Best way to randomize an array with .NET

Question

What is the best way to randomize an array of strings with .NET? My array contains about 500 strings and I'd like to create a new Array with the same strings but in a random order.

Please include a C# example in your answer.

Answer 1

The following implementation uses the Fisher-Yates algorithm AKA the Knuth Shuffle. It runs in O(n) time and shuffles in place, so is better performing than the 'sort by random' technique, although it is more lines of code. See here for some comparative performance measurements. I have used System.Random, which is fine for non-cryptographic purposes.*

static class RandomExtensions
{
    public static void Shuffle<T> (this Random rng, T[] array)
    {
        int n = array.Length;
        while (n > 1) 
        {
            int k = rng.Next(n--);
            T temp = array[n];
            array[n] = array[k];
            array[k] = temp;
        }
    }
}

Usage:

var array = new int[] {1, 2, 3, 4};
var rng = new Random();
rng.Shuffle(array);
rng.Shuffle(array); // different order from first call to Shuffle

* For longer arrays, in order to make the (extremely large) number of permutations equally probable it would be necessary to run a pseudo-random number generator (PRNG) through many iterations for each swap to produce enough entropy. For a 500-element array only a very small fraction of the possible 500! permutations will be possible to obtain using a PRNG. Nevertheless, the Fisher-Yates algorithm is unbiased and therefore the shuffle will be as good as the RNG you use.

Answer 2

First, you should use the most upvoted answer to this question so far, which is https://stackoverflow.com/a/110570/1757994.

You can shuffle using LINQ.

var myShuffledArray = myArray.Shuffle().

Add the following code as a file called EnumerableExtensions.cs to your project:

public static class EnumerableExtensions
{
    
    public static IList<T> Shuffle<T>(this IEnumerable<T> sequence)
    {
        return sequence.Shuffle(new Random());
    }

    public static IList<T> Shuffle<T>(this IEnumerable<T> sequence, Random randomNumberGenerator)
    {
        if (sequence == null)
        {
            throw new ArgumentNullException("sequence");
        }

        if (randomNumberGenerator == null)
        {
            throw new ArgumentNullException("randomNumberGenerator");
        }

        T swapTemp;
        List<T> values = sequence.ToList();
        int currentlySelecting = values.Count;
        while (currentlySelecting > 1)
        {
            int selectedElement = randomNumberGenerator.Next(currentlySelecting);
            --currentlySelecting;
            if (currentlySelecting != selectedElement)
            {
                swapTemp = values[currentlySelecting];
                values[currentlySelecting] = values[selectedElement];
                values[selectedElement] = swapTemp;
            }
        }

        return values;
    }
}

Adapted from https://github.com/microsoft/driver-utilities/blob/master/src/Sarif.Driver/EnumerableExtensions.cs

It is a bad idea to use informally authored shuffle algorithms, since they are hard to analyze for flaws.

If you're using C# 7.0 or higher, this other approach is slow for many elements but works correctly in all environments C# is used:

var rnd = new Random();
var myRandomArray = myArray
 .Select(x => (x, rnd.Next()))
 .OrderBy(tuple => tuple.Item2)
 .Select(tuple => tuple.Item1)
 .ToArray();    

This is almost as verbose as the Fisher-Yates shuffle, which is much faster for large numbers of elements. This creates random numbers for each element, then sorts by the random number.

This answer used to say something along the lines of .OrderBy(x => random()).

Experienced developers will correctly know that .OrderBy(x => random()) alone is wrong. So if you commit code that contains it, you will look bad.

Why is it wrong? OrderBy expects the keySelector, the function that it is passed, to be pure. That means the keySelector should return the same answer every time it is called and cause no side effects.

C# implementations like Unity's do not cache the result of the selector passed to OrderBy. Games tend to shuffle things. It is hard to generalize across all CLR implementations, so it's better to just use the correct thing. In order to not look bad with code you author, you should use Fisher-Yates instead.

For Visual Basic and versions of C# earlier than 7.0, use Fisher-Yates. A correct implementation using only LINQ is more verbose than implementing Fisher-Yates, so you will have written more code to do a worse implementation.

Answer 3

You're looking for a shuffling algorithm, right?

Okay, there are two ways to do this: the clever-but-people-always-seem-to-misunderstand-it-and-get-it-wrong-so-maybe-its-not-that-clever-after-all way, and the dumb-as-rocks-but-who-cares-because-it-works way.

Dumb way

• Create a duplicate of your first array, but tag each string should with a random number.
• Sort the duplicate array with respect to the random number.

This algorithm works well, but make sure that your random number generator is unlikely to tag two strings with the same number. Because of the so-called Birthday Paradox, this happens more often than you might expect. Its time complexity is O(n log n).

