(C++) std::rand

Function for drawing a random positive integer from zero to RAND_MAX. RAND_MAX is a #defined constant in cstdlib.

std::_lrand is like std::rand, except it returns random numbers in a larger range. Check out the Boost C++ library for other random number generators.

The code below draws 10 different random numbers.

include <iostream>


int main() 
{
  for (int i=0; i!=10; ++i)
  {
    std::cout << std::rand() << std::endl;
  }
}

As 'true' random number generation is impossible for a device as non-random as a computer, it uses this algorithm to mimic inpredictability as close as possible. This algorithm will always generate the same sequence from the same seed. The seed is the starting point of the rand sequence. The srand function sets this seed. The code below demonstrates that after the same seed (zero in this case), the first 'randomly drawn' number is always the same.

include <iostream>


int main () 
{
  for (int i=0; i!=10; ++i)
  {
    std::srand(0);
    std::cout << std::rand() << std::endl;
  }
}

Note when using multithreading

As srand and rand use a global/static variable and therefore is not suitable for multithreading. Check out the Boost C++ library for other random number generators that do support multithreading.

How to: get a broken random number from zero to one?

Knowing that std::rand has RAND_MAX as a maximum value, just divide by this value, after casting it to a double.

double uniform()
{
  return static_cast<double>(std::rand())/static_cast<double>(RAND_MAX);
}

How to: get a random number from a normal distribution?

See normal distribution.

'Rand' links

Code links

last edited (March 27, 2007) by bilderbikkel, Number of views: 9322, Current Rev: 23 (Diff)

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