Цезарь шифр (гниль)

Я новичок в программировании на C ++. Недавно мое внимание привлекли шифры Цезаря. Я закончил тем, что сделал текстовый конвертер rot , где N — любое значение от 2 до 25. Имейте в виду, что я все еще новичок в программировании и не разбираюсь в материалах продвинутого уровня. Мы будем благодарны за все ваши отзывы. К вашему сведению, это было разработано в Visual Studio 2019.

#include <iostream>
#include <string>

std::string user_input;//user input string
std::string output_str;//output ciphertext
int x;//loop counter 
int corres_num[1500000] = {};//corresponding number according to alphabets
int cipher_value;//the cipher rotation value

void termclr()//terminal clear function
{
#ifdef __linux__
system("clear");
#elif _WIN32
system("cls");
#endif
}

void convertion()//user input convertion
{
std::string output_str_assign = "";//assigns individual ciphered characters to main ciphertext
int calc = 0;//contributes in assigning <output> 
int output = 0;//assigns individual cipher characters
if (user_input[x] == 'A' || user_input[x] == 'a') { corres_num[x] = 1; }
else if (user_input[x] == 'B' || user_input[x] == 'b') { corres_num[x] = 2; }
else if (user_input[x] == 'C' || user_input[x] == 'c') { corres_num[x] = 3; }
else if (user_input[x] == 'D' || user_input[x] == 'd') { corres_num[x] = 4; }
else if (user_input[x] == 'E' || user_input[x] == 'e') { corres_num[x] = 5; }
else if (user_input[x] == 'F' || user_input[x] == 'f') { corres_num[x] = 6; }
else if (user_input[x] == 'G' || user_input[x] == 'g') { corres_num[x] = 7; }
else if (user_input[x] == 'H' || user_input[x] == 'h') { corres_num[x] = 8; }
else if (user_input[x] == 'I' || user_input[x] == 'i') { corres_num[x] = 9; }
else if (user_input[x] == 'J' || user_input[x] == 'j') { corres_num[x] = 10; }
else if (user_input[x] == 'K' || user_input[x] == 'k') { corres_num[x] = 11; }
else if (user_input[x] == 'L' || user_input[x] == 'l') { corres_num[x] = 12; }
else if (user_input[x] == 'M' || user_input[x] == 'm') { corres_num[x] = 13; }
else if (user_input[x] == 'N' || user_input[x] == 'n') { corres_num[x] = 14; }
else if (user_input[x] == 'O' || user_input[x] == 'o') { corres_num[x] = 15; }
else if (user_input[x] == 'P' || user_input[x] == 'p') { corres_num[x] = 16; }
else if (user_input[x] == 'Q' || user_input[x] == 'q') { corres_num[x] = 17; }
else if (user_input[x] == 'R' || user_input[x] == 'r') { corres_num[x] = 18; }
else if (user_input[x] == 'S' || user_input[x] == 's') { corres_num[x] = 19; }
else if (user_input[x] == 'T' || user_input[x] == 't') { corres_num[x] = 20; }
else if (user_input[x] == 'U' || user_input[x] == 'u') { corres_num[x] = 21; }
else if (user_input[x] == 'V' || user_input[x] == 'v') { corres_num[x] = 22; }
else if (user_input[x] == 'W' || user_input[x] == 'w') { corres_num[x] = 23; }
else if (user_input[x] == 'X' || user_input[x] == 'x') { corres_num[x] = 24; }
else if (user_input[x] == 'Y' || user_input[x] == 'y') { corres_num[x] = 25; }
else if (user_input[x] == 'Z' || user_input[x] == 'z') { corres_num[x] = 26; }
else { output_str_assign = user_input[x]; }//keeping user input intact if user input is not within a-z
calc = corres_num[x] + cipher_value;//assigning <calc> value
if (calc > 26)//setting value if <calc> goes over 26
{
    output = calc - 26;
}
else if (calc <= 26 && calc > 0)//maintaing <calc> value if within range
{
    output = calc;
}
switch (output)//assigning individual cipher characters
{
case 1:if (isupper(user_input[x])) { output_str_assign = "A"; }
      else if (islower(user_input[x])) { output_str_assign = "a"; } break;
case 2:if (isupper(user_input[x])) { output_str_assign = "B"; }
      else if (islower(user_input[x])) { output_str_assign = "b"; } break;
case 3:if (isupper(user_input[x])) { output_str_assign = "C"; }
      else if (islower(user_input[x])) { output_str_assign = "c"; } break;
case 4:if (isupper(user_input[x])) { output_str_assign = "D"; }
      else if (islower(user_input[x])) { output_str_assign = "d"; } break;
case 5:if (isupper(user_input[x])) { output_str_assign = "E"; }
      else if (islower(user_input[x])) { output_str_assign = "e"; } break;
case 6:if (isupper(user_input[x])) { output_str_assign = "F"; }
      else if (islower(user_input[x])) { output_str_assign = "f"; } break;
case 7:if (isupper(user_input[x])) { output_str_assign = "G"; }
