Thread: Passing random seed/device to a function

  1. #1
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    Passing random seed/device to a function

    Hello! I am trying to pass a seed to several functions, each generating one random number from different distribution, e.g. This does not work, as each iteration (function call) produces the same number. But why? Thank you for your help!

    Code:
    #include <iostream>
    #include <random>
    
    using std::cout;
    using std::endl;
    
    typedef std::mt19937 rng_type;
    rng_type rng;
    
    auto D(rng_type r, int n)
    {
    	std::uniform_int_distribution<rng_type::result_type> dist(1, n);
    	rng_type::result_type random_number = dist(r);
    	return random_number;
    }
    
    auto Scatter(rng_type r)
    {
    	std::uniform_real_distribution<rng_type::result_type> dist(0, 1);
    	rng_type::result_type random_number = dist(r);
    	return random_number;
    }
    
    int main()
    {
      rng_type::result_type const seedval = std::random_device{}();
      rng.seed(seedval);
      
      for(int i = 0; i <= 10; ++i) cout << D(rng, 6) << ", " << Scatter(rng) << endl;
      
      return 0;
    }

  2. #2
    and the hat of int overfl Salem's Avatar
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    > auto D(rng_type r, int n)
    > auto Scatter(rng_type r)
    Because you create brand new random number generators through their copy constructors each time you call the function.

    Make each of your r parameters a reference.

    It's like you were expecting this to produce different results.
    Code:
    int foo ( int a ) {
      a += 1;
      return a;
    }
    int main ( ) {
      int x = 42;
      for ( int i = 0 ; i < 10 ; i++ ) cout << foo(x) << endl;
    }
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
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    Quote Originally Posted by Salem View Post
    > auto D(rng_type r, int n)
    > auto Scatter(rng_type r)
    Because you create brand new random number generators through their copy constructors each time you call the function.

    Make each of your r parameters a reference.
    OK thx that fixed it! The other problem was that the output of Scatter was recast to a fixed 0, so I got rid of the types in the functions.

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