I'm making a Caesar cipher, currently I have a shift of three that I want to encrypt the message with. If any letter has "x, y or z" it will give me an out of bounds array error (because the shift is 3).
How can I pass the error by going back to the start of the array, but ending with the remainder of the shift?
This is my code currently:
using System;
using System.Text;
//caesar cipher
namespace Easy47
{
class Program
{
static void Main(string[] args)
{
const string alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
var input = Input(alphabet);
Encrypt(3, input, alphabet);
Console.WriteLine();
}
private static void Encrypt(int shift, string input, string alphabet)
{
var message = new StringBuilder();
foreach (char letter in input)
{
for (int i = 0; i < alphabet.Length; i++)
{
if (letter == alphabet[i])
{
message.Append(alphabet[i + shift]);
}
}
}
Console.WriteLine("\n" + message);
}
private static string Input(string alphabet)
{
Console.Write("Input your string\n\n> ");
string input = Console.ReadLine().ToUpper();
return input;
}
}
}
You use the modulo operator:
var i = 255
var z = i % 200 // z == 55
in your case here:
for (int i = 0; i < alphabet.Length; i++)
{
if (letter == alphabet[i])
{
message.Append(alphabet[ (i + shift) % alphabet.Length]);
}
}
If after your addition of shift
the index is bigger then alphabet.Length
it will start at 0 again.
Unrelated, but your loop is not very efficient. A message of "ZZZZZ"
would go 5 times through your full alphabet to get translated. You should use a Dictionary as lookup. You can create it at the start before you translate the message and then your lookup is very fast - thats what dictionarys excel at. O(1) lookups :o)
If you know a little about linq, this should be understandable:
// needs: using System.Linq;
private static void Encrypt(int shift, string input, string alphabet)
{
var message = new StringBuilder();
// create a string that is shifted by shift characters
// skip: skips n characters, take: takes n characters
// string.Join reassables the string from the enumerable of chars
var moved = string.Join("",alphabet.Skip(shift))+string.Join("",alphabet.Take(shift));
// the select iterates through your alphabet, c is the character you currently handle,
// i is the index it is at inside of alphabet
// the rest is a fancy way of creating a dictionary for
// a->d
// b->e
// etc using alphabet and the shifted lookup-string we created above.
var lookup = alphabet
.Select( (c,i)=> new {Orig=c,Chiff=moved[i]})
.ToDictionary(k => k.Orig, v => v.Chiff);
foreach (char letter in input)
{
// if the letter is not inside your alphabet, you might want to add
// it "as-is" in a else-branch. (Numbers or dates or .-,?! f.e.)
if (lookup.ContainsKey(letter))
{
message.Append(lookup[letter]);
}
}
Console.WriteLine("\n" + message);
}
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