How to generate a random number in C

Question

I m trying to make a game with dice  and I need to have random numbers in it  to simulate the sides of the die   I know how to make it between 1 and 6    Using   include  lt cstdlib gt    include  lt ctime gt    include  lt iostream gt   using namespace std   int main           srand  unsigned time 0         int i      i    rand   6  1       cout  lt  lt  i  lt  lt    n        doesn t work very well  because when I run the program a few times  here s the output I get   6 1 1 1 1 1 2 2 2 2 5 2   So I want a command that will generate a different random number each time  not the same one 5 times in a row   Is there a command that will do this

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for random every RUN file  size t randomGenerator size t min  size t max        std  mt19937 rng      rng seed std  random device             rng seed std  chrono  high resolution clock  now   time since epoch   count         std  uniform int distribution lt std  mt19937  result type gt  dist min  max        return dist rng

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Generate a different random number each time  not the same one six times in a row    Use case scenario  I likened Predictability s problem to a bag of six bits of paper  each with a  value from 0 to 5 written on it  A piece of paper is drawn from the bag each time a new value is required  If the bag is empty  then the numbers are put back into the bag      from this  I can create an algorithm of sorts   Algorithm  A bag is usually a Collection  I chose a bool    otherwise known as a boolean array  bit plane or bit map  to take the role of the bag   The reason I chose a bool   is because the index of each item is already the value of each piece of paper  If the papers required anything else written on them then I would have used a Dictionary lt string  bool gt  in its place  The boolean value is used to keep track of whether the number has been drawn yet or not   A counter called RemainingNumberCount is initialised to 5 that counts down as a random number is chosen  This saves us from having to count how many pieces of paper are left each time we wish to draw a new number   To select the next random value I m using a for  loop to scan through the bag of indexes  and a counter to count off when an index is false called NumberOfMoves   NumberOfMoves is used to choose the next available number  NumberOfMoves is first set to be a random value between 0 and 5  because there are 0  5 available steps we can make through the bag  On the next iteration NumberOfMoves is set to be a random value between 0 and 4  because there are now 0  4 steps we can make through the bag  As the numbers are used  the available numbers reduce so we instead use rand      RemainingNumberCount   1  to calculate the next value for NumberOfMoves    When the NumberOfMoves counter reaches zero  the for  loop should as follows    Set the current Value to be the same as for  loop s index  Set all the numbers in the bag to false  Break from the for  loop    Code  The code for the above solution is as follows    put the following three blocks into the main  cpp file one after the other    include  stdafx h   include  lt ctime gt    include  lt iostream gt   include  lt string gt   class RandomBag   public      int Value   -1       RandomBag             ResetBag                void NextValue             int BagOfNumbersLength   sizeof BagOfNumbers    sizeof  BagOfNumbers            int NumberOfMoves   rand      RemainingNumberCount   1            for  int i   0  i  lt  BagOfNumbersLength  i                            if  BagOfNumbers i     0                    NumberOfMoves--                   if  NumberOfMoves    -1                                        Value   i                       BagOfNumbers i    1                       break                                               if  RemainingNumberCount    0                RemainingNumberCount   5               ResetBag                       else                         RemainingNumberCount--               std  string ToString             return std  to string Value           private      bool BagOfNumbers 6         int RemainingNumberCount       int NumberOfMoves       void ResetBag             RemainingNumberCount   5           NumberOfMoves   rand     6           int BagOfNumbersLength   sizeof BagOfNumbers    sizeof  BagOfNumbers            for  int i   0  i  lt  BagOfNumbersLength  i                            BagOfNumbers i    0               A Console class  I create this Console class because it makes it easy to redirect output    Below in the code     Console  WriteLine  The next value is     randomBag ToString          can be replaced by      std  cout  lt  lt   The next value is     randomBag ToString    lt  lt  std  endl        and then this Console class can be deleted if desired   class Console   public      static void WriteLine std  string s            std  cout  lt  lt  s  lt  lt  std  endl               Main method  Example usage as follows   int main         srand  unsigned time 0       Initialise random seed based on current time      RandomBag randomBag       Console  WriteLine  First set of six    n         randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          Console  WriteLine   nSecond set of six    n         randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          Console  WriteLine   nThird set of six    n         randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          randomBag NextValue         Console  WriteLine  The next value is     randomBag ToString          Console  WriteLine   nProcess complete  n         system  pause         Example output  When I ran the program  I got the following output   First set of six     The next value is 2 The next value is 3 The next value is 4 The next value is 5 The next value is 0 The next value is 1  Second set of six     The next value is 3 The next value is 4 The next value is 2 The next value is 0 The next value is 1 The next value is 5  Third set of six     The next value is 4 The next value is 5 The next value is 2 The next value is 0 The next value is 3 The next value is 1  Process complete   Press any key to continue         Closing statement  This program was written using Visual Studio 2017  and I chose to make it a Visual C   Windows Console Application project using  Net 4 6 1   I m not doing anything particularly special here  so the code should work on earlier versions of Visual Studio too

