Question

NO LINKS

Answer 1

Answer:

#include <iostream>

#include <string>

#include <regex>

using namespace std;

int main()

{

   cout << "Enter a 5-digit number: ";

   string number;

   cin >> number;

   bool valid = regex_search(number, regex("^\\d{4}[02468]$"));

   if (valid) {

       valid = stoi(number.substr(0, 1)) + stoi(number.substr(1, 1))  

           < stoi(number.substr(3, 1)) + stoi(number.substr(4, 1));

   }

   cout << number << (valid ? " is valid" : " is invalid");

}

Explanation:

Regular expressions can do all of your checking except for the sum of digits check. The checks are i.m.o. easiest if you don't treat the input as a number, but as a string with digits in it.

The regex means:

^ start of string

\d{4} exactly 4 digits

[02468] one of 0, 2, 4, 6 or 8 (this is what makes it even)

$ end of string