(C++) How do I display an error when more than one "." is used in a calcualtor?

I am making a calculator using command line arguments, and one of the problems I am having is that I can’t find a way to display an error to inputs that have more than one ".". 3.33 can be accepted, but 3.3.3.2 cannot because its an invalid number.

int main(int argc, char* argv[]) {
if (argc == 1) {

    cout << "E\n";
    return 0;
}
if (argc <= 2) {
    cout << "P\n";
    return 0;
}
if (argc > 4) {
    cout << "P\n";
    return 0;
}

if (argc == 3) {
    cout << endl << (atof(argv[1]) + atof(argv[2])) << endl;
    return 0;
}

else if (argc == 4)
{
    //Addition operation  
    if (argv[3][0] == 'a')
        cout << endl << (atoi(argv[1]) + atoi(argv[2])) << endl;

    //Subtraction operation
    else if (argv[3][0] == 's')
        cout << endl << (atof(argv[1]) - atof(argv[2])) << endl;

    //Multiplication operation
    else if (argv[3][0] == 'm')
        cout << endl << (atof(argv[1]) * atof(argv[2])) << endl;

    //Division operation
    else if (argv[3][0] == 'd')
        if (argv[2][0] == '0') {
            cout << endl << "error";
            return 0;
        }
        else {
            cout << endl << (atof(argv[1]) / atof(argv[2])) << endl;
        }


    //Exponential operation
    else if (argv[3][0] == 'p')
        if (argv[2][0] > - 1.00 && argv[2][0] < 1.00) {
            cout << endl << "Y";
            return 0;
        }
        else if (argv[1][0] == '-') {
            cout << endl << "Y";
            return 0;
        }

        else 
            cout << endl << pow(atof(argv[1]), atof(argv[2])) << endl;
        

    else
        cout << endl << "V" << endl;
    //Any other operator
}

}

>Solution :

One way for checking the validity of a string is to use regular expressions, for example, you can use this.

You can write something like this to extract your operands:

float op1, op2;
std::string p1 = argv[1];
std::string p2 = argv[2];
std::regex pattern("[+-]?([0-9]*[.])?[0-9]+");

if (std::regex_match(p1, pattern) && 
    std::regex_match(p2, pattern)) {
  op1 = atof(argv[1]);
  op2 = atof(argv[2]);
  // ...
} else {
  cout << "Error!" << std::endl;
}

Alternatively, you could use the return value of atof to check if an error happened:

Return value

Floating point value corresponding to the contents of str
on success. If the converted value falls out of range of corresponding
return type, range error occurs and HUGE_VAL, HUGE_VALF or HUGE_VALL
is returned. If no conversion can be performed, ​0​ is returned and
*str_end is set to str.