rand returns the same number each time the program is run

Question

In this rather basic C   code snippet involving random number generation   include  lt iostream gt  using namespace std   int main         cout  lt  lt   rand     100       return 0      Why am I always getting an output of 41  I m trying to get it to output some random number between 0 and 100  Maybe I m not understanding something about how the rand function works

User · Answer

You are not seeding the number   Use This    include  lt iostream gt   include  lt ctime gt   using namespace std   int main         srand static cast lt unsigned int gt  time 0         cout  lt  lt   rand     100   lt  lt  endl      return 0      You only need to seed it once though  Basically don t seed it every random number

User · Answer

srand   seeds the random number generator  Without a seed  the generator is unable to generate the numbers you are looking for  As long as one s need for random numbers is not security-critical  e g  any sort of cryptography   common practice is to use the system time as a seed by using the time   function from the  lt ctime gt  library as such  srand time 0    This will seed the random number generator with the system time expressed as a Unix timestamp  i e  the number of seconds since the date 1 1 1970   You can then use rand   to generate a pseudo-random number   Here is a quote from a duplicate question      The reason is that a random number generated from the rand   function isn t   actually random  It simply is a transformation  Wikipedia gives a better   explanation of the meaning of pseudorandom number generator  deterministic   random bit generator  Every time you call rand   it takes the seed and or the   last random number s  generated  the C standard doesn t specify the algorithm   used  though C  11 has facilities for specifying some popular algorithms   runs   a mathematical operation on those numbers  and returns the result  So if the   seed state is the same each time  as it is if you don t call srand with a truly   random number   then you will always get the same  random  numbers out       If you want to know more  you can read the following       http   www dreamincode net forums topic 24225-random-number-generation-102       http   www dreamincode net forums topic 29294-making-pseudo-random-number-generators-more-random

User · Answer

You need to change the seed   int main          srand time NULL        cout  lt  lt   rand     101       return 0      the srand seeding thing is true also for  a c language code     See also  http   xkcd com 221

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random functions like borland complier  using namespace std   int sys random int min  int max       return  rand      max - min 1    min      void sys randomize         srand time 0

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For what its worth you are also only generating numbers between 0 and 99  inclusive   If you wanted to generate values between 0 and 100 you would need   rand     101   in addition to calling srand   as mentioned by others

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You need to  seed  the generator  Check out this short video  it will clear things up   https   www thenewboston com videos php cat 16 amp video 17503

[c++] rand() returns the same number each time the program is run

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