Random integer in VB NET

Question

I need to generate a random integer between 1 and n  where n is a positive whole number  to use for a unit test  I don t need something overly complicated to ensure true randomness - just an old-fashioned random number   How would I do that

User · Accepted Answer

To get a random integer value between 1 and N  inclusive  you can use the following   CInt Math Ceiling Rnd     n     1

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Public Function RandomNumber ByVal n As Integer  As Integer      initialize random number generator     Dim r As New Random System DateTime Now Millisecond      Return r Next 1  n  End Function

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All the answers so far have problems or bugs  plural  not just one   I will explain  But first I want to compliment Dan Tao s insight to use a static variable to remember the Generator variable so calling it multiple times will not repeat the same   over and over  plus he gave a very nice explanation  But his code suffered the same flaw that most others have  as i explain now   MS made their Next   method rather odd  the Min parameter is the inclusive minimum as one would expect  but the Max parameter is the exclusive maximum as one would NOT expect  in other words  if you pass min 1 and max 5 then your random numbers would be any of 1  2  3  or 4  but it would never include 5  This is the first of two potential bugs in all code that uses Microsoft s Random Next   method   For a simple answer  but still with other possible but rare problems  then you d need to use   Private Function GenRandomInt min As Int32  max As Int32  As Int32     Static staticRandomGenerator As New System Random     Return staticRandomGenerator Next min  max   1  End Function    I like to use Int32 rather than Integer because it makes it more clear how big the int is  plus it is shorter to type  but suit yourself    I see two potential problems with this method  but it will be suitable  and correct  for most uses  So if you want a simple solution  i believe this is correct   The only 2 problems i see with this function is  1  when Max   Int32 MaxValue so adding 1 creates a numeric overflow  altho  this would be rare  it is still a possibility  2  when min   max   1  when min   10 and max   5 then the Next function throws an error  this may be what you want  but it may not be either  or consider when min   5 and max   4  by adding 1  5 is passed to the Next method  but it does not throw an error  when it really is an error  but Microsoft  NET code that i tested returns 5  so it really is not an  exclusive  max when the max   the min  but when max  lt  min for the Random Next   function  then it throws an ArgumentOutOfRangeException  so Microsoft s implementation is really inconsistent and buggy too in this regard   you may want to simply swap the numbers when min   max so no error is thrown  but it totally depends on what is desired  if you want an error on invalid values  then it is probably better to also throw the error when Microsoft s exclusive maximum  max   1  in our code equals minimum  where MS fails to error in this case   handling a work-around for when max   Int32 MaxValue is a little inconvenient  but i expect to post a thorough function which handles both these situations  and if you want different behavior than how i coded it  suit yourself  but be aware of these 2 issues   Happy coding   Edit  So i needed a random integer generator  and i decided to code it  right   So if anyone wants the full functionality  here s one that actually works   But it doesn t win the simplest prize with only 2 lines of code  But it s not really complex either         lt summary gt      Generates a random Integer with any  inclusive  minimum or  inclusive  maximum values  with full range of Int32 values       lt  summary gt       lt param name  inMin  gt Inclusive Minimum value  Lowest possible return value  lt  param gt       lt param name  inMax  gt Inclusive Maximum value  Highest possible return value  lt  param gt       lt returns gt  lt  returns gt       lt remarks gt  lt  remarks gt  Private Function GenRandomInt inMin As Int32  inMax As Int32  As Int32     Static staticRandomGenerator As New System Random     If inMin  gt  inMax Then Dim t   inMin   inMin   inMax   inMax   t     If inMax  lt  Int32 MaxValue Then Return staticRandomGenerator Next inMin  inMax   1        now max   Int32 MaxValue  so we need to work around Microsoft s quirk of an exclusive max parameter      If inMin  gt  Int32 MinValue Then Return staticRandomGenerator Next inMin - 1  inMax    1   okay  this was the easy one        now min and max give full range of integer  but Random Next   does not give us an option for the full range of integer        so we need to use Random NextBytes   to give us 4 random bytes  then convert that to our random int      Dim bytes 3  As Byte   4 bytes  0 to 3     staticRandomGenerator NextBytes bytes    4 random bytes     Return BitConverter ToInt32 bytes  0    return bytes converted to a random Int32 End Function

User · Answer

You should create a pseudo-random number generator only once   Dim Generator As System Random   New System Random     Then  if an integer suffices for your needs  you can use   Public Function GetRandom myGenerator As System Random  ByVal Min As Integer  ByVal Max As Integer  As Integer  min is inclusive  max is exclusive  dah   Return myGenerator Next Min  Max   1  End Function   as many times as you like  Using the wrapper function is justified only because the maximum value is exclusive - I know that the random numbers work this way but the definition of  Next is confusing   Creating a generator every time you need a number is in my opinion wrong  the pseudo-random numbers do not work this way   First  you get the problem with initialization which has been discussed in the other replies  If you initialize once  you do not have this problem   Second  I am not at all certain that you get a valid sequence of random numbers  rather  you get a collection of the first number of multiple different sequences which are seeded automatically based on computer time  I am not certain that these numbers will pass the tests that confirm the randomness of the sequence