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Here is a solution  Create a function that returns the random number and place it outside the main function to make it global  Hope this helps   include  lt iostream gt   include  lt cstdlib gt   include  lt ctime gt  int rollDie    using std  cout  int main         srand  unsigned time 0        int die1      int die2      for  int n 10  n gt 0  n--       die1   rollDie        die2   rollDie        cout  lt  lt  die1  lt  lt         lt  lt  die2  lt  lt         lt  lt  die1   die2  lt  lt    n     system  pause    return 0    int rollDie        return  rand   6  1

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Here is a simple random generator with approx  equal probability of generating positive and negative values around 0     int getNextRandom const size t lim               int nextRand   rand     lim          int nextSign   rand     lim          if  nextSign  lt  lim   2              return -nextRand          return nextRand           int main                srand time NULL            int r   getNextRandom 100           cout  lt  lt  r  lt  lt  endl          return 0

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Using modulo may introduce bias into the random numbers  depending on the random number generator  See this question for more info  Of course  it s perfectly possible to get repeating numbers in a random sequence   Try some C  11 features for better distribution    include  lt random gt   include  lt iostream gt   int main         std  random device dev      std  mt19937 rng dev         std  uniform int distribution lt std  mt19937  result type gt  dist6 1 6      distribution in range  1  6       std  cout  lt  lt  dist6 rng   lt  lt  std  endl      See this question answer for more info on C  11 random numbers  The above isn t the only way to do this  but is one way

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Can get full Randomer class code for generating random numbers from here   If you need random numbers in different parts of the project you can create a separate class Randomer to incapsulate all the random stuff inside it   Something like that   class Randomer          random seed by default     std  mt19937 gen       std  uniform int distribution lt size t gt  dist    public              some convenient ctors              Randomer size t min  size t max  unsigned int seed   std  random device                gen  seed   dist  min  max                  if you want predictable numbers     void SetSeed unsigned int seed            gen  seed seed              size t operator               return dist  gen               Such a class would be handy later on   int main         Randomer randomer 0  10       std  cout  lt  lt  randomer    lt  lt    n       You can check this link as an example how i use such Randomer class to generate random strings  You can also use Randomer if you wish

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I know how to generate random number in C   without using any headers  compiler intrinsics or whatever   include  lt cstdio gt     Just for printf int main         auto val   new char 0x10000       auto num   reinterpret cast lt unsigned long long gt  val       delete   val      num   num   0x1000   10      printf  quot  llu n quot   num      I got the following stats after run for some period of time  0  5268 1  5284 2  5279 3  5242 4  5191 5  5135 6  5183 7  5236 8  5372 9  5343  Looks random  How it works   Modern compilers protect you from buffer overflow using ASLR  address space layout randomization   So you can generate some random numbers without using any libraries  but it is just for fun  Do not use ASLR like that

