how I do it.

gistfile1.cpp

#include <iostream>
#include <string>
#include <math.h>
using namespace std ;

bool checkForDouble(string const& s) {
  istringstream ss(s);
  double d;
  return (ss >> d) && (ss >> std::ws).eof();
}

int main (void) {
  string inS;
  double doubS;
  while (1) {
    cout << "S (must be between 0 and 1):  ";
    getline(cin, inS);
    
    if (!checkForDouble(inS)) {
      cout << "I need a number for this to work.  Try again." << endl;
      continue;
    }
    stringstream strStream;
    strStream << inS;
    strStream >> doubS;
    if (doubS < 0 || doubS > 1) {
      cout << "It must be between 0 and 1.  Try again." << endl ;
      continue;
    }
    
    break;
  }
  
  cout << "CONGRATS ON FOLLOWING DIRECTIONS!  YOU ENTERED:  " << doubS << endl ;
    
  return 0;
}