How do I generate a random int number

Question

How do I generate a random integer in C

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I ve tried all of these solutions excluding the COBOL answer    lol  None of these solutions were good enough  I needed randoms in a fast for int loop and I was getting tons of duplicate values even in very wide ranges  After settling for kind of random results far too long I decided to finally tackle this problem once and for all    It s all about the seed    I create a random integer by parsing out the non-digits from Guid  then I use that to instantiate my Random class   public int GenerateRandom int min  int max        var seed   Convert ToInt32 Regex Match Guid NewGuid   ToString       d    Value       return new Random seed  Next min  max       Update  Seeding isn t necessary if you instantiate the Random class once  So it d be best to create a static class and call a method off that   public static class IntUtil      private static Random random      private static void Init              if  random    null  random   new Random             public static int Random int min  int max             Init          return random Next min  max            Then you can use the static class like so    for var i   0  i  lt  1000  i         int randomNumber   IntUtil Random 1 100      Console WriteLine randomNumber        I admit I like this approach better

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Random random   new Random     int randomNumber   random Next  lowerBound upperBound

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I used below code to have a random number  not recommended    var random   new Random  int DateTime Now Ticks   var randomValue   random Next startValue  endValue   1

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Every time you do new Random   it is initialized   This means that in a tight loop you get the same value lots of times  You should keep a single Random instance and keep using Next on the same instance     Function to get random number private static readonly Random getrandom   new Random     public static int GetRandomNumber int min  int max        lock getrandom     synchronize               return getrandom Next min  max

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I wanted to demonstrate what happens when a new random generator is used every time  Suppose you have two methods or two classes each requiring a random number  And naively you code them like   public class A       public A                 var rnd new Random            ID rnd Next              public int ID   get  private set      public class B       public B                 var rnd new Random            ID rnd Next              public int ID   get  private set        Do you think you will get two different IDs  NOPE  class Program       static void Main string   args                A a new A            B b new B             int ida a ID  idb b ID             ida   1452879101            idb   1452879101           The solution is to always use a single static random generator  Like this   public static class Utils       public static readonly Random random new Random       public class A       public A                 ID Utils random Next              public int ID   get  private set      public class B       public B                 ID Utils random Next              public int ID   get  private set

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The easiest way is probably just Random range 1  3  This would generate a number between 1 and 2

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int n   new Random   Next      You can also give minimum and maximum value to Next   function  Like    int n   new Random   Next 5  10

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While this is okay   Random random   new Random    int randomNumber   random Next     You d want to control the limit  min and max mumbers  most of the time  So you need to specify where the random number starts and ends   The Next   method accepts two parameters  min and max   So if i want my random number to be between say 5 and 15  I d just do  int randomNumber   random Next 5  16

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I always have methods that generate random numbers which help for various purposes  I hope this may help you too   public class RandomGenerator           public int RandomNumber int min  int max                    var random   new Random              return random Next min  max                  public string RandomString int size  bool lowerCase                    var builder   new StringBuilder              var random    new Random              char ch             for  int i   0  i  lt  size  i                              ch   Convert ToChar Convert ToInt32 Math Floor 26   random NextDouble     65                   builder Append ch                          if  lowerCase                return builder ToString   ToLower              return builder ToString

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The numbers generated by the inbuilt Random class  System Random  generates pseudo random numbers   If you want true random numbers  the closest we can get is  secure Pseudo Random Generator  which can be generated by using the Cryptographic classes in C  such as RNGCryptoServiceProvider   Even so  if you still need true random numbers you will need to use an external source such as devices accounting for radioactive decay as a seed for an random number generator  Since  by definition  any number generated by purely algorithmic means cannot be truly random

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Try these simple steps to create random numbers   Create function   private int randomnumber int min  int max        Random rnum   new Random        return rnum Next min  max       Use the above function in a location where you want to use random numbers  Suppose you want to use it in a text box   textBox1 Text   randomnumber 0  999  ToString      0 is min and 999 is max  You can change the values to whatever you want

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Just as a note for future reference   If you re using  NET Core  multiple Random instances isn t as dangerous as before  I m aware that this question is from 2010  but since this question is old but has some attraction  I think it s a good thing to document the change   You may refer to this question I made a while back   Did Microsoft change Random default seed   Basically  they have changed the default seed from Environment TickCount to Guid NewGuid   GetHashCode    so if you create 2 instances of Random it won t display the same numbers   You can see the file diffs from  NET Framework  NET Core  2 0 0   here  https   github com dotnet coreclr pull 2192 commits 9f6a0b675e5ac0065a268554de49162c539ff66d  It isn t as safe as RNGCryptoServiceProvider  but at least it won t give you weird results