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If you are using boost libs you can obtain a random generator in this way    include  lt iostream gt   include  lt string gt      Used in randomization  include  lt ctime gt   include  lt boost random mersenne twister hpp gt   include  lt boost random uniform int distribution hpp gt   include  lt boost random variate generator hpp gt   using namespace std  using namespace boost   int current time nanoseconds        struct timespec tm      clock gettime CLOCK REALTIME   amp tm       return tm tv nsec     int main  int argc  char  argv          unsigned int dice rolls   12      random  mt19937 rng current time nanoseconds         random  uniform int distribution lt  gt  six 1 6        for unsigned int i 0  i lt dice rolls  i             cout  lt  lt  six rng   lt  lt  endl            Where the function current time nanoseconds   gives the current time in nanoseconds which is used as a seed      Here is a more general class to get random integers and dates in a range    include  lt iostream gt   include  lt ctime gt   include  lt boost random mersenne twister hpp gt   include  lt boost random uniform int distribution hpp gt   include  lt boost random variate generator hpp gt   include  boost date time posix time posix time hpp   include  boost date time gregorian gregorian hpp    using namespace std  using namespace boost  using namespace boost  posix time  using namespace boost  gregorian    class Randomizer   private      static const bool debug mode   false      random  mt19937 rng           The private constructor so that the user can not directly instantiate     Randomizer             if debug mode  true               this- gt rng    random  mt19937             else              this- gt rng    random  mt19937 current time nanoseconds                           int current time nanoseconds            struct timespec tm          clock gettime CLOCK REALTIME   amp tm           return tm tv nsec                C   03                        Dont forget to declare these two  You want to make sure they        are unacceptable otherwise you may accidentally get copies of        your singleton appearing      Randomizer Randomizer const amp           Don t Implement     void operator  Randomizer const amp       Don t implement  public      static Randomizer amp  get instance               The only instance of the class is created at the first call get instance               and will be destroyed only when the program exits         static Randomizer instance          return instance            bool method     return true          int rand unsigned int floor  unsigned int ceil           random  uniform int distribution lt  gt  rand    random  uniform int distribution lt  gt   floor ceil           return  rand  rng                   Is not considering the millisecons     time duration rand time duration            boost  posix time  time duration floor 0  0  0  0           boost  posix time  time duration ceil 23  59  59  0           unsigned int rand seconds   rand floor total seconds    ceil total seconds             return seconds rand seconds               date rand date from epoch to now            date now   second clock  local time   date            return rand date from epoch to ceil now              date rand date from epoch to ceil date ceil date           date epoch   ptime date 1970 1 1   date            return rand date in interval epoch  ceil date              date rand date in interval date floor date  date ceil date           return rand ptime in interval ptime floor date   ptime ceil date   date               ptime rand ptime from epoch to now            ptime now   second clock  local time            return rand ptime from epoch to ceil now              ptime rand ptime from epoch to ceil ptime ceil date           ptime epoch   ptime date 1970 1 1            return rand ptime in interval epoch  ceil date              ptime rand ptime in interval ptime floor date  ptime ceil date           time duration const diff   ceil date - floor date          long long gap seconds   diff total seconds            long long step seconds   Randomizer  get instance   rand 0  gap seconds           return floor date   seconds step seconds

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include  lt iostream gt   include  lt cstdlib gt   include  lt ctime gt   int main         srand time NULL        int random number   std  rand       rand   return a number between  0  and RAND MAX     std  cout  lt  lt  random number      return 0      http   en cppreference com w cpp numeric random rand