      else if (islower(user_input[x])) { output_str_assign = "g"; } break;
case 8:if (isupper(user_input[x])) { output_str_assign = "H"; }
      else if (islower(user_input[x])) { output_str_assign = "h"; } break;
case 9:if (isupper(user_input[x])) { output_str_assign = "I"; }
      else if (islower(user_input[x])) { output_str_assign = "i"; } break;
case 10:if (isupper(user_input[x])) { output_str_assign = "J"; }
       else if (islower(user_input[x])) { output_str_assign = "j"; } break;
case 11:if (isupper(user_input[x])) { output_str_assign = "K"; }
       else if (islower(user_input[x])) { output_str_assign = "k"; } break;
case 12:if (isupper(user_input[x])) { output_str_assign = "L"; }
       else if (islower(user_input[x])) { output_str_assign = "l"; } break;
case 13:if (isupper(user_input[x])) { output_str_assign = "M"; }
       else if (islower(user_input[x])) { output_str_assign = "m"; } break;
case 14:if (isupper(user_input[x])) { output_str_assign = "N"; }
       else if (islower(user_input[x])) { output_str_assign = "n"; } break;
case 15:if (isupper(user_input[x])) { output_str_assign = "O"; }
       else if (islower(user_input[x])) { output_str_assign = "o"; } break;
case 16:if (isupper(user_input[x])) { output_str_assign = "P"; }
       else if (islower(user_input[x])) { output_str_assign = "p"; } break;
case 17:if (isupper(user_input[x])) { output_str_assign = "Q"; }
       else if (islower(user_input[x])) { output_str_assign = "q"; } break;
case 18:if (isupper(user_input[x])) { output_str_assign = "R"; }
       else if (islower(user_input[x])) { output_str_assign = "r"; } break;
case 19:if (isupper(user_input[x])) { output_str_assign = "S"; }
       else if (islower(user_input[x])) { output_str_assign = "s"; } break;
case 20:if (isupper(user_input[x])) { output_str_assign = "T"; }
       else if (islower(user_input[x])) { output_str_assign = "t"; } break;
case 21:if (isupper(user_input[x])) { output_str_assign = "U"; }
       else if (islower(user_input[x])) { output_str_assign = "u"; } break;
case 22:if (isupper(user_input[x])) { output_str_assign = "V"; }
       else if (islower(user_input[x])) { output_str_assign = "v"; } break;
case 23:if (isupper(user_input[x])) { output_str_assign = "W"; }
       else if (islower(user_input[x])) { output_str_assign = "w"; } break;
case 24:if (isupper(user_input[x])) { output_str_assign = "X"; }
       else if (islower(user_input[x])) { output_str_assign = "x"; } break;
case 25:if (isupper(user_input[x])) { output_str_assign = "Y"; }
       else if (islower(user_input[x])) { output_str_assign = "y"; } break;
case 26:if (isupper(user_input[x])) { output_str_assign = "Z"; }
       else if (islower(user_input[x])) { output_str_assign = "z"; } break;
}
output_str += output_str_assign;//adding to output ciphertext
}
int main()
{
bool run = true;//run validator
char run_again;//user choice for run again
while (run)//will run while <run> is true
{
    std::cout << std::endl;
    std::cout << " Cipher value (rot[2-25]): ";
    std::cin >> cipher_value;//user input
    while (cipher_value > 25 || cipher_value < 2)//checking illegal input 
    {
        std::cout << " Invalid...cipher value can only be between 2-25: ";
        std::cin >> cipher_value;//user input
    }
    std::cin.ignore(1000, 'n');
    std::cout << std::endl << std::endl;
    std::cout << " Input====> ";
    std::getline(std::cin, user_input);//user input
    for (x = 0; x < user_input.size(); x++)
    {
        user_input[x];
        convertion();//calling string convertion
    }
    std::cout << " Output===> " << output_str << std::endl;
    std::cout << std::endl << std::endl;
    std::cout << " Press any key to continue . . .";
    _getwch();
    std::cout << std::endl;
    termclr();//clear screen
    std::cout << std::endl;
    std::cout << " Run again? (Y/N): ";
    run_again = toupper(_getwche());//user input
    std::cout << std::endl;
    while (run_again != 'Y' && run_again != 'N')//checking illegal input
    {
        std::cout << " Invalid...try again (Y/N): ";
        run_again = toupper(_getwche());//user input
        std::cout << std::endl;
    }
    termclr();//clear screen
    output_str = "";
    if (run_again == 'Y')
    {
        run = true;//will run again
    }
    else if (run_again == 'N')
    {
        run = false;//wont run again
    }
}
return 0;
}