How to create an array of non repeating random numbers

Question

I have a lottery application in C# which takes in the number of numbers to draw and also the maximum number to draw.I have coded up to creating an array holding the required random numbers but I need them to be unique and am having trouble doing it.I would be very grateful if someone could give me some advice on this,Thanks

Here is my code so far:

class Lottery

{

  static int[] numberHolder; //array to be filled with numbers up to an 
                             //amount entered by the user eg 42 Max

  static int[] drawHolder;   //array to hold the each random number 
                             //drawn from the pool of numbers eg 7 numbers

    public Lottery() //Lottery class Constructor
    {

    }


    //method which takes in a number limit and amount of numbers to be drawn
    public String drawNumbers(int numLimit, int numAmount) 
    {

        Random RandomNumber = new Random();

        for (int i = 0; i < numLimit ; i++) //loop to fill up numberHolder array
                                            // with predefined limit of numbers

        {
            numberHolder[i] = i++;
        }

        for (int i = 0; i < numAmount; i++)
        {


            // code to pick unique random numbers no greater than numAmount 
            // and add them to the drawHolder[] array
            drawHolder[i] = RandomNumber.Next(1, numLimit);

        }





        //return the drawHolder array to String
        return null;
    }





}

Answer 1

In my opinion, you should change your approach.

Instead of thinking "I'll generate random indexes to pick my numbers", where you have to be sure you don't get any duplicates, I would simply shuffle the array and take the X first you need. That way, you don't need to worry about indexes or duplicates.

So your second for loop would be changed to

drawHolder = numberHolder.OrderBy(x => new Guid()).Take(numAmount);

(Please note I've used new Guid() so you can remove your RandomNumber declaration line. As discussed before, a GUID is a unique value and not meant for to be used as a random gen. You could also use x => RandomNumber.Next(), but if you really need a strong and reliable shuffe, read about Fisher-Yates)

You can also replace your numberHolder array with a simple Enumerable.Range

So your whole code would become (And please note I've changed your method name to use C# conventions, method names should be in PascalCase)

public string DrawNumbers(int numLimit, int numAmount) 
{
    drawHolder = Enumerable.Range(0, numLimit).OrderBy(x => new Guid()).Take(numAmount);

    return string.Join(", ", drawHolder);
}