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The most fundamental problem of your test application is that you call srand once and then call rand one time and exit  The whole point of srand function is to initialize the sequence of pseudo-random numbers with a random seed  It means that if you pass the same value to srand in two different applications  with the same srand rand implementation  then you will get exactly the same sequence of rand   values read after that in both applications  However in your example application pseudo-random sequence consists only of one element - the first element of a pseudo-random sequence generated from seed equal to current time of 1 sec precision  What do you expect to see on output then  Obviously when you happen to run application on the same second - you use the same seed value - thus your result is the same of course  as Martin York already mentioned in a comment to the question   Actually you should call srand seed  one time and then call rand   many times and analyze that sequence - it should look random  EDIT  Oh I get it  Apparently verbal description is not enough  maybe language barrier or something          OK  Old-fashioned C code example based on the same srand   rand   time   functions that was used in the question   include  lt stdlib h gt   include  lt time h gt   include  lt stdio h gt   int main void        unsigned long j      srand   unsigned time NULL          for  j   0  j  lt  100500    j                 int n              skip rand   readings that would make n 6 non-uniformly distributed            assuming rand   itself is uniformly distributed from 0 to RAND MAX             while    n   rand      gt  RAND MAX -  RAND MAX-5  6                bad value retrieved so get next one               printf   quot  d  t d n quot   n  n   6   1               return 0         THAT sequence from a single run of the program is supposed to look random  Please NOTE that I don t recommend to use rand srand functions in production for the reasons explained below and I absolutely don t recommend to use function time as a random seed for the reasons that IMO already should be quite obvious  Those are fine for educational purposes and to illustrate the point sometimes but for any serious use they are mostly useless  EDIT2  When using C or C   standard library it is important to understand that as of now there is not a single standard function or class producing actually random data definitively  guaranteed by the standard   The only standard tool that approaches this problem is std  random device that unfortunately still does not provide guarantees of actual randomness  Depending on the nature of application you should first decide if you really need truly random  unpredictable  data  Notable case when you do most certainly need true randomness is information security - e g  generating symmetric keys  asymmetric private keys  salt values  security tokens  etc  However security-grade random numbers is a separate industry worth a separate article  In most cases Pseudo-Random Number Generator is sufficient - e g  for scientific simulations or games  In some cases consistently defined pseudo-random sequence is even required - e g  in games you may choose to generate exactly same maps in runtime to avoid storing lots of data  The original question and reoccurring multitude of identical similar questions  and even many misguided  quot answers quot  to them  indicate that first and foremost it is important to distinguish random numbers from pseudo-random numbers AND to understand what is pseudo-random number sequence in the first place AND to realize that pseudo-random number generators are NOT used the same way you could use true random number generators   Intuitively when you request random number - the result returned shouldn t depend on previously returned values and shouldn t depend if anyone requested anything before and shouldn t depend in what moment and by what process and on what computer and from what generator and in what galaxy it was requested  That is what word  quot random quot  means after all - being unpredictable and independent of anything - otherwise it is not random anymore  right  With this intuition it is only natural to search the web for some magic spells to cast to get such random number in any possible context       THAT kind of intuitive expectations IS VERY WRONG and harmful in all cases involving Pseudo-Random Number Generators - despite being reasonable for true random numbers  While the meaningful notion of  quot random number quot  exists  kind of  - there is no such thing as  quot pseudo-random number quot   A Pseudo-Random Number Generator actually produces pseudo-random number sequence  Pseudo-random sequence is in fact always deterministic  predetermined by its algorithm and initial parameters  i e  there is actually nothing random about it  When experts talk about quality of PRNG they actually talk about statistical properties of the generated sequence  and its notable sub-sequences   For example if you combine two high quality PRNGs by using them both in turns - you may produce bad resulting sequence - despite them generating good sequences each separately  those two good sequences may simply correlate to each other and thus combine badly   Specifically rand   srand s  pair of functions provide a singular per-process non-thread-safe    pseudo-random number sequence generated with implementation-defined algorithm  Function rand   produces values in range  0  RAND MAX   Quote from C11 standard  ISO IEC 9899 2011    The srand function uses the argument as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to rand  If srand is then called with the same seed value  the sequence of pseudo-random numbers shall be repeated  If rand is called before any calls to srand have been made  the same sequence shall be generated as when srand is first called with a seed value of 1   Many people reasonably expect that rand   would produce a sequence of semi-independent uniformly distributed numbers in range 0 to RAND MAX  Well it most certainly should  otherwise it s useless  but unfortunately not only standard doesn t require that - there is even explicit disclaimer that states  quot there is no guarantees as to the quality of the random sequence produced quot   In some historical cases rand srand implementation was of very bad quality indeed  Even though in modern implementations it is most likely good enough - but the trust is broken and not easy to recover  Besides its non-thread-safe nature makes its safe usage in multi-threaded applications tricky and limited  still possible - you may just use them from one dedicated thread   New class template std  mersenne twister engine lt  gt   and its convenience typedefs - std  mt19937 std  mt19937 64 with good template parameters combination  provides per-object pseudo-random number generator defined in C  11 standard  With the same template parameters and the same initialization parameters different objects will generate exactly the same per-object output sequence on any computer in any application built with C  11 compliant standard library  The advantage of this class is its predictably high quality output sequence and full consistency across implementations  Also there are more PRNG engines defined in C  11 standard - std  linear congruential engine lt  gt   historically used as fair quality srand rand algorithm in some C standard library implementations  and std  subtract with carry engine lt  gt   They also generate fully defined parameter-dependent per-object output sequences  Modern day C  11 example replacement for the obsolete C code above   include  lt iostream gt   include  lt chrono gt   include  lt random gt   int main         std  random device rd         seed value is designed specifically to make initialization        parameters of std  mt19937  instance of std  mersenne twister engine lt  gt          different across executions of application     std  mt19937  result type seed   rd                    std  mt19937  result type              std  chrono  duration cast lt std  chrono  seconds gt                   std  chrono  system clock  now   time since epoch                     count                  std  mt19937  result type              std  chrono  duration cast lt std  chrono  microseconds gt                   std  chrono  high resolution clock  now   time since epoch                     count           std  mt19937 gen seed        for  unsigned long j   0  j  lt  100500    j             Yes  Generating single pseudo-random number makes no sense        even if you use std  mersenne twister engine instead of rand          and even when your seed quality is much better than time NULL                       std  mt19937  result type n             reject readings that would make n 6 non-uniformly distributed         while    n   gen      gt  std  mt19937  max   -                                       std  mt19937  max   - 5   6                bad value retrieved so get next one               std  cout  lt  lt  n  lt  lt    t   lt  lt  n   6   1  lt  lt    n              return 0     The version of previous code that uses std  uniform int distribution lt  gt   include  lt iostream gt   include  lt chrono gt   include  lt random gt   int main         std  random device rd      std  mt19937  result type seed   rd                    std  mt19937  result type              std  chrono  duration cast lt std  chrono  seconds gt                   std  chrono  system clock  now   time since epoch                     count                  std  mt19937  result type              std  chrono  duration cast lt std  chrono  microseconds gt                   std  chrono  high resolution clock  now   time since epoch                     count           std  mt19937 gen seed       std  uniform int distribution lt unsigned gt  distrib 1  6        for  unsigned long j   0  j  lt  100500    j                 std  cout  lt  lt  distrib gen   lt  lt                  std  cout  lt  lt    n       return 0