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You can try with random seed value using below   var rnd   new Random 11111111     note  seed value is 11111111  string randomDigits   rnd Next     var requestNumber     SD- randomDigits

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Beware that new Random   is seeded on current timestamp   If you want to generate just one number you can use   new Random   Next  int MinValue  int MaxValue    For more information  look at the Random class  though please note      However  because the clock has finite resolution  using the parameterless constructor to create different Random objects in close succession creates random number generators that produce identical sequences of random numbers   So do not use this code to generate a series of random number

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I will assume that you want a uniformly distributed random number generator like below  Random number in most of programming language including C  and C   is not properly shuffled before using them  This means that you will get the same number over and over  which isn t really random   To avoid to draw the same number over and over  you need a seed  Typically  ticks in time is ok for this task  Remember that you will get the same number over and over if you are using the same seed every time  So try to use varying seed always  Time is a good source for seed because they chage always   int GetRandomNumber int min  int max        Random rand   new Random  int DateTime Now Ticks       return rand Next min  max       if you are looking for random number generator for normal distribution  you might use a Box-Muller transformation  Check the answer by yoyoyoyosef in Random Gaussian Variable Question  Since you want integer  you have to cast double value to integer at the end    Random rand   new Random      reuse this if you are generating many double u1   1 0-rand NextDouble      uniform 0 1  random doubles double u2   1 0-rand NextDouble    double randStdNormal   Math Sqrt -2 0   Math Log u1              Math Sin 2 0   Math PI   u2     random normal 0 1  double randNormal            mean   stdDev   randStdNormal    random normal mean stdDev 2    Random Gaussian Variables

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Random rand   new Random    int name   rand Next     Put whatever values you want in the second parentheses make sure you have set a name by writing prop and double tab to generate the code

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Quick and easy for inline   use bellow code   new Random   Next min  max       for example unique name strName          new Random   Next 100  999

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Sorry  OP indeed requires a random int value  but for the simple purpose to share knowledge if you want a random BigInteger value you can use following statement   BigInteger randomVal   BigInteger Abs BigInteger Parse Guid NewGuid   ToString   Replace  -       NumberStyles AllowHexSpecifier

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Use one instance of Random repeatedly     Somewhat better code    Random rng   new Random    for  int i   0  i  lt  100  i          Console WriteLine GenerateDigit rng          static int GenerateDigit Random rng           Assume there d be more logic here really     return rng Next 10       This article takes a look at why randomness causes so many problems  and how to address them  http   csharpindepth com Articles Chapter12 Random aspx

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For strong random seed I always use CryptoRNG and not Time   using System  using System Security Cryptography   public class Program       public static void Main                 var random   new Random GetSeed             Console WriteLine random Next                public static int GetSeed                  using  var rng   new RNGCryptoServiceProvider                          var intBytes   new byte 4               rng GetBytes intBytes               return BitConverter ToInt32 intBytes  0

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Numbers calculated by a computer through a deterministic process  cannot  by definition  be random   If you want a genuine random numbers  the randomness comes from atmospheric noise or radioactive decay   You can try for example RANDOM ORG  it reduces performance

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You could use Jon Skeet s StaticRandom method inside the MiscUtil class library that he built for a pseudo-random number    using MiscUtil       for  int i   0  i  lt  100       Console WriteLine StaticRandom Next

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create a Random object  Random rand   new Random      and use it  int randomNumber   rand Next min  max     you don t have to initialize new Random   every time you need a random number  initiate one Random then use it as many times as you need inside a loop or whatever

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I wanted to add a cryptographically secure version    RNGCryptoServiceProvider Class  MSDN or dotnetperls   It implements IDisposable   using  RNGCryptoServiceProvider rng   new RNGCryptoServiceProvider         byte   randomNumber   new byte 4    4 for int32    rng GetBytes randomNumber      int value   BitConverter ToInt32 randomNumber  0

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Why not use int randomNumber   Random Range start range  end range

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Modified answer from here   If you have access to an Intel Secure Key compatible CPU  you can generate real random numbers and strings using these libraries  https   github com JebteK RdRand and https   www rdrand com   Just download the latest version from here  include Jebtek RdRand and add a using statement for it  Then  all you need to do is this      Check to see if this is a compatible CPU bool isAvailable   RdRandom GeneratorAvailable        Generate 10 random characters string key         RdRandom GenerateKey 10        Generate 64 random characters  useful for API keys  string apiKey      RdRandom GenerateAPIKey        Generate an array of 10 random bytes byte   b           RdRandom GenerateBytes 10       Generate a random unsigned int uint i             RdRandom GenerateUnsignedInt      If you don t have a compatible CPU to execute the code on  just use the RESTful services at rdrand com  With the RdRandom wrapper library included in your project  you would just need to do this  you get 1000 free calls when you signup    string ret   Randomizer GenerateKey  lt length gt     lt key gt     uint ret     Randomizer GenerateUInt   lt key gt     byte   ret   Randomizer GenerateBytes  lt length gt     lt key gt