Clever way

I'll describe this as a recursive algorithm:

To shuffle an array of size n (indices in the range [0..n-1]):

if n = 0

• do nothing

if n > 0

• (recursive step) shuffle the first n-1 elements of the array
• choose a random index, x, in the range [0..n-1]
• swap the element at index n-1 with the element at index x

The iterative equivalent is to walk an iterator through the array, swapping with random elements as you go along, but notice that you cannot swap with an element after the one that the iterator points to. This is a very common mistake, and leads to a biased shuffle.

Time complexity is O(n).

Answer 4

This algorithm is simple but not efficient, O(N²). All the "order by" algorithms are typically O(N log N). It probably doesn't make a difference below hundreds of thousands of elements but it would for large lists.

var stringlist = ... // add your values to stringlist

var r = new Random();

var res = new List<string>(stringlist.Count);

while (stringlist.Count >0)
{
   var i = r.Next(stringlist.Count);
   res.Add(stringlist[i]);
   stringlist.RemoveAt(i);
}

The reason why it's O(N²) is subtle: List.RemoveAt() is a O(N) operation unless you remove in order from the end.

Answer 5

You can also make an extention method out of Matt Howells. Example.

   namespace System
    {
        public static class MSSystemExtenstions
        {
            private static Random rng = new Random();
            public static void Shuffle<T>(this T[] array)
            {
                rng = new Random();
                int n = array.Length;
                while (n > 1)
                {
                    int k = rng.Next(n);
                    n--;
                    T temp = array[n];
                    array[n] = array[k];
                    array[k] = temp;
                }
            }
        }
    }

Then you can just use it like:

        string[] names = new string[] {
                "Aaron Moline1", 
                "Aaron Moline2", 
                "Aaron Moline3", 
                "Aaron Moline4", 
                "Aaron Moline5", 
                "Aaron Moline6", 
                "Aaron Moline7", 
                "Aaron Moline8", 
                "Aaron Moline9", 
            };
        names.Shuffle<string>();

Answer 6

Just thinking off the top of my head, you could do this:

public string[] Randomize(string[] input)
{
  List<string> inputList = input.ToList();
  string[] output = new string[input.Length];
  Random randomizer = new Random();
  int i = 0;

  while (inputList.Count > 0)
  {
    int index = r.Next(inputList.Count);
    output[i++] = inputList[index];
    inputList.RemoveAt(index);
  }

  return (output);
}

Answer 7

Randomizing the array is intensive as you have to shift around a bunch of strings. Why not just randomly read from the array? In the worst case you could even create a wrapper class with a getNextString(). If you really do need to create a random array then you could do something like

for i = 0 -> i= array.length * 5
   swap two strings in random places

The *5 is arbitrary.

Answer 8

 public static void Shuffle(object[] arr)
    {
        Random rand = new Random();
        for (int i = arr.Length - 1; i >= 1; i--)
        {
            int j = rand.Next(i + 1);
            object tmp = arr[j];
            arr[j] = arr[i];
            arr[i] = tmp;
        }
    }

Answer 9

Generate an array of random floats or ints of the same length. Sort that array, and do corresponding swaps on your target array.

This yields a truly independent sort.

Answer 10

Ok, this is clearly a bump from my side (apologizes...), but I often use a quite general and cryptographically strong method.

public static class EnumerableExtensions
{
    static readonly RNGCryptoServiceProvider RngCryptoServiceProvider = new RNGCryptoServiceProvider();
    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> enumerable)
    {
        var randomIntegerBuffer = new byte[4];
        Func<int> rand = () =>
                             {
                                 RngCryptoServiceProvider.GetBytes(randomIntegerBuffer);
                                 return BitConverter.ToInt32(randomIntegerBuffer, 0);
                             };
        return from item in enumerable
               let rec = new {item, rnd = rand()}
               orderby rec.rnd
               select rec.item;
    }
}

Shuffle() is an extension on any IEnumerable so getting, say, numbers from 0 to 1000 in random order in a list can be done with

Enumerable.Range(0,1000).Shuffle().ToList()

This method also wont give any surprises when it comes to sorting, since the sort value is generated and remembered exactly once per element in the sequence.

Answer 11

Random r = new Random();
List<string> list = new List(originalArray);
List<string> randomStrings = new List();

while(list.Count > 0)
{
int i = r.Random(list.Count);
randomStrings.Add(list[i]);
list.RemoveAt(i);
}

Answer 12

Jacco, your solution ising a custom IComparer isn't safe. The Sort routines require the comparer to conform to several requirements in order to function properly. First among them is consistency. If the comparer is called on the same pair of objects, it must always return the same result. (the comparison must also be transitive).