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Whenever you do a basic web search for random number generation in the C   programming language this question is usually the first to pop up  I want to throw my hat into the ring to hopefully better clarify the concept of pseudo-random number generation in C   for future coders that will inevitably search this same question on the web   The Basics  Pseudo-random number generation involves the process of utilizing a deterministic algorithm that produces a sequence of numbers whose properties approximately resemble random numbers  I say approximately resemble  because true randomness is a rather elusive mystery in mathematics and computer science  Hence  why the term pseudo-random is utilized to be more pedantically correct   Before you can actually use a PRNG  i e   pseudo-random number generator  you must provide the algorithm with an initial value often referred too as the seed  However  the seed must only be set once before using the algorithm itself       Proper way  seed  1234       Seed set only once    for  x in range  0  10       PRNG  seed       Will work as expected      Wrong way  for  x in rang  0  10        seed  1234       Seed reset for ten iterations    PRNG  seed       Output will be the same      Thus  if you want a good sequence of numbers  then you must provide an ample seed to the PRNG   The Old C Way  The backwards compatible standard library of C that C   has  uses what is called a linear congruential generator found in the cstdlib header file  This PRNG functions through a discontinuous piecewise function that utilizes modular arithmetic  i e   a quick algorithm that likes to use the modulo operator      The following is common usage of this PRNG  with regards to the original question asked by  Predictability    include  lt iostream gt   include  lt cstdlib gt   include  lt ctime gt   int main  void       int low dist    1    int high dist   6    std  srand    unsigned int  std  time  nullptr        for  int repetition   0  repetition  lt  10    repetition       std  cout  lt  lt  low dist   std  rand       high dist - low dist    lt  lt  std  endl    return 0      The common usage of C s PRNG houses a whole host of issues such as    The overall interface of std  rand   isn t very intuitive for the proper generation of pseudo-random numbers between a given range  e g   producing numbers between  1  6  the way  Predictability wanted  The common usage of std  rand   eliminates the possibility of a uniform distribution of pseudo-random numbers  because of the Pigeonhole Principle  The common way std  rand   gets seeded through std  srand    unsigned int  std  time  nullptr     technically isn t correct  because time t is considered to be a restricted type  Therefore  the conversion from time t to unsigned int is not guaranteed    For more detailed information about the overall issues of using C s PRNG  and how to possibly circumvent them  please refer to Using rand    C C     Advice for the C standard library   s rand   function   The Standard C   Way  Since the ISO IEC 14882 2011 standard was published  i e   C  11  the random library has been apart of the C   programming language for a while now  This library comes equipped with multiple PRNGs  and different distribution types such as  uniform distribution  normal distribution  binomial distribution  etc  The following source code example demonstrates a very basic usage of the random library  with regards to  Predictability s original question    include  lt iostream gt   include  lt cctype gt   include  lt random gt   using u32      uint least32 t   using engine   std  mt19937   int main  void       std  random device os seed    const u32 seed   os seed       engine generator  seed      std  uniform int distribution lt  u32  gt  distribute  1  6       for  int repetition   0  repetition  lt  10    repetition       std  cout  lt  lt  distribute  generator    lt  lt  std  endl    return 0      The 32-bit Mersenne Twister engine  with a uniform distribution of integer values was utilized in the above example   The name of the engine in source code sounds weird  because its name comes from its period of 2 19937-1    The example also uses std  random device to seed the engine  which obtains its value from the operating system  If you are using a Linux system  then std  random device returns a value from  dev urandom    Take note  that you do not have to use std  random device to seed any engine  You can use constants or even the chrono library  You also don t have to use the 32-bit version of the std  mt19937 engine  there are other options  For more information about the capabilities of the random library  please refer to cplusplus com  All in all  C   programmers should not use std  rand   anymore  not because its bad  but because the current standard provides better alternatives that are more straight forward and reliable  Hopefully  many of you find this helpful  especially those of you who recently web searched generating random numbers in c

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This code produces random numbers from n to m   int random int from  int to       return rand      to - from   1    from      example   int main        srand time 0        cout  lt  lt  random 0  99   lt  lt    n

[c++] How to generate a random number in C++?

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