Is it possible to make a (sparse) matrix with the C++ Eigen library similar to this elegant python code I need to translate?
(np.random.rand(100,100) < 0.1) * np.random.rand(100,100)
eg a matrix filled with a certain proportion of random values.
Directly adapted from the Eigen Documentation , and not quite that concise:
std::default_random_engine gen;
std::uniform_real_distribution<double> dist(0.0,1.0);
int rows=100;
int cols=100;
std::vector<Eigen::Triplet<double> > tripletList;
for(int i=0;i<rows;++i)
for(int j=0;j<cols;++j)
{
auto v_ij=dist(gen); //generate random number
if(v_ij < 0.1)
{
tripletList.push_back(T(i,j,v_ij)); //if larger than treshold, insert it
}
}
SparseMatrixType mat(rows,cols);
mat.