  cout << "value: " << value << endl;
  cout << "timeUnit: " << timeUnit << endl;
}

Output:

mathOperator: +
value: 50
timeUnit: years

In the output you can see me successfully extract the values I need, the math operator, the value and the time unit.

Is there a way to do the same with myString2? Perhaps in a loop? I can extract the math operator, the value, but the time unit simply extracts everything else, and I cannot think of a way to get around that. Much appreciated.

Answer 1

The problem is that timeUnit is a string, so >> will extract anything until the first space, which you haven't in your string.

Alternatives:

• you could extract parts using getline(), which extracts strings until it finds a separator. Unfortunately, you don't have one potential separator, but 2 (+ and -).
• you could opt for using regex directly on the string
• you could finally split the strings using find_first_of() and substr().

As an illustration, here the example with regex:

  regex rg("([\\+-][0-9]+[A-Za-z]+)", regex::extended);
  smatch sm;
  while (regex_search(myString2, sm, rg)) {
      cout <<"Found:"<<sm[0]<<endl;
      myString2 = sm.suffix().str(); 
      //... process sstring sm[0]
  }

Here a live demo applying your code to extract ALL the elements.

Answer 2

You could something more robust like <regex> like in the example below:

#include <iostream>
#include <regex>
#include <string>

int main () {
  std::regex e ("(\\+|\\-)((\\d)+)(years|months|weeks|minutes|seconds)");
  std::string str("+50years-4months+3weeks+5minutes");
  std::sregex_iterator next(str.begin(), str.end(), e);
  std::sregex_iterator end;
  while (next != end) {
    std::smatch match = *next;
    std::cout << "Expression: " << match.str() << "\n";
    std::cout << "  mathOperator : " << match[1] << std::endl;
    std::cout << "  value        : " << match[2] << std::endl;
    std::cout << "  timeUnit     : " << match[4] << std::endl;
    ++next;
  }
}

Output:

Expression: +50years
  mathOperator : +
  value        : 50
  timeUnit     : years
Expression: -4months
  mathOperator : -
  value        : 4
  timeUnit     : months
Expression: +3weeks
  mathOperator : +
  value        : 3
  timeUnit     : weeks
Expression: +5minutes
  mathOperator : +
  value        : 5
  timeUnit     : minutes

LIVE DEMO

Answer 3

I'd use getline for the timeUnit, but since getline can take only one delimiter, I'd search the string separately for mathOperator and use that:

string myString2 = "+50years-4months+3weeks+5minutes";
stringstream ss (myString2);
size_t pos=0;

ss >> mathOperator;
do
  {
    cout << "mathOperator: " << mathOperator << endl;

    ss >> value;
    cout << "value: " << value << endl;

    pos = myString2.find_first_of("+-", pos+1);
    mathOperator = myString2[pos];
    getline(ss, timeUnit, mathOperator);
    cout << "timeUnit: " << timeUnit << endl;
  }
while(pos!=string::npos);