Another Caesar Cipher

Hello,

I'm currently learning C++ and have decided to write a Caesar implementation. I have the code working but lack error checking (dealing with spaces, going back to start of alphabet) and am not sure how best to do it. Here's my method so far:

Code:

#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;
}

I don't really want a bunch of if statements inside the while loop, but cannot think of any other way. Can somebody suggest something please?

Use something like isalpha() to determine whether the character is a letter.
As for wrap-around, look at modulo arithmetic.

I'm familiar with modular arithmetic already but I'm not sure how to implement a mod 26 function inside the code.

As I'm using ASCII to shift the letters, could I just say something like:

Code:

Psuedo:

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

Obviously it would be two different statements. The problem that I'm having is that surely that would just assign the value of the new letter to be either A or Z, how would I make it 'wrap', is there a function or snippet that could help me please?

Code:

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

This assumes that all characters are upper-case, otherwise it'll get more complex.

--
Mats

Originally Posted by matsp

Code:

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

This assumes that all characters are upper-case, otherwise it'll get more complex.

--
Mats

Thanks, can you explain what this does though please as I'm a bit confused :P

Well it should have been ? : to begin with ( not : : )

Try writing it out in a longer form.
c = c - 'A';
c = c + offset;
etc.

Originally Posted by Swordsman

Thanks, can you explain what this does though please as I'm a bit confused :P

Ok, so to write it in a different way:

Code:

// 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.
}

Here's the values of the variable c at step 0, 1, 2 and 3 for some varying values of c, where offset = 3

Code:

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