CODEWITHSUNDEEP
c++c++11templatesiostream
Lampros
Lampros

Reputation: 392

Similar function to Input("msg") from python in C++

I am looking to make this function in c++, input("x = ");, somewhat like in python, this function prints the message in the () and the expects input. It can take only bool,str,int,double. I thought of making a struct input like so

struct input {
 
 std::string str;
 int num;
 double dub;
 bool boolean;
 input(const char *s) {
   std::cout << s;
   std::cin >> ### ; //here is my problem 
 }
};

But that's as I far as I got. I tried templates but still couldn't figure it out.

Extracting the input shouldn't be so difficult, I will figure that out, for now I just want to see how to get my data in the struct.

Upvotes: 1

Views: 328

Answers (2)

Lampros
Lampros

Reputation: 392

What i did was make a class,

template <typename T> using couple = std::pair<bool, T>;
struct input {
  std::string inp;
  couple<std::string> str = {false, ""};
  couple<int> num = {false, 0};
  couple<double> dbl = {false, 0};
  couple<bool> bl = {false, 0};

  input(const char *s) {
    std::cout << s;
    std::cin >> inp;
    fillinput(inp);
  }
  bool check_bool(std::string inputVal) {
    transform(inputVal.begin(), inputVal.end(), inputVal.begin(), ::tolower);
    if (inputVal == "false") {
      bl.first = true;
      bl.second = false;
      return true;
    }
    if (inputVal == "true") {
      bl.first = bl.second = true;
      return true;
    }
    return false;
  }
  bool check_double(std::string inputVal) {
    int dots = std::count(inputVal.begin(), inputVal.end(), '.');
    bool digits = std::all_of(inputVal.begin(), inputVal.end(), ::isdigit);
    if (dots == 1 && digits) {
      dbl.first = true;
      dbl.second = std::stod(inputVal);
      return true;
    }
    return false;
  }
  bool check_int(std::string inputVal) {
    bool digits = std::all_of(inputVal.begin(), inputVal.end(), ::isdigit);
    if (digits) {
      num.first = true;
      num.second = std::stoi(inputVal);
      return true;
    }
    return false;
  }
  void fillinput(std::string Input) {
    if (check_int(Input) || (check_bool(Input)) || (check_double(Input)))
      return;
    else {
      str.first = true;
      str.second = Input;
    }
  }
};

It takes some more work to make this usable but this is the way. I know its not good code but it is what it is.

Upvotes: 0

463035818_is_not_an_ai
463035818_is_not_an_ai

Reputation: 122994

Generally translating Python to C++ is difficult due to the fundamentally different type systems. However, thats not an issue here. From documentation:

If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that. When EOF is read, EOFError is raised.

Python's input does'nt do any conversion. It just returns a string. The EOF exception can be enabled, but it requires a bit of boilerplate to reset the streams exceptions settings when an exception is actually thrown. It can be done with the help of a RAII class whose only purpose is to reset the streams previous exception settings, no matter whether the function returns or throws.

struct eof_exception_enabled {
    std::istream & inp;
    std::ios::iostate old;
    eof_exception_enabled(std::istream& inp) : inp(inp), old(inp.exceptions()) {
        inp.exceptions(old | std::ios::eofbit);
    }
    ~eof_exception_enabled() {
        inp.exceptions(old);
    }
};


std::string input(std::string prompt = "") {
    std::cout << prompt;
    std::string res;
    auto inp = eof_exception_enabled{std::cin};
    std::getline(inp.inp, res);
    return res;
}

Actually it would be more idiomatic to pass also the input stream to input so that different streams can be used. Not sure how much sense this would make for a function that prompts the user and typically reads from standard input.

If you want to enable conversions you can use a function template that utilizes a stringstream for the conversion:

template <typename T>
T from_string(const std::string& str) {
    std::stringstream ss{str};
    T t;
    ss >> t;
    return t;
}

Usage would be

auto age = from_string<int>( input("enter your age:"));

However, correctly handling errors due to unexpected input requires a bit more. from_string does just return 0 when extraction from the stream fails for integer types. Though, now we are talking about the conversion, which also in Python is not done by input.

Upvotes: 4

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