Fill an array with unique random integers

Question

I want to fill a small array with unique values from a bigger array. How do I check the uniqueness?

Here's what I have:

int[] numbers45 = new int[45];

for (int i = 0; i <= 44; i++)             // I create a big array
{
    numbers45[i] = i + 1;                
}         

Random r = new Random();
int[] draw5 = new int[5];                 //new small array 

Console.WriteLine("The 5 draws are:");

for (int i = 1; i <= 4; i++)
{
    draw5[i] = numbers45[r.Next(numbers45.Length)];  //I fill the small array with values from the big one. BUT the values might not be unique.
    Console.WriteLine(draw5[i]);
}

You can either have separate table to track drawn values and draw until you have only unique values, or you can remove values that you already have drawn and then draw from numbers45.Length - i. — Przemek Marcinkiewicz, Dec 14 '17 at 11:20

Jite · Answer 1 · 2017-12-14T11:29:14.130

There are multiple ways to do what you are asking.

First off, though, I would recommend to use one of the classes which wraps the array type and adds some extra functionality you could use (in this case a List would probably be a perfect structure to use)!

One way to handle this is to check if the value is already in the draw5 array. This can be done with (for example) the List<T>.Contains(T) function, and if it exists, try another.

Personally though, I would probably have randomized the first array with the OrderBy linq method and just return a random number, like:

numbers45.OrderBy(o => random.Next());

That way the numbers are already random and unique when it is supposed to be added to the second list.

And a side note: Remember that arrays indexes starts on index 0. In your second loop, you start at 1 and go to 4, that is, you wont set a value to the first index.
You could just run for (int i=0;i<5;i++) to get it right.

random.Next() is not optimal as as a tie retains prior order — paparazzo, Dec 14 '17 at 11:31

paparazzo · Answer 2 · 2017-12-14T14:08:21.927

-1

HashSet<int> hs = new HashSet<int>();
int next = random.next(45);
while(hs.Length <=5)
{
    if(!hs.Contains(array[next]))
        hs.Add(array[next]);
    next = random next(45);
}

edited Dec 14 '17 at 14:08

answered Dec 14 '17 at 11:36

paparazzo

44,497
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FYI: [Somebody flagged](https://stackoverflow.com/review/low-quality-posts/18253855) your answer and it went into the Low Quality Posts queue - presumably because your answer is just code without accompanying text to explain. – Wai Ha Lee Dec 14 '17 at 12:12
a) Does this code compile? b) What does `!hs.Contains(array[next])` achieve? Could it be removed? – mjwills Dec 14 '17 at 12:21
[*When to flag an answer as “not an answer”?*](https://meta.stackoverflow.com/q/265552/1364007) - directed at the flagger, not mjwills – Wai Ha Lee Dec 14 '17 at 12:38

score -1 · Answer 3 · answered Dec 14 '17 at 12:41

-1

Inspired by Jite's answer, I changed to use Guid to randomize the numbers

var randomized = numbers45.OrderBy(o => Guid.NewGuid().ToString());

Then you could take the draws by:

var draws = randomized.Take(5).ToArray;

answered Dec 14 '17 at 12:41

Evan Huang

1,245
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Fill an array with unique random integers

3 Answers3