Preventing user from inputting certain chars in c++

Question

I'm quite new to c++ so please understand that my question may be silly.

I need to create a function which takes from the user a table and fills it with only specific characters. Let's say that the user needs to input his name. If the user inputs a charater from A to Z (or a to z) the character should be displayed on the screen and in that case- everything is fine. The problem is- when the user inputs a forbidden character (for instance 1-9) this shouldn't be displayed on the screen and the cursor should stay in the same position).

Do you guys know how to do this?

Perhaps you can use `getchar` to test if the character is valid and then use `putch()` to delete it if it isn't? — Pradhan, Dec 29 '14 at 15:01

score 1 · Answer 1 · answered Dec 29 '14 at 15:19

1

May be you can use this to do your job:

char ch;

while(ch = getch())
{
    if((ch>='A' && ch<='Z') || (ch>='a' && ch<='z'))
    {
        cout << ch;
    }
}

This will print only [A-Z][a-z]. You can also store your required char to use further.

answered Dec 29 '14 at 15:19

Shahriar

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This answer assumes ASCII. – Lightness Races in Orbit Dec 29 '14 at 17:18
If the user inputs a charater from A to Z (or a to z) the character should be displayed on the screen. – Shahriar Dec 29 '14 at 17:26

score 1 · Answer 2 · answered Dec 29 '14 at 16:26

On Windows you can use conio.h.

Also, you can overload the istream::operator>> function to make solution more elegant and easy to use:

Complete example:

#include <iostream>
#include <conio.h>

using namespace std;


struct person_t
{
    string name;
    string last_name;
};

// This is the function you're looking for.
void get_filtered_string(string &str)
{
    char c;
    str = "";

    do
    {
        c = _getch();

        if (('A' <= c && c <= 'Z') || ('a' <= c && c <= 'z'))
        {
            putchar(c); // 1
            str.push_back(c);
        }

    } while (c != '\r'); // 2

}


istream &operator>>(istream &stream, person_t &person)
{
    string str = "";

    cout << "Enter name: ";
    get_filtered_string(str);
    person.name = str;
    cout << endl;

    cout << "Enter last name: ";
    get_filtered_string(str);
    person.last_name = str;
    cout << endl;

    return stream;
}

int main()
{
    person_t person;
    cin >> person;
    cout << person.name.c_str() << " " << person.last_name.c_str() << endl;
    return 0;
}

Output character to screen.
In Windows when you hit Enter you're introducing two characters '\r' and '\n' in that order. Thats why we check here for '\r'.

Thank you very much for this solution, it worked. The only thing that can be added is to be able to use backspace and other text editing keys but this can be easily implemented by editing the IF. Thanks a lot — Mateusz Kukieła, Dec 29 '14 at 17:03

Preventing user from inputting certain chars in c++

2 Answers2