Enter valid double input C++

Question

I have been trying to get this correct for a while now. I finally gave up, I need help. So, what I'm trying to do is to get the user to provide me with valid double input with only one decimal point, e.g - "12.1". If a user enters 12.1.1 I want the program to start all over again till a valid double input is provided.

Thanks!

#include <iostream>
#include <cctype>
#include <string>

using namespace std;

double ReadDouble(string prompt);

int main() {
    ReadDouble("Enter double: \n");

}

double ReadDouble(string prompt) {
    string input;
    bool done = false;
    do {
        cout << prompt;
        cin >> input;
        done = true;
        for (int i = 0; i < input.length(); i++) {
            int count = 0;
            if (isdigit(input[i]) == 0) {
                if (ispunct(input[i]) != 0) {
                    count++;
                    count > 1 ? done = false : done = true;
                } else {
                    done = false;
                }
            }
        }
    } while (done == false);

    double retVal = stod(input.c_str());
    cout << retVal << endl;
    return retVal;
}

Answer 1

I know you want to manually parse the user's input yourself, but using a regex simplifies the process. This matches any number of digits, optionally a period, then another optional digit.

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

bool ValidateDouble(const std::string& str) {
  const std::basic_regex dbl (R"(^\d+\.?\d?$)");
  return std::regex_match(str, dbl);
}

double ReadDouble(const std::string& prompt) {
  std::string input;
  bool match = false;

  do {
    std::cout << prompt;
    std::getline(std::cin, input);
    match = ValidateDouble(input);
  } while (!match);

  return std::stod(input);
}

int main() {
  double d = ReadDouble("Enter double: ");
  std::cout << d << '\n';
}

Answer 2

made some minor changes in your code, maybe it can meet your requirements. Here is the code:

#include <iostream>
#include <cctype>
#include <string>
using namespace std;

double ReadDouble(string prompt);

int main() {
    ReadDouble("Enter double: \n");
}

double ReadDouble(string prompt) {
    string input;
    bool done = false;
    do {
        cout << prompt;
        cin >> input;
        done = false;
        int count = 0;
        for (int i = 0; i < input.length(); i++) {

            if (isdigit(input[i]) == 0) {
                if (ispunct(input[i]) != 0) {
                    count++;
                    continue;
                }
                else
                {
                    count++;
                    break;
                }
            }
            else
                continue;   
        }
        count > 1 ? done = false : done = true;

    } while (done == false);

    double retVal = stod(input.c_str());
    cout << retVal << endl;
    return retVal;
}

If enter "12.1.1" and the program will ask you to reinput the double until the correct double entered, maybe like this:

Enter double:
12.1.1
Enter double:
12.1.33
Enter double:
12.1
12.1
press any key to continue. . .