Question

Hey im validating a string.

string getString(string q)
{
    string input;

    do
    {
        cout << q.c_str() << endl;
        cin >> input;
    } while (!isalpha(input));

    return input;
}

When using while(!isalpha(input)); input gives that error.

Can anyone help me with this?

Answer 1

The other answer describes what the problem is, but here’s a solution that makes use of algorithms from the standard library instead of writing your own (example requires C++11)

bool all_alpha( std::string const& s )
{
  return std::all_of( s.cbegin(), s.cend(), static_cast<int(*)(int)>(std::isalpha) );
}

The above function will return true only if all characters in the string are alphabetic. If you only want to disallow numeric characters, I’d use a slightly different function.

bool any_digit( std::string const& s )
{
  return std::any_of( s.cbegin(), s.cend(), static_cast<int(*)(int)>(std::isdigit) );
}

or

bool no_digits( std::string const& s )
{
  return std::none_of( s.cbegin(), s.cend(), static_cast<int(*)(int)>(std::isdigit) );
}

Use these functions to validate the input that you receive from the user.

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If you can’t use C++11 features, the functions can be modified to use std::find_if instead, and compare the return value of find_if to s.end() to determine success / failure.