Caesar Cipher Shift (using alphabet array)

Question

I am currently writing a Caesar Cipher program in C# for my assignment and I am having a problem.

I am approaching this task using an array where I store the whole a

Answer 1

I took a look at your code and made a slight adjustment. First of all, I converted it to a method that lets you pass in the string and the amount you want to shift, so that you can either call it in a loop from 0 to 25 to see all the permutations, or you can just get a single value. I also check to see if each character is actually in the array, and if it isn't, then don't change it (in your code you were only checking for '\' and ' ' characters:

public static string ShiftText(string input, int shiftAmount)
{
    if (input == null) return null;

    char[] alphabet =
    {
        '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'
    };

    shiftAmount %= 26; // Ensure shift is between 0 and 25

    var shiftedText = string.Empty;

    foreach (var character in input)
    {
        var index = Array.IndexOf(alphabet, character);

        if (index < 0)
        {
            // This character isn't in the array, so don't change it
            shiftedText += character;
        }
        else
        {
            var newIndex = index - shiftAmount;

            // If it's negative, wrap around to end of array
            if (newIndex < 0) newIndex += 26;

            shiftedText += alphabet[newIndex];
        }
    }

    return shiftedText;
}

---

But another way to do this that works for upper AND lower case, and which is less code, is to simply test if char.IsLetter(character), and then shift the ASCII value of the character within the same 0-25 range.

For example, this does the same as the code above, only it works for lower case letters as well. The difference here is that before we compare the character to our lowest valued character ('a' or 'A'), we test if char.IsLower() first. This way we stay within the ASCII range for this character set:

/// 

/// This method takes the input string and shifts all letter characters 
/// to the left (subtracts) by the amount specified in shiftAmount, so 
/// if shiftAmount = 1, then 'M' becomes 'L', and 'a' becomes 'z'.
/// 
/// The input string to apply changes to
/// A value from 0 to 25, used to shift the characters
/// The modified (shifted) string
public static string ShiftText(string input, int shiftAmount)
{
    if (input == null) return null;

    // Ensure shift is between 0 and 25
    shiftAmount %= 26; 

    var result = string.Empty;

    // Loop through input and update result with shifted letters
    foreach (var character in input)
    {
        if (!char.IsLetter(character))
        {
            // If the character isn't a letter, don't change it
            result += character;
        }
        else
        {
            var newChar = (char) (character - shiftAmount);
            // Adjust newChar to stay within this character range
            if (newChar < (char.IsLower(character) ? 'a' : 'A')) newChar += (char) 26;
            result += newChar;
        }
    }

    return result;
}