Why does rand() yield the same sequence of numbers on every run?

by

The seed for the random number generator is not set.

If you call srand((unsigned int)time(NULL)) then you will get more random results:

#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;

int main() {
    srand((unsigned int)time(NULL));
    cout << rand() << endl;
    return 0;
}

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/

More Related Contents:

  1. Why do people say there is modulo bias when using a random number generator?
  2. Generate random numbers uniformly over an entire range
  3. Random float number generation
  4. Recommended way to initialize srand?
  5. rand() returns same values when called within a single function
  6. Generate random numbers using C++11 random library
  7. Why do I get the same sequence for every run with std::random_device with mingw gcc4.8.1?
  8. Using stdlib’s rand() from multiple threads
  9. Same random numbers every loop iteration
  10. generate random double numbers in c++
  11. Need for predictable random generator
  12. What is the optimal algorithm for generating an unbiased random integer within a range?
  13. If we seed c++11 mt19937 as the same on different machines, will we get the same sequence of random numbers
  14. How do you generate a random double uniformly distributed between 0 and 1 from C++?
  15. How do I scale down numbers from rand()?
  16. Boost random number generator
  17. How often should I call srand() in a C++ application?
  18. What’s the Right Way to use the rand() Function in C++?
  19. Consistent pseudo-random numbers across platforms
  20. Getting big random numbers in C/C++
  21. Program is generating same random numbers on each run? [duplicate]
  22. srand(time(NULL)) generating similar results [duplicate]
  23. Generating a random integer from a range
  24. Why is the new random library better than std::rand()?
  25. Does std::mt19937 require warmup?
  26. Vary range of uniform_int_distribution
  27. Why Aren’t `std::uniform_int_distribution` and `std::uniform_int_distribution` Allowed?
  28. How to make sure that std::random_shuffle always produces a different result?
  29. How does calling srand more than once affect the quality of randomness?
  30. What difference between rand() and random() functions?

Leave a Comment