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The question looks very simple but the answer is  bit complicated  If you see almost everyone has suggested to use the Random class and some have suggested to use the RNG crypto class  But then when to choose what  For that we need to first understand the term RANDOMNESS and the philosophy behind it  I would encourage you to watch this video which goes in depth in the philosophy of RANDOMNESS using C  https   www youtube com watch v tCYxc-2-3fY First thing let us understand the philosophy of RANDOMNESS   When we tell a person to choose between RED   GREEN and YELLOW  what happens internally  What makes a person choose RED or YELLOW or GREEN   Some initial thought goes into the persons mind which decides his choice  it can be favorite color   lucky color and so on  In other words some initial trigger which we term in RANDOM as SEED This SEED is the beginning point  the trigger which instigates him to select the RANDOM value  Now if a SEED is easy to guess then those kind of random numbers are termed as PSEUDO and when a seed is difficult to guess those random numbers are termed  SECURED random numbers  For example a person chooses is color depending on weather and sound combination then  it would be difficult to guess the initial seed   Now let me make an important statement -     Random    class generates only PSEUDO random number and to generate SECURE random number we need to use    RNGCryptoServiceProvider    class   Random class takes seed values from your CPU clock which is very much predictable   So in other words RANDOM class of C  generates pseudo random numbers   below is the code for the same  var random   new Random    int randomnumber   random Next    While the RNGCryptoServiceProvider class uses OS entropy to generate seeds  OS entropy is a random value which is generated using sound  mouse click  and keyboard timings  thermal temp etc  Below goes the code for the same  using  RNGCryptoServiceProvider rg   new RNGCryptoServiceProvider            byte   rno   new byte 5           rg GetBytes rno           int randomvalue   BitConverter ToInt32 rno  0       To understand OS entropy see this video from 14 30 https   www youtube com watch v tCYxc-2-3fY where the logic of OS entropy is explained  So putting in simple words RNG Crypto generates SECURE random numbers

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If you want a CSRNG to generate random numbers between a min and max  this is for you  It will initialize Random classes with secure random seeds       class SecureRandom   Random               public static byte   GetBytes ulong length                        RNGCryptoServiceProvider RNG   new RNGCryptoServiceProvider                byte   bytes   new byte length               RNG GetBytes bytes               RNG Dispose                return bytes                    public SecureRandom     base BitConverter ToInt32 GetBytes 4   0                                public int GetRandomInt int min  int max                        int treashold   max - min              if treashold    Math Abs treashold                                 throw new ArithmeticException  The minimum value can t exceed the maximum value                               if  treashold    0                                throw new ArithmeticException  The minimum value can t be the same as the maximum value                               return min    Next     treashold                     public static int GetRandomIntStatic int min  int max                        int treashold   max - min              if  treashold    Math Abs treashold                                 throw new ArithmeticException  The minimum value can t exceed the maximum value                               if treashold    0                                throw new ArithmeticException  The minimum value can t be the same as the maximum value                               return min    BitConverter ToInt32 GetBytes 4   0    treashold

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The Random class is used to create random numbers   Pseudo-random that is of course     Example   Random rnd   new Random    int month    rnd Next 1  13       creates a number between 1 and 12 int dice     rnd Next 1  7        creates a number between 1 and 6 int card     rnd Next 52          creates a number between 0 and 51   If you are going to create more than one random number  you should keep the Random instance and reuse it  If you create new instances too close in time  they will produce the same series of random numbers as the random generator is seeded from the system clock

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There are a number utility functions or services that are better cached in the same way that System Random should be  so it lends itself to a generic implementation  static public class CachedService lt T gt  where T   new         static public T Get   get      new T       To use for random  or similar   CachedService lt System Random gt  Get Next 999

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This is the class I use  Works like RandomNumber GenerateRandom 1  666   internal static class RandomNumber       private static Random r   new Random        private static object l   new object        private static Random globalRandom   new Random         ThreadStatic      private static Random localRandom      public static int GenerateNewRandom int min  int max                return new Random   Next min  max             public static int GenerateLockedRandom int min  int max                int result          lock  RandomNumber l                        result   RandomNumber r Next min  max                     return result            public static int GenerateRandom int min  int max                Random random   RandomNumber localRandom          if  random    null                        int seed              lock  RandomNumber globalRandom                                seed   RandomNumber globalRandom Next                              random    RandomNumber localRandom   new Random seed                      return random Next min  max

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Random r   new Random    int n   r Next

[c#] How do I generate a random int number?

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