What will I have to write in my program so that it would only accept commas in their proper location (i.e. cannot input 10,00.0 , 100,0.2334, etc)?

I am taking a summer course in computer science, and I get assigned two projects a week, so bear with me if I'm getting some terminology wrong.

This week, I was able to get the first one done, but not the second one. In the second assignment, we were required to rewrite the ReadDouble function so that it's more user-friendly; user-friendly as in allowing the user to input commas along with numbers. Additionally,

1. we were required to allow the first character to be a number, plus or minus sign, or a decimal point.

2. All other characters can be a number, comma, or decimal point (if there already wasn't one).

3. As mentioned, commas must be written in properly (we can't allow the user to input 10,00.244, 343,2.334, or some nonsensical number like that).

4. No commas should be allowed after the decimal point.

5. Only one decimal point should be allowed in the number.

So far, I was able to able to meet 1), 2), and 5). 3) and 4)? Not so much.

The underlying issue is that I don't know what classes, objects, and whatnot I should be using to make the program read the string input and determine if the commas were inserted properly. I have an idea that I would need to use something similar to the "input.length()" code, set it as a variable that can be compared in an if statement to ensure that the amount of digits until the next comma can be used is met.

I also tried writing a for loop that would check after the decimal place for commas or any other invalid character, but I didn't know what to write down as its initialization. How would I get the for loop to start looking from the decimal after it's aware that one decimal exists?

Another major issue I am encountering is that when I input something like 1.2, it is displayed as 12, meaning that "atof(convert.cstr())" has stripped the decimal from the return value. However, when I enter it as just .2, it comes out as 0.2.

I will provide the code of what I have written so far along with the code of what a friend has suggested to me.

My code:

#include <iostream> 
#include <cstdlib>
#include <string>
#include <climits>

using namespace std;

// Main Menu Error Prototype
double ReadDouble(string prompt);

double ReadDouble(string prompt)
{
    string input;
    string convert;
    bool isValid = true;

    do {
        // Reset the flag to valid input
        isValid = true;

        // Read input from user
        cout << prompt;
        cin >> input;

        // Validate user input
        // Check the first character is a number, + or -
        int decimal = 0;
        if (input[0] != '+' && input[0] != '-' && input[0] != '.' && isdigit(input[0]) == 0) {
            cout << "Error! Input was not an integer.\n";
            isValid = false;
        }
        else {
            if (input[0] == '.') {
                decimal++;
                //cout << "." << endl;
            }
            convert = input.substr(0, 1);
        }

        // check that the remaining characters are numeric
        long len = input.length();
        for (long index = 1; index < len && isValid == true && decimal <= 1; index++) {
            if (input[index] == ',') {
                ;  // do nothing if character is a ','
            }
            else if (input[index] == '.') {
                decimal++; // do nothing if character is a '.'
                if (decimal > 1) {
                    cout << "Error! You can have only one decimal point.\n";
                    isValid = false;
                }
            }
            else if (isdigit(input[index]) == 0) {
                cout << "Error! Input was not an integer.\n";
                isValid = false;
            }
            
            else {
                convert += input.substr(index, 1);
            }
            
        }


        // Start looking where the decimal starts 

        /*
        long decimal=input.find('.');
        for (decimal; decimal < len && isValid==true; decimal++) {
            if (input[decimal] =='.') {
                ; // do nothing if character is a '.'
            }

        }
        */
        //cout << "\nDecimal value is " << decimal << endl; -- Test Code
    } while (isValid == false);

    
    double returnvalue = atof(convert.c_str());
    
    return returnvalue;
}

int main()
{
    double x = ReadDouble("Enter a value: ");
    cout << "Value entered was " << x << endl;
    return 0;
}

My friend's incomplete code:

ReadDouble(){
     isValid = true

     do{
            get user input and set it to a variable called input
            set output variable to a variable called output
            bool hasDecimal = false;
            int digitsUntilNextComma = 3

            for(i = 0; i < input length; i++){
                    if(input[i] == ','){
                             if((i < 3 && i > 0) || digitsUntilNextComma == 0){
                                     digitsUntilNextComma = 3;
                                     output += input[i];
                             }else{ //if it gets to here the comma was in a bad place like ,123 or 12,12,123
                                        isValid = false; 
                                        i = input length  //breaks out of for loop
                              }
                    } else if(input[i] == '.'){
                             if(i < 3 || digitsUntilNextComma == 0){
                                     if(hasDecimal){ //if this is true then the input had 2 decimals 
                                               isValid = false; 
                                               i = input length  //breaks out of for loop
                                      }else{
                                              hasDecimal = true;
                                              digitsUntilNextComma = 3;
                                              output += input[i];
                                       }
                             }else{ //if it gets to here, a previous comma was in a bad place like 12,34.56
                                      isValid = false; 
                                      i = input length  //breaks out of for loop
                              }
                    }else{
                            output += input[i];
                            digitsUntilNextComma--;
                    }
            }
          
     }while(isValid == false)
}

I hope what I provided wasn't too vague or messy. Again, I had little exposure to programming in the past, so forgive me if I mess some terminology up.