Failure to meet these requirements can cause any number of problems in the sorting routine including the possibility of an infinite loop.

Regarding the solutions that associate a random numeric value with each entry and then sort by that value, these are lead to an inherent bias in the output because any time two entries are assigned the same numeric value, the randomness of the output will be compromised. (In a "stable" sort routine, whichever is first in the input will be first in the output. Array.Sort doesn't happen to be stable, but there is still a bias based on the partitioning done by the Quicksort algorithm).

You need to do some thinking about what level of randomness you require. If you are running a poker site where you need cryptographic levels of randomness to protect against a determined attacker you have very different requirements from someone who just wants to randomize a song playlist.

For song-list shuffling, there's no problem using a seeded PRNG (like System.Random). For a poker site, it's not even an option and you need to think about the problem a lot harder than anyone is going to do for you on stackoverflow. (using a cryptographic RNG is only the beginning, you need to ensure that your algorithm doesn't introduce a bias, that you have sufficient sources of entropy, and that you don't expose any internal state that would compromise subsequent randomness).

Answer 13

This post has already been pretty well answered - use a Durstenfeld implementation of the Fisher-Yates shuffle for a fast and unbiased result. There have even been some implementations posted, though I note some are actually incorrect.

I wrote a couple of posts a while back about implementing full and partial shuffles using this technique, and (this second link is where I'm hoping to add value) also a follow-up post about how to check whether your implementation is unbiased, which can be used to check any shuffle algorithm. You can see at the end of the second post the effect of a simple mistake in the random number selection can make.

Answer 14

You don't need complicated algorithms.

Just one simple line:

Random random = new Random();
array.ToList().Sort((x, y) => random.Next(-1, 1)).ToArray();

Note that we need to convert the Array to a List first, if you don't use List in the first place.

Also, mind that this is not efficient for very large arrays! Otherwise it's clean & simple.

Answer 15

This is a complete working Console solution based on the example provided in here:

class Program
{
    static string[] words1 = new string[] { "brown", "jumped", "the", "fox", "quick" };

    static void Main()
    {
        var result = Shuffle(words1);
        foreach (var i in result)
        {
            Console.Write(i + " ");
        }
        Console.ReadKey();
    }

   static string[] Shuffle(string[] wordArray) {
        Random random = new Random();
        for (int i = wordArray.Length - 1; i > 0; i--)
        {
            int swapIndex = random.Next(i + 1);
            string temp = wordArray[i];
            wordArray[i] = wordArray[swapIndex];
            wordArray[swapIndex] = temp;
        }
        return wordArray;
    }         
}

Answer 16

        int[] numbers = {0,1,2,3,4,5,6,7,8,9};
        List<int> numList = new List<int>();
        numList.AddRange(numbers);

        Console.WriteLine("Original Order");
        for (int i = 0; i < numList.Count; i++)
        {
            Console.Write(String.Format("{0} ",numList[i]));
        }

        Random random = new Random();
        Console.WriteLine("\n\nRandom Order");
        for (int i = 0; i < numList.Capacity; i++)
        {
            int randomIndex = random.Next(numList.Count);
            Console.Write(String.Format("{0} ", numList[randomIndex]));
            numList.RemoveAt(randomIndex);
        }
        Console.ReadLine();

Answer 17

Could be:

Random random = new();

string RandomWord()
{
    const string CHARS = "abcdefghijklmnoprstuvwxyz";
    int n = random.Next(CHARS.Length);
    return string.Join("", CHARS.OrderBy(x => random.Next()).ToArray())[0..n];
}

Answer 18

Although Random's Next() method is much faster, you can also use this technique:

string[] myArray = <insert code that defines your array>;
var newArray = myArray.OrderBy(x => Guid.NewGuid()).ToArray();

There might be some odd reason why you'd use a GUID rather than use an Int for randomization... I think most practical purposes would rule out this solution, but added since it's a technically valid approach.

Answer 19

private ArrayList ShuffleArrayList(ArrayList source)
{
    ArrayList sortedList = new ArrayList();
    Random generator = new Random();

    while (source.Count > 0)
    {
        int position = generator.Next(source.Count);
        sortedList.Add(source[position]);
        source.RemoveAt(position);
    }  
    return sortedList;
}

Answer 20

Here's a simple way using OLINQ:

// Input array
List<String> lst = new List<string>();
for (int i = 0; i < 500; i += 1) lst.Add(i.ToString());

// Output array
List<String> lstRandom = new List<string>();

// Randomize
Random rnd = new Random();
lstRandom.AddRange(from s in lst orderby rnd.Next(100) select s);