Converting string to float with atof() in C++

Question

I'm new to C++ and I need help converting my string variable into a number. I need to print the person's age and her next year age on the screen but I can't figure out how. I can't use the string to do math. So can you please shoot me a couple examples please?

I tried to do it this way:

  //converts F to C
  ctemp=(ftemp-32)*5.0/9.0;

  //calculates age
  age2 = age + 1 ; age2 = atof (age2.c_str()); age = atof (age.c_str());


  cout<<name<<" is "<<age<<" now, and will be " <<age2<<" a year from now.\n"<<"Its "<<ftemp<<" in "<<city<<"--- That's "<<setprecision(2)<<ctemp<<" degrees C"<<endl;

Edit:

int main() { 
    int city; 
    double ftemp, ctemp,age,age2;
    string buf,name; //Ask user to enter age, name, temperature and city 
    cout<<"Enter your age\n"; 
    cin>>buf; 
    age = atof(buf.c_str()); 
    cout<<"Enter your full name please\n"; 
    getline(cin,name); 
    cout<<"Enter the outside temperature please\n"; 
    cin>>buf; 
    ftemp=atof(buf.c_str()); 
    cout<<"Enter the current city you are in\n"; 
    cin>>buf; 
    city=atoi(buf.c_str());
}

Answer 1

You can use stringstreams to convert the string to the data type you want:

#include <sstream>
#include <string>
#include <iostream>

int main() {
  std::string num_str = "10.5";

  std::istringstream iss(num_str);

  float f_val = 0;

  iss >> f_val;

  //this will output 10.5 (as expected)
  std::cout << f_val << std::endl;

  return 0;
}

Which will assign f_val to the value of the string.

This approach is preferred in that it has built-in checking of conversion, and it can work on any of the fundamental C++ types. Hence, if you can can create a template function to perform the checking for you and return that particular value of the type you desire:

#include <sstream>
#include <iostream>
#include <string>
#include <stdexcept>

template<typename T>
T ConvertStringToNumber(const std::string& str)  
{  
  std::istringstream ss(str);  
  T number = 0;  

  ss >> number;  

  if (ss.fail( )) 
  {
    throw std::invalid_argument("ConvertStringToNumber:" + str);
  }

   return number;  
}

int main(int argc, char argv[]){
  std::string num_str = "10.5";
  float f = ConvertStringToNumber<float>(num_str);

  std::cout << f << std::endl;

  return 0;
}