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Use System Random   Dim MyMin As Integer   1  MyMax As Integer   5  My1stRandomNumber As Integer  My2ndRandomNumber As Integer    Create a random number generator Dim Generator As System Random   New System Random      Get a random number  gt   MyMin and  lt   MyMax My1stRandomNumber   Generator Next MyMin  MyMax   1    Note  Next function returns numbers  less than  max  so pass in max   1 to include max as a possible value    Get another random number  don t create a new generator  use the same one  My2ndRandomNumber   Generator Next MyMin  MyMax   1

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Function xrand   As Long         Dim r1 As Long   Now Day  amp  Now Month  amp  Now Year  amp  Now Hour  amp  Now Minute  amp  Now Second  amp  Now Millisecond         Dim RAND As Long   Math Max r1  r1   2          Return RAND End Function    BBOYSE  This its the best way  from scratch  P

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If you are using Joseph s answer which is a great answer  and you run these back to back like this   dim i   GetRandom 1  1715  dim o   GetRandom 1  1715    Then the result could come back the same over and over because it processes the call so quickly   This may not have been an issue in  08  but since the processors are much faster today  the function doesn t allow the system clock enough time to change prior to making the second call     Since the System Random   function is based on the system clock  we need to allow enough time for it to change prior to the next call   One way of accomplishing this is to pause the current thread for 1 millisecond   See example below   Public Function GetRandom ByVal min as Integer  ByVal max as Integer  as Integer     Static staticRandomGenerator As New System Random     max    1     Return staticRandomGenerator Next If min  gt  max  max  min   If min  gt  max  min  max   End Function

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Dim rnd As Random   New Random rnd Next n

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As has been pointed out many times  the suggestion to write code like this is problematic   Public Function GetRandom ByVal Min As Integer  ByVal Max As Integer  As Integer     Dim Generator As System Random   New System Random       Return Generator Next Min  Max  End Function   The reason is that the constructor for the Random class provides a default seed based on the system s clock  On most systems  this has limited granularity -- somewhere in the vicinity of 20 ms  So if you write the following code  you re going to get the same number a bunch of times in a row   Dim randoms 1000  As Integer For i As Integer   0 to randoms Length - 1     randoms i    GetRandom 1  100  Next   The following code addresses this issue   Public Function GetRandom ByVal Min As Integer  ByVal Max As Integer  As Integer       by making Generator static  we preserve the same instance          i e   do not create new instances with the same seed over and over          between calls       Static Generator As System Random   New System Random       Return Generator Next Min  Max  End Function   I threw together a simple program using both methods to generate 25 random integers between 1 and 100  Here s the output   Non-static  70 Static  70 Non-static  70 Static  46 Non-static  70 Static  58 Non-static  70 Static  19 Non-static  70 Static  79 Non-static  70 Static  24 Non-static  70 Static  14 Non-static  70 Static  46 Non-static  70 Static  82 Non-static  70 Static  31 Non-static  70 Static  25 Non-static  70 Static  8 Non-static  70 Static  76 Non-static  70 Static  74 Non-static  70 Static  84 Non-static  70 Static  39 Non-static  70 Static  30 Non-static  70 Static  55 Non-static  70 Static  49 Non-static  70 Static  21 Non-static  70 Static  99 Non-static  70 Static  15 Non-static  70 Static  83 Non-static  70 Static  26 Non-static  70 Static  16 Non-static  70 Static  75

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Microsoft Example Rnd Function  https   msdn microsoft com en-us library f7s023d2 28v vs 90 29 aspx  1- Initialize the random-number generator   Randomize     2 - Generate random value between 1 and 6    Dim value As Integer   CInt Int  6   Rnd      1

