Xtykutl

The program only needs numbers and/or letters as input from the user. otherwise, the program must be terminated. I don't know how to limit the input to numbers and letters only.

Here is my program:

#include <iostream>

#include <string>

using namespace std;



int main()

{

    string input, reversed = "";



    cout << "Enter string: "; 

    cin >> input;



    for (int i = input.length() - 1; i < input.length(); i--)

    {

        reversed += input[i];

    }

    cout << reversed << endl;



    if (reversed == input)  cout << "It is a palindrome!"; 

    else            cout << "No, it is not a palindrome!";

    return 0;

}

The simplest way you can achieve what you want is checking every character. As @Pete Becker pointed out, you may use isalnum to check if the character is a number or a letter character:

#include <iostream>

#include <string>



int main () {

  std::string s = "Hello W>orld";



  for (auto c : s) {

    if (!isalnum(c)) {

      std::cout << "Found : '" << c << "'" << std::endl;

    }

  }



  return 0;

}

Output:

Found : ' '

Found : '>'

As @Someprogrammerdude mentioned in the comments, using std::all_of and an appropriate binary predicate(lambda function), you can do it easily as follows.

Secondly, for checking the string input to its reverse string, just create a temporary string using the reverse iterators of std::string and check it. That way, you do not need to have another variable.

Hope the comments will help you to understand more options.
SEE LIVE

#include <iostream>

#include <cctype> // std::isdigit and std::isalpha and std::isalnum

#include <string>

#include <algorithm> // std::all_of



int main()

{

    std::string input; std::cin >> input;



    const auto check = (const char eachCar)->bool{ return std::isalnum(eachCar); };

    /* change return statement of lambda to

        std::isdigit(eachCar)  ---> for only digits

        std::isalpha(eachCar)  ---> for only letters

    */

    if(std::all_of(input.cbegin(), input.cend(), check))

    {

        if(input == std::string(input.crbegin(), input.crend()))

            std::cout << "It is a palindrome!";

        else  std::cout << "No, it is not a palindrome!";

    }

    return 0;

}

Input:

123kk321

Output:

It is a palindrome!

Assume the input has no space, you can do like this:

char c; string s; bool chk = true;

while (cin >> c) 

{

    if (('0' <= c && c <= '9') || ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z')) 

        s.push_back(c);

    else chk = false;

}

if (chk == true) cout << "Valid string";

else cout << "Invalid string";

The above code will terminate when there is no character left in the input.