Error with string validation function

Question

Hey im trying to validate a string. Basically what i want it to do is prevent the user from entering anything other than a string. Here is my code:

**getString**

string getString(string str)
{
    string input;
    do
    {
        cout << str.c_str() << endl;
        cin >> input;
    }
    while(!isalpha(input));
    return input;
}

Errors

Error   2   error LNK2019: unresolved external symbol "public: bool __thiscall Validator::getString(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >)" (?getString@Validator@@QAE_NV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@Z) referenced in function "public: void __thiscall Player::information(void)" (?information@Player@@QAEXXZ)    C:\Users\Conor\Documents\College\DKIT - Year 2 - Repeat\DKIT - Year 2 - Semester 1 - Repeat\Games Programming\MaroonedCA2\MaroonedCA2\Player.obj    MaroonedCA2
Error   3   error LNK1120: 1 unresolved externals   C:\Users\Conor\Documents\College\DKIT - Year 2 - Repeat\DKIT - Year 2 - Semester 1 - Repeat\Games Programming\MaroonedCA2\Debug\MaroonedCA2.exe MaroonedCA2
    4   IntelliSense: no suitable conversion function from "std::string" to "int" exists    c:\Users\Conor\Documents\College\DKIT - Year 2 - Repeat\DKIT - Year 2 - Semester 1 - Repeat\Games Programming\MaroonedCA2\MaroonedCA2\Validator.cpp 72  17  MaroonedCA2

Main

cout << "What is your name ?\n";
    name = validator.getString();<------This skips.

    cout << "\nWhat is your age? ";
    age = validator.getNum();

    string character = "What is your sex M/F?";
    sex = validator.getChar(character);


    cout <<"Name:\n"<< name<<" Age:\n" << age<< " Sex:\n"<< sex <<"\n";

New getString function.

string Validator :: getString()
{
   string input;
    do 
    {
    } 
    while (
    std::find_if_not( 
        std::begin(input), //from beginning
        std::end(input), //to end
        isalpha //check for non-alpha characters
    ) != std::end(input) //continue if non-alpha character is found
);
    return input;
}

You can output a string; you don't have to convert it to a C string first. Anyway, I'm guessing you forgot to prefix it with Validator::. — Qaz
– Qaz, Commented Nov 28, 2012 at 19:01
@chris Yeah i forgot the prefix. But i still have an error for no suitable conversion. The variable input on the line while(!isalpha(input)); shows this. — Pendo826
– Pendo826, Commented Nov 28, 2012 at 19:06

Qaz · Accepted Answer · 2012-11-28 19:36:11Z

2

The first problem described is that this function belongs to a class, but you forgot to specify that:

string Validator::getString(string str)
       ^^^^^^^^^^^

Next, isalpha takes an int (due to C reasons), and, as far as I know, there is no version for std::string. You can, however, use standard algorithms to do this:

do {
    ...
} while (
    std::find_if_not( 
        std::begin(input), //from beginning
        std::end(input), //to end
        isalpha //check for non-alpha characters
    ) != std::end(input) //continue if non-alpha character is found
);

This find_if_not call will search through the string and check if any non-alpha characters are found by comparing the return value to the ending iterator of the string. If they're equal, the string is clean. You might also have to cast isalpha because it expects a predicate taking a char, not an int.

For some samples using this algorithm, see here. Note that due to the version of GCC on there, std::begin() and std::end() were replaced, and the != was changed to == due to the reversed logic of the function (you'd use it like do {} while (!ok(...));).

edited Nov 28, 2012 at 19:36

answered Nov 28, 2012 at 19:27

Qaz

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5 Comments

Pendo826 Over a year ago

Is there an import for the find_if_not ?

Qaz Over a year ago

@Pendo826, find_if_not also part of C++11, so you'll need the appropriate version of your compiler and argument to use it. Using C++03, I'm pretty sure using std::find_if with std::not1 wrapping isalpha would work.

Pendo826 Over a year ago

@chris im getting a problem. When i run the code using this cout << "What is your name ?\n"; name = validator.getString(); It skips the input altogether.

Qaz Over a year ago

@Pendo826, Could you post a small, complete sample of code that causes that to happen?

Pendo826 Over a year ago

I have also added the new function you have helped me with.

Jimmy Lu · Accepted Answer · 2012-11-28 19:22:44Z

1

isalpha takes in an int not a string.

One way to make sure every character in your string is a letter would be to do something like this.

bool isAlpha = false;
while (!isAlpha) {
// take in input blah blah


    isAlpha = true;
    for (unsigned i = 0; i < input.length(); ++i) {
      if (!isalpha(input[i]))
          isAlpha = false;
    }
}

edited Nov 28, 2012 at 19:22

answered Nov 28, 2012 at 19:10

Jimmy Lu

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3 Comments

Jimmy Lu Over a year ago

yeah sorry, should be fixed now.

Pendo826 Over a year ago

Just ran the code with the update. It skips the string altogether. This is the way i done it in the main cout << "What is your name ?\n"; name = validator.getString();

Pendo826 Over a year ago

Yeah i dont quite understand it.

Thomas Matthews · Accepted Answer · 2012-11-28 20:51:07Z

0

A more efficient method would be to search your string for numeric digits, using the std::string functions:

static const char digits[] = "0123456789";
isAlpha = text.find_first_of(digits, 0); // start from position 0.

answered Nov 28, 2012 at 20:51

Thomas Matthews

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3 Comments

Qaz Over a year ago

This would return true for anything except finding it at position 0. isAlpha = text.find_first_of(digits, 0) == std::string::npos;. I don't know exactly which characters the OP doesn't want, but it would work well if all of those characters are included (or doing a whitelist instead).

Thomas Matthews Over a year ago

@chris: my understanding is that the 2nd parameter of find_first_of is the starting position to search from, inclusive. Also, the first position in the string is zero.

Qaz Over a year ago

Yes, I meant the return value. The only false return possible is if it's found at index 0. Otherwise, the return value will be the index (not 0), or npos (also not 0), meaning isAlpha will mostly always be true.

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