#include <iostream>
#include <string>

using namespace std;

string sEncrypt(string& sInput, int iShift);

int main(int argc, char *argv[])
{
    // Declare variables
    string sInput; //text to encrypt, user inputted
    int iShift = 0; // shift value, user inputted

    
    cout << "Please enter the string to encrypt: ";
    getline(cin, sInput); //getline used to preserve spaces
    cout << "\n Please input the shift value: ";
    cin >> iShift;
    // pass values to Encryption functon
    sEncrypt(sInput, iShift);
    cout << "Encrypted String = " << sInput << endl;
    system("PAUSE");
    return EXIT_SUCCESS;
}

string sEncrypt(string& sInput, int iShift) //pass variable by reference
{
    int iArrayValue = 0; //simple integer variable used to traverse the string array
    
       while(iArrayValue < sInput.length())
    {
            sInput[iArrayValue] = int(sInput[iArrayValue]) + iShift; //use ASCII values to change letter
            iArrayValue++;
    }
    return sInput;
}

Psuedo:

if(letter value < 'A' or > 'Z')
letter value = 'Z' or 'A'

c = (isascii(c)):(((c-'A'+offset) % 26) + 'A':c;

// c is a single char we need to convert - I'll leave it to you to "dig out the char, and store it back into the string"
if (isalpha(c))   // Is it A..Z? // 0.
   c = (c - 'A') + offset;   // 1. This is a value offset..25+offset  (offset = A, 25+offset = Z). 
   c %= 26; // 2. This takes the remainder of division by 26, which is 0..25
   c += 'A';  // 3. Make it back into a letter.
}

0    1    2     3
'A'  3    3    'D'
'F'  8    8    'I'
'W' 25   25    'Z'
'Z' 28    2    'C'