Two parts of my decoding program aren't working as expected - c++

Question

I came here for help(with a more specific part) accouple days ago, but the solution I got didn't quite work. Basically I'm writing a program that serves 3 purposes: decodes a Rot 13 cypher, decodes a Rot 6 cypher, and put user input through an equation "x=2n-1" where n is the user input.
Rot 13 works fine, but Rot 6 outputs gibberish, and the equation outputs a letter (putting "8" gives you o instead of 15)
I know that this could be done in less functions, and I probably don't need an list, but this is for an assignment, and I need them
I know that I am not great at this, but any help would be great
#include <iostream>
#include <string>
#include <list>
#include <array>
using namespace std;
string coffeeCode(string input) { //Coffee code= 2n-1 where n=a number in a string
double index{};
input[index] = 2*input[index]-1;
return input;
};
string rot6(string input) {
int lower[] = { 'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z',' ' };
int upper[] = { 'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z' };
int inputSize = input.size(); // rot 6 rotates letters so that a{0}->g[6]
int index{}; // m[12]->s[18]
// n->t
while (index != inputSize) { // z->f
if (input[index] >= lower[0] && input[index] <= lower[19])
input[index] = input[index] + 6;
else if (input[index] >= lower[20] && input[index] <= lower[25])
input[index] = input[index] - 20;
else if (input[index] >= upper[0] && input[index] <= upper[19])
input[index] = input[index] + 6;
else if (input[index] <= upper[20] && input[index] <= upper[25])
input[index] = input[index] - 20;
index++;
}
return input;
}
string rot13(string input) { //Decodes into rot 13
int lower[] = { 'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z',' ' };
int upper[] = { 'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z' };
int inputSize = input.size();
int index{};
while (index != inputSize) {
if (input[index] >= lower[0] && input[index] <= lower[12])
input[index] = input[index] + 13;
else if (input[index] >= lower[13] && input[index] <= lower[25])
input[index] = input[index] - 13;
else if (input[index] >= upper[0] && input[index] <= upper[12])
input[index] = input[index] + 13;
else if (input[index] <= upper[13] && input[index] <= upper[25])
input[index] = input[index] - 13;
index++;
}
return input;
}
int main() {
string plaintext;
string ans13;
string ans6;
string ansCoffee;
cout << "Whats the message Spy Guy: ";
getline(cin, plaintext);
ans13 = rot13(plaintext);
ans6 = rot6(plaintext);
ansCoffee = coffeeCode(plaintext);
cout << "One of these is your decoded message" << endl << "In Rot 13: " << ans13 << endl << "In Rot 6: " << ans6 << endl
<< "In Coffee Code: " << ansCoffee << endl;
return 0;
}

Answer 1

Your best chance at understanding what you are doing is reading about how c++ handles characters, and what ASCII is.
[http://www.asciitable.com/]
Just a quick summary:
The ASCII table assigns every character in the english alphabet a number(or a code), and that number is what C++ actually stores in memory.
So, when you do char c = 'a' that is equivalent to doing char c = 97.
The ASCII table is pretty organised too, so all capital letters are in alphabetical order starting from 65(which is A) to 90(which is Z). The same is true for non capital letters, which go from 97 to 122, and for digits, which go from 48 to 57.
This can be used to determine what kind of character a variable is:
if ('a' <= input[index] && input[index] <= 'z') {
// It's lower case
}
if('A' <= input[index] && input[index] <= 'Z') {
// It's upper case
}
Note that when you put a singe character in single quotes the compiler will replace it with it's ASCII code, so you don't actually need to memorize the table. Think about how you could construct an if to determine whether a character is a digit.
Here is how I would implement the rot6. It may not bet the best, but I think it's alright.
string rot6(string input) {
for (int index = 0; index < input.size(); ++ index) {
if ('a' <= input[index] && input[index] <= 'z') { // It's lower case
input[index] = 'a' + ((input[index] - 'a') + 6) % 26;
}
else if ('A' <= input[index] && input[index] <= 'Z') { // It's upper case
input[index] = 'A' + ((input[index] - 'A') + 6) % 26;
}
else if ('0' <= input[index] && input[index] <= '9') { // It's a digit
input[index] = '0' + ((input[index] - '0') + 6) % 10;
}
else { // It's an error
return "Error, bad input!";
}
}
return input;
}