(F# versus. C#) Primitive form of ROT13

Program.cs

using System;
using System.Linq;

namespace ConsoleApplication10
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(EncryptText("THE QUICK BROWN FOX JUMPED OVER THE LAZY DOG"));
        }

        private static string EncryptText(string text)
        {
            return text.Aggregate("", (s, c) => s + Encrypt(c));
        }

        private static char Encrypt(char letter)
        {
            if (char.IsLetter(letter))
            {
                var a = letter - 'A';
                var b = (a + 13) % 26;
                var c = b + 'A';
                return (char) c;
            }
            else
            {
                return letter;
            }
        }

    }
}

Program.fs

open System

let rot13Encrypt (letter : char) =
    if Char.IsLetter(letter) then
        let newLetter =
            (int letter)
            |> (fun letterIdx -> letterIdx - (int 'A'))
            |> (fun letterIdx -> (letterIdx + 13) % 26)
            |> (fun letterIdx -> letterIdx + (int 'A'))
            |> char
        newLetter
    else
        letter


let encryptText (text : char[]) =
    for idx = 0 to text.Length - 1 do
        let letter = text.[idx]
        text.[idx] <- rot13Encrypt letter

let text =
    Array.ofSeq "THE QUICK BROWN FOX JUMPED OVER THE LAZY DOG"
    
encryptText(text)
printfn "%s" <| new String(text)