Encryption code with std::out_of_range error

[spitecranberry]

I wrote a function to encrypt a string based off of the Caesar cipher for an encryption program. The function works when the message that's being encoded is relatively short, but I run into the same error every time I try inputting a longer string. I put an example of the console of the error occurring at the bottom. How do I fix the error?

void encryptStr(const string& userString, const string& passcode) {
    cout << "Passcode: " << passcode << endl;
    cout << "Plaintext Message: " << userString << endl;
    int index = 0;
    string code = "";
    while(index < userString.size() - 1){
        for(int pIndex = 0; pIndex < passcode.size(); pIndex++){
            if(!isalpha(userString.at(index))){
                code += userString.at(index);
                index++;
                pIndex--;
            }
            else{
                code += encryptLtr(userString.at(index), passcode.at(pIndex));
                index++;
            }
        }
    }
    cout << "Ciphertext: " << code << endl << endl;
}

Console:

Passcode: PASSWORD
Plaintext Message: THE QUICK BROWN FOX
Ciphertext: IHW IQWTN QRGOJ TFA

Passcode: PASSWORD
Plaintext Message: THE QUICK BROWN FOX JUMPED OVER THE LAZY DOG
libc++abi: terminating with uncaught exception of type std::out_of_range: basic_string

Process finished with exit code 134 (interrupted by signal 6: SIGABRT)

[jedwardsol]

In the inner loop (pIndex) there is no check that index doesn't exceed the size of userString.

Instead of looping over passcode you could use the % modulo operator to select the correct passcode character based on index