Related
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.
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.
If you're on .NET 3.5, you can use the following IEnumerable coolness:
Random rnd=new Random();
string[] MyRandomArray = MyArray.OrderBy(x => rnd.Next()).ToArray();
Edit: and here's the corresponding VB.NET code:
Dim rnd As New System.Random
Dim MyRandomArray = MyArray.OrderBy(Function() rnd.Next()).ToArray()
Second edit, in response to remarks that System.Random "isn't threadsafe" and "only suitable for toy apps" due to returning a time-based sequence: as used in my example, Random() is perfectly thread-safe, unless you're allowing the routine in which you randomize the array to be re-entered, in which case you'll need something like lock (MyRandomArray) anyway in order not to corrupt your data, which will protect rnd as well.
Also, it should be well-understood that System.Random as a source of entropy isn't very strong. As noted in the MSDN documentation, you should use something derived from System.Security.Cryptography.RandomNumberGenerator if you're doing anything security-related. For example:
using System.Security.Cryptography;
...
RNGCryptoServiceProvider rnd = new RNGCryptoServiceProvider();
string[] MyRandomArray = MyArray.OrderBy(x => GetNextInt32(rnd)).ToArray();
...
static int GetNextInt32(RNGCryptoServiceProvider rnd)
{
byte[] randomInt = new byte[4];
rnd.GetBytes(randomInt);
return Convert.ToInt32(randomInt[0]);
}
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).
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.
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>();
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);
}
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.
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;
}
}
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.
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.
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);
}
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).
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.
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.
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;
}
}
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();
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];
}
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);
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;
}
how can I get two strings from two arrays randomly. I have something, but It doesn't pick randomly.
string[] firstNames = { "John", "Michael", "Victor", "Mark", "Alex", "Steven", "Zack", "Howard", "Xzavier", "Brendan", "Dustin", "Carl" };
string[] lastNames = { "Adams", "McGregor", "Piasta", "Semmel", "Laris", "Sever", "Bourque", "Percy", "Shanabarger", "Bobak", "Adair" };
string[] shuffled = firstNames.OrderBy(n => Guid.NewGuid()).ToArray();
string[] shuffled2 = firstNames.OrderBy(n => Guid.NewGuid()).ToArray();
Random firstn = new Random();
Random lastn = new Random();
int index = firstn.Next(firstNames.Length);
int index2 = lastn.Next(lastNames.Length);
Console.WriteLine($"The random name is {firstNames[index]}" + $" {lastNames[index2]}");
I am trying to make a random full name generator. I have tried this code multiple times and the results were: John Adams, Victor Piasta, and so on! And as you can see they are not random.
Looks pretty random to me..
With 150 iterations you still get a performance of roughly 90 of 132 possible combinations.
Testing code:
HashSet<string> names = new HashSet<string>(150);
Random rndm = new Random();
for (int i = 0; i < 150; i++)
{
names.Add($"{firstNames[rndm.Next(firstNames.Length)]} " +
$"{lastNames[rndm.Next(lastNames.Length)]}");
}
Console.WriteLine($"{names.Count} / {firstNames.Length * lastNames.Length}");
Console.ReadKey();
Don't forget that true random is not possible to simulate... so you will always have some kind of repetitive distribution with higher numbers.
Random works slightly different in .NET-Framework to .NET-Core
In .NET Framework, the default seed value is time-dependent. In .NET Core, the default seed value is produced by the thread-static, pseudo-random number generator.
From: https://learn.microsoft.com/en-us/dotnet/api/system.random?view=netcore-3.1#Instantiate
In the .NET-Framework only, on most Windows systems, Random objects created within 15 milliseconds of one another are likely to have identical seed values.
Use a single Random-instance on .NET-Framework to avoid this behaviour.
Just change to below so that you are using the same Random instance for both
int index2 = firstn.Next(lastNames.Length);
Random does not actually generate random numbers, but pseudo-random numbers. That means, you need a starting value, the so-called seed. From that value the first number is calculated. From that, one could calculate another number, and so on. These numbers look like random, but actually they are determined by the seed. If you have the same seed, you always get the same sequence of numbers.
Your Random class has actually a constructor where you can deliver a seed. You can try it out here: https://dotnetfiddle.net/0bHDNg For the same seed, you will always get the same sequence of numbers.
If you use the constructor without a parameter, the current time is taken as the seed. This has the big advantage, that you always get a different sequence of numbers when you restart the program. But in your example, you have a problem: If the time between creating the two instances of Random has not changed, because the computer is so fast, they will have both the same seed and thus both the same sequence of numbers. That's why index and index2 both have the same value. Just use the same instance of Random for generating both indices, and they don't have necessarily the same value:
Random random = new Random();
int index = random .Next(firstNames.Length);
int index2 = random .Next(lastNames.Length);
Console.WriteLine($"The random name is {firstNames[index]}" + $" {lastNames[index2]}");
I'm writing a trivia game player app in C#, where the user must pick 5 randomly selected trivia questions from a set of 20 available questions.
