CODEWITHSUNDEEP
c++matrixeigen
wlfbck
wlfbck

Reputation: 527

MatrixXf::Random always returning same matrices

I just played around with Eigen a bit and noticed that MatrixXf::Random(3,3) always returns the same matrices, first one is always this for example:
0.680375 0.59688 -0.329554
-0.211234 0.823295 0.536459
0.566198 -0.604897 -0.444451

Is that intended behaviour or am i just overseeing something really simple? (My experience with mathematic libraries is close to zero)

Code i used:

for(int i = 0; i < 5; i++) {
        MatrixXf A = MatrixXf::Random(3, 3);
        cout << A <<endl;
}

Upvotes: 13

Views: 10104

Answers (4)

GPrathap
GPrathap

Reputation: 7810

How about this way?

#include<iostream>
#include<random>
#include <Eigen/Dense>



int main(){

  std::random_device rd;
  std::mt19937 gen(rd());  
  std::uniform_real_distribution<float> dis(0, 1);

  Eigen::MatrixXf m = Eigen::MatrixXf::Zero(10,10).unaryExpr([&](float dummy){return dis(gen);});
  cout<<"Uniform random matrix:\n"<<m<<endl;
  cout<<"Mean: "<<m.mean()<<endl;

  return 0;
}

Upvotes: 1

davidhigh
davidhigh

Reputation: 15468

Instead of srand you can also use a nullary expression together with modern C++11 random number generation:

//see https://en.cppreference.com/w/cpp/numeric/random/uniform_real_distribution
std::random_device rd;
std::mt19937 gen(rd());  //here you could set the seed, but std::random_device already does that
std::uniform_real_distribution<float> dis(-1.0, 1.0);

Eigen::MatrixXf A = Eigen::MatrixXf::NullaryExpr(3,3,[&](){return dis(gen);});

This also allows to use more complex distributions such as a normal distribution.

Upvotes: 28

s.heinekamp
s.heinekamp

Reputation: 41

@orian:

std::srand(unsigned seed) is not an Eigen function. The complete code should work like that:

    std::srand((unsigned int) time(0));
    for(int i = 0; i < 5; i++) {   
            MatrixXf A = MatrixXf::Random(3, 3);
            cout << A <<endl;
        }

Upvotes: 2

ggael
ggael

Reputation: 29205

Yes, that's the intended behaviour. Matrix::Random uses the random number generator of the standard library, so you can initialize the random number sequence with srand(unsigned int seed), for instance:

srand((unsigned int) time(0));

Upvotes: 23

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