Need to remove duplicate values from C# program

Question

I need some help with a C# program that i am creating. So in this scenario i am inputting duplicate values into the program. For Example, a,b,b,c,c.
The exercise is that

Answer 1

Choose right data structure at beginning, use HashSet instead of array since the operations are mainly looking up & inserting.

Answer 2

Keep it simple, and although a HashSet is nice semantically, it's not needed for 5 elements (it's actually slower than a List in that case). Worse, it requires a parallel structure to track the characters (assuming you care about order).

Clearly none of these considerations matter for such a small example but it's good to learn them up front and don't always jump to big-O notation when actually measured performance and memory consumption should be your guide for most practical applications.

Instead you can simply do:-

List<char> chars = new List<char>(5);
while (chars.Count < 5)
{
   char c = Console.ReadKey().KeyChar;
   if (!char.IsLetter(c)) continue;
   if (chars.Contains(char)) continue;
   chars.Add(char);
}

Plus whatever error messages you want to add.

Answer 3

Use Any linq expression to validate duplicates. char.TryParse will validates input and returns true when succeeded.

public static void Main()
{
    char[] arr = new char[5];

    //User input
    Console.WriteLine("Please Enter 5 Letters only: ");

    for (int i = 0; i < arr.Length; i++)
    {
         char input;

        if(char.TryParse(Console.ReadLine(), out input) && !arr.Any(c=>c == input))
        {           

            arr[i] = input;
        }
        else
        {
            Console.WriteLine( "Error : Either invalid input or a duplicate entry.");
            i--;
        }
    }

    Console.WriteLine("You have entered the following inputs: ");
    //display
    for(int i = 0; i<arr.Length; i++)
    {

        Console.WriteLine(arr[i]);
    }

}

Working Code

Answer 4

Using a hashtable (Generic Dictionary) is an efficient way to determine if an entered character has already been encountered.

Also, the Char.IsLetter method in the .NET framework is a great way to check for bad data.

static void Main(string[] args) {
    Dictionary<char, bool> charsEntered = new Dictionary<char, bool>();

    Console.WriteLine("Please enter 5 characters, each on a separate line.");
    while (charsEntered.Count() < 5) {
        Console.WriteLine("Enter a character:");
        char[] resultChars = Console.ReadLine().ToCharArray();

        if(resultChars.Length != 1 || !Char.IsLetter(resultChars[0])) {
            Console.WriteLine("Bad Entry. Try again.");
        } else {
            char charEntered = resultChars[0];
            if (charsEntered.ContainsKey(charEntered))
                Console.WriteLine("Character already encountered. Try again.");
            else
                charsEntered[charEntered] = true;
        }
    }

    Console.WriteLine("The following inputs were entered:");
    Console.WriteLine(String.Join(", ", charsEntered.Keys));
    Console.ReadLine();
}

Answer 5

Elaborating on Shelvin's answer of using HashSet

HashSet<char> chars = new HashSet<char>();

//User input
Console.WriteLine("Please Enter 5 Letters only: ");
for (int i = 0; i < 5; )
{
     char c = Convert.ToChar(Console.ReadLine());
     if(!("abcdefghijklmnopqrstuvwxyz".Contains(c.ToString().ToLower())))
     {
          Console.WriteLine("Please enter an alphabet");
          continue;
     }
     else if (!chars.Contains(c))
     {
          chars.Add(c);
          i++;
     }
     else
     {
          Console.WriteLine("Duplicate value please try again");
          continue;
     }
}
//display
Console.WriteLine("You have entered the following inputs: ");
foreach(char c in chars)
    Console.WriteLine(c.ToString());

Console.Read();