Question

I’m writing a program that solves Caesar ciphers in C++. It takes a string of the alphabet and shifts it to the left each loop: “abc….yz” –> “bcd…..yza”. The problem is after another loop it goes: “bcd…..yza” –> “cde…..yzaa”.

char temp;       // holds the first character of string
string letters = "abcdefghijklmnopqrstuvwxyz";
while (true)
{
    temp = letters[0];
    for (int i = 0; i < 26; i++)
    {
        if (i == 25)
        {
            letters += temp;
        }
        letters[i] = letters[i + 1];
        cout << letters[i];
    }
    cin.get();
}

Copy and paste that code and you’ll see what I’m talking about. How do I fix this mysterious problem?

Answer 1

I think you need letters to be 27 characters, not 26, and instead of letters += temp (which grows the string every time), use letters[26] = temp[0].

…at which point you can just ditch temp entirely:

string letters = "abcdefghijklmnopqrstuvwxyz.";
while (true)
{
    letters[26] = letters[0];
    for (int i = 0; i < 26; i++)
    {

        letters[i] = letters[i + 1];
        cout << letters[i];
    }
    cin.get();
}

[edit]

Although the more natural way to handle this is to use arithmetic on the characters themselves. The expression 'a' + ((c - 'a' + n) % 26) will shift a char c by n places Caesar-style.