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Just for reference  VB NET Fuction definition for RND and RANDOMIZE  which should give the same results of BASIC  1980 years  and all versions after is   Public NotInheritable Class VBMath       Methods     Private Shared Function GetTimer   As Single         Dim now As DateTime   DateTime Now         Return CSng      60   now Hour    now Minute    60    now Second     CDbl now Millisecond    1000        End Function      Public Shared Sub Randomize           Dim timer As Single   VBMath GetTimer         Dim projectData As ProjectData   ProjectData GetProjectData         Dim rndSeed As Integer   projectData m rndSeed         Dim num3 As Integer   BitConverter ToInt32 BitConverter GetBytes timer   0          num3      num3 And  amp HFFFF  Xor  num3  gt  gt   amp H10    lt  lt  8          rndSeed     rndSeed And -16776961  Or num3          projectData m rndSeed   rndSeed     End Sub      Public Shared Sub Randomize ByVal Number As Double          Dim num2 As Integer         Dim projectData As ProjectData   ProjectData GetProjectData         Dim rndSeed As Integer   projectData m rndSeed         If BitConverter IsLittleEndian Then             num2   BitConverter ToInt32 BitConverter GetBytes Number   4          Else             num2   BitConverter ToInt32 BitConverter GetBytes Number   0          End If         num2      num2 And  amp HFFFF  Xor  num2  gt  gt   amp H10    lt  lt  8          rndSeed     rndSeed And -16776961  Or num2          projectData m rndSeed   rndSeed     End Sub      Public Shared Function Rnd   As Single         Return VBMath Rnd 1       End Function      Public Shared Function Rnd ByVal Number As Single  As Single         Dim projectData As ProjectData   ProjectData GetProjectData         Dim rndSeed As Integer   projectData m rndSeed         If  Number  lt  gt  0  Then             If  Number  lt  0  Then                 Dim num1 As UInt64    BitConverter ToInt32 BitConverter GetBytes Number   0  And  amp HFFFFFFFF                  rndSeed   CInt   num1    num1  gt  gt   amp H18   And CULng  amp HFFFFFF                End If             rndSeed   CInt    rndSeed    amp H43FD43FD     amp HC39EC3  And  amp HFFFFFF           End If         projectData m rndSeed   rndSeed         Return  CSng rndSeed    1 677722E 07       End Function  End Class   While the Random CLASS is   Public Class Random       Methods      lt   DynamicallyInvokable gt        Public Sub New           Me New Environment TickCount      End Sub       lt   DynamicallyInvokable gt        Public Sub New ByVal Seed As Integer          Me SeedArray   New Integer  amp H38  - 1             Dim num4 As Integer   If  Seed   -2147483648    amp H7FFFFFFF  Math Abs Seed           Dim num2 As Integer     amp H9A4EC86 - num4          Me SeedArray  amp H37    num2         Dim num3 As Integer   1         Dim i As Integer         For i   1 To  amp H37 - 1             Dim index As Integer      amp H15   i  Mod  amp H37              Me SeedArray index    num3             num3    num2 - num3              If  num3  lt  0  Then                 num3    num3    amp H7FFFFFFF              End If             num2   Me SeedArray index          Next i         Dim j As Integer         For j   1 To 5 - 1             Dim k As Integer             For k   1 To  amp H38 - 1                 Me SeedArray k     Me SeedArray k  - Me SeedArray  1     k   30  Mod  amp H37                     If  Me SeedArray k   lt  0  Then                     Me SeedArray k     Me SeedArray k     amp H7FFFFFFF                  End If             Next k         Next j         Me inext   0         Me inextp    amp H15         Seed   1     End Sub      Private Function GetSampleForLargeRange   As Double         Dim num As Integer   Me InternalSample         If   Me InternalSample Mod 2    0  Then             num   -num         End If         Dim num2 As Double   num         num2    num2   2147483646          Return  num2   4294967293      End Function      Private Function InternalSample   As Integer         Dim inext As Integer   Me inext         Dim inextp As Integer   Me inextp         If    inext  gt    amp H38  Then             inext   1         End If         If    inextp  gt    amp H38  Then             inextp   1         End If         Dim num As Integer    Me SeedArray inext  - Me SeedArray inextp           If  num    amp H7FFFFFFF  Then             num -  1         End If         If  num  lt  0  Then             num    num    amp H7FFFFFFF          End If         Me SeedArray inext    num         Me inext   inext         Me inextp   inextp         Return num     End Function       lt   DynamicallyInvokable gt        Public Overridable Function  Next    As Integer         Return Me InternalSample     End Function       lt   DynamicallyInvokable gt        Public Overridable Function  Next  ByVal maxValue As Integer  As Integer         If  maxValue  lt  0  Then             Dim values As Object     New Object      maxValue                Throw New ArgumentOutOfRangeException  maxValue   Environment GetResourceString  ArgumentOutOfRange MustBePositive   values           End If         Return CInt  Me Sample   maxValue       End Function       lt   DynamicallyInvokable gt        Public Overridable Function  Next  ByVal minValue As Integer  ByVal maxValue As Integer  As Integer         If  minValue  gt  maxValue  Then             Dim values As Object     New Object      minValue    maxValue                Throw New ArgumentOutOfRangeException  minValue   Environment GetResourceString  Argument MinMaxValue   values           End If         Dim num As Long    maxValue - minValue          If  num  lt    amp H7FFFFFFF  Then             Return  CInt  Me Sample   num     minValue          End If         Return  CInt CLng  Me GetSampleForLargeRange   num      minValue      End Function       lt   DynamicallyInvokable gt        Public Overridable Sub NextBytes ByVal buffer As Byte            If  buffer Is Nothing  Then             Throw New ArgumentNullException  buffer           End If         Dim i As Integer         For i   0 To buffer Length - 1             buffer i    CByte  Me InternalSample Mod  amp H100           Next i     End Sub       lt   DynamicallyInvokable gt        Public Overridable Function NextDouble   As Double         Return Me Sample     End Function       lt   DynamicallyInvokable gt        Protected Overridable Function Sample   As Double         Return  Me InternalSample   4 6566128752457969E-10      End Function         Fields     Private inext As Integer     Private inextp As Integer     Private Const MBIG As Integer    amp H7FFFFFFF     Private Const MSEED As Integer    amp H9A4EC86     Private Const MZ As Integer   0     Private SeedArray As Integer   End Class

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