I use this code to generate an index that the app will use to fetch the questions:
private void GenerateRandomNumbers(int RandomIndex)
{
Random rand = new Random();
HashSet<int> check = new HashSet<int>();
for (int i = 1; i < RandomIndex; i++)
{
int curValue = rand.Next(1, RandomIndex);
while (check.Contains(curValue))
{
curValue = rand.Next(1, RandomIndex);
}
ChosenQuestions.Add(Questions[curValue]);
check.Add(curValue);
}
check.Clear();
}
The problem is that when the user plays the game a second time (by clicking the "PLAY AGAIN" button) and the above code is executed again, it keeps picking the same random numbers, though they are not necessarily in the same order. this causes the same trivia questions to be chosen every time. I want a fresh new set of totally random numbers every time the above code executes. How can I modify the code to accomplish this?
NOTE: I did copy this code from somewhere else here on Stack Overflow, so it's not my own original code.
Assuming RandomIndex is the number of values to pick. You want this:
rand.Next(1, RandomIndex);
to be
rand.Next(0, Questions.Length);
There is an off by one error in the loop as well. Let's just fix the whole thing:
private void GenerateRandomNumbers(int RandomIndex)
{
Random rand = new Random();
HashSet<int> check = new HashSet<int>();
for (int i = 0; i < RandomIndex; i++)
{
int curValue = rand.Next(0, Questions.Length);
while (check.Contains(curValue))
{
curValue = rand.Next(0, Questions.Length);
}
ChosenQuestions.Add(Questions[curValue]);
check.Add(curValue);
}
}
Also, if you call this in a tight loop, you will get the same values picked repeatedly. This because Random chooses a seed based on the system clock which only upadates every 15 ms or so.
I think that a shuffle would be better approach rather than checking whether indexes have already been picked.
You could seed RNG with different seeds
Random rand = new Random(newSeed);
Seed could be taken from current time, for example
Try using just one global Random object, and not creating a new Random() every time the user plays. This will ensure that isntead of initializing a new seed every time, the numbers will be a sequence created by one seed. This will make them closer to real randomness.
I'm working on Pong in C# w/ XNA.
I want to use a random number (within a range) to determine things such as whether or not the ball rebounds straight, or at an angle, and how fast the ball moves when it hits a paddle.
I want to know how to implement it.
Use the Random class. For example:
Random r = new Random();
int nextValue = r.Next(0, 100); // Returns a random number from 0-99
Unless you need cryptographically secure numbers, Random should be fine for you... but there are two gotchas to be aware of:
You shouldn't create a new instance every time you need one. If you create an instance without specifying a seed, it will use the current time as the seed - which means if you create several instances in quick succession, many of them will produce the same sequence of numbers. Typically you create a long-lasting instance of Random and reuse it.
It's not thread-safe. If you need to generate random numbers from multiple threads, you should think about having one instance per thread. Read this blog post for more information - but make sure you read the comments as well, as they have very useful information.
Random rnd = new Random();
rnd.Next(minValue, maxValue);
i.e.
rnd.Next(1,10);
Use the Random object's Next method that takes a min and max and returns a value in that range:
var random = new Random();
int randomNum = random.Next(min, max);
While you can use the Random class like all the other are suggesting, the Random class only uses psuedo-random number generation. The RandomNumberGenerator, which can be found in the System.Security.Cryptography namespace, creates actual random numbers.
How To Use:
RandomNumberGenerator rng = RandomNumberGenerator.Create();
byte[] rand = new byte[25]; //Set the length of this array to
// the number of random numbers you want
rng.GetBytes(rand);
More Info: http://msdn.microsoft.com/en-us/library/system.security.cryptography.randomnumbergenerator(v=VS.80).aspx
Here is my random generator
private static Random rnd = new Random(Environment.TickCount);
private int RandomNum(int Lower, int Upper)
{
return rnd.Next(Lower, Upper);//MyRandomNumber;
}
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.
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.
If you're on .NET 3.5, you can use the following IEnumerable coolness:
Random rnd=new Random();
string[] MyRandomArray = MyArray.OrderBy(x => rnd.Next()).ToArray();
Edit: and here's the corresponding VB.NET code:
Dim rnd As New System.Random
Dim MyRandomArray = MyArray.OrderBy(Function() rnd.Next()).ToArray()
Second edit, in response to remarks that System.Random "isn't threadsafe" and "only suitable for toy apps" due to returning a time-based sequence: as used in my example, Random() is perfectly thread-safe, unless you're allowing the routine in which you randomize the array to be re-entered, in which case you'll need something like lock (MyRandomArray) anyway in order not to corrupt your data, which will protect rnd as well.
Also, it should be well-understood that System.Random as a source of entropy isn't very strong. As noted in the MSDN documentation, you should use something derived from System.Security.Cryptography.RandomNumberGenerator if you're doing anything security-related. For example:
using System.Security.Cryptography;
...
RNGCryptoServiceProvider rnd = new RNGCryptoServiceProvider();
string[] MyRandomArray = MyArray.OrderBy(x => GetNextInt32(rnd)).ToArray();
...
static int GetNextInt32(RNGCryptoServiceProvider rnd)
{
byte[] randomInt = new byte[4];
rnd.GetBytes(randomInt);
return Convert.ToInt32(randomInt[0]);
}
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).
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.
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>();
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);
}
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.
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;
}
}
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.
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.
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);
}
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).
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.
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.
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;
}
}
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();
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];
}
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);
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;
}