#include<iostream>
#include<istream>
#include<cctype>
 
using namespace std;
 
int main()
{
	const int OFFSET = 3;
    string tekst;
    char c;
 
    cout << "Wprowadz tekst: ";
    getline(cin, tekst);
    cout << "\nOryginalny tekst: " << tekst;
 
    for (int i = 0, epos = tekst.length(); i < epos; i++)
    {
 
          c = tekst[i];
          if (islower(c)) {
            c = 'a' + (c - 'a' + OFFSET) % ('z'-'a'+1);
            tekst[i] = c;
          } else if (isupper(c)) {
            c = 'A' + (c - 'A' + OFFSET) % ('Z'-'A'+1);
            tekst[i] = c;
          }
    }
 
    cout << "\nZmieniony tekst: " << tekst;
 
}

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VG8gamVzdCBUZVN0IHp6elpaWnl5eQ==

To jest TeSt zzzZZZyyy