How can I verify user inputted a string into std::cin?

Question

students[#] is an array of structs and students[#].name is a string in the struct, and I want to make sure that when the user inputs the name that it's verified to be a string of letters.

Can someone tell me a (simple?) way to get that job done?

I have tried the following to no avail.

        bool checkString(const char *str)
        {
            bool isLetters = true;

            while (*str != '\0')
            {
                if (*str < '65' || *str > '122')
                {
                   //checking to see if it's over or below ASCII alpha characters
                    isLetters = false;
                }

                if (*str < '97' && *str > '90')
                {
                     // checking to see if it's between capital and 
                     //lowercase alpha char's
                    isLetters = false;
                }

                ++str;
            }

            return isLetters;
        }

        main
        {
        //...
                for (int i = 0; i < numStudents; ++i)
                {
                    valid = true;
                    do
                    {
                        valid = true;
                        cout << "Enter NAME for student #" << i + 1 << ":" << endl;
                        cin >> students[i].name;
                        if (cin.fail() || !checkString(students[i].name.c_str()))
                            // tried to see if i could convert it to a c-style 
                            //string to check char by char
                        {
                            cin.clear();
                            cin.ignore();
                            cout << "Please enter a valid input." << endl;
                            valid = false;
                        }
                    } while (!valid);
        //...
        return 0;
        }

        `

The only thing you can enter into cin is a string. That string may then be parsed into something else though - like a int or double for example. — Jesper Juhl
– Jesper Juhl, Commented Feb 7, 2018 at 6:05
Oh I see so it may be much more work than it's worth if it's just for personal satisfaction then, because the only way I could see really doing it is through extensive switch statements — Micah Johnson
– Micah Johnson, Commented Feb 7, 2018 at 6:26
@MicahJohnson you can do it with function isalpha and a loop. isalpha tests if one character is a letter. You could also do it with a regex which is even simpler. — john
– john, Commented Feb 7, 2018 at 6:31
@john might just be out of my league, but when I come across those terms in my learning I'll keep that in mind. Thanks — Micah Johnson
– Micah Johnson, Commented Feb 7, 2018 at 7:20
What are '65' and '122' meant to be? Either test if it's less than the character, *str < 'A', or the integer value, *str < 65. Not '65' (which is a multi-character constant, and not what you want). Better still, don't assume ASCII and use isalpha like the answer below. — Jonathan Wakely
– Jonathan Wakely, Commented Feb 7, 2018 at 13:25

john · Accepted Answer · 2018-02-07 08:09:01Z

1

ASCII codes are integers, don't put them in quotes.

bool checkString(const char *str)
{
    bool isLetters = true;
    while (*str != '\0')
    {
        if (*str < 65 || *str > 122)
        {
           //checking to see if it's over or below ASCII alpha characters
            isLetters = false;
        }

        if (*str < 97 && *str > 90)
        {
             // checking to see if it's between capital and 
             //lowercase alpha char's
            isLetters = false;
        }
        ++str;
    }
    return isLetters;
}

You are still doing the hard way though, the function isalpha is designed for this task

#include <cctype>

bool checkString(const char *str)
{
    bool isLetters = true;
    while (*str != '\0')
    {
        if (!isalpha(*str)))
            isLetters = false;
        ++str;
    }
    return isLetters;
}

Then you could return immediately when you realise it's not a letter, no point carrying on checking

#include <cctype>

bool checkString(const char *str)
{
    while (*str != '\0')
    {
        if (!isalpha(*str)))
            return false;
        ++str;
    }
    return true;
}

answered Feb 7, 2018 at 8:09

john

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2 Comments

Jonathan Wakely Over a year ago

Since this is processing untrusted input from outside the program, that needs to be isalpha((unsigned char)*str) or you get undefined behaviour if the input contains values outside the range [0,127] and char is signed. And strictly speaking if you include <cctype> you should use std::isalpha (or add a using-declaration, or include <ctype.h> instead).

Micah Johnson Over a year ago

Thanks, that worked. Feel silly since I didn't try taking them out of quotes. But isalpha is overall much more simple.

Pranav Totala · Accepted Answer · 2018-02-07 13:02:50Z

0

You will have to check if each char is an alphabet by using ascii values.

bool checkString(string stdunt)
    {
        bool isLetters = true;
        for (int i = 0; i < stdunt.length(); i++)
            {
                    if (stdunt[i] < 65 || stdunt[i] > 122)
                    { return false; }

                    if (stdunt[i] < 97 && stdunt[i] > 90)
                    { return false; }
            }
        return isLetters;
    }

int main()
{

int numStudents=5;
string students[numStudents];
    for (int i = 0; i < numStudents; i++)
    {
        bool valid = true;


    do{
            valid = true;
            cout << "Enter NAME for student #" << i + 1 << ":" << endl;
            cin >> students[i];
//            cout <<students[i];
            if (cin.fail() || !checkString(students[i]))
            {
                cin.clear();
                cin.ignore();
                cout << "Please enter a valid input." << endl;
                valid = false;
            }
      } while (!valid);
    }
    cout << "Complete";
    return 0;
}
`

edited Feb 7, 2018 at 13:02

answered Feb 7, 2018 at 6:37

Pranav Totala

1582 silver badges15 bronze badges

2 Comments

Micah Johnson Over a year ago

Just edited code with attempt at doing this, no luck.

Pranav Totala Over a year ago

Just edited the answer, hopefully it is what you want. You can use pointers if the string size is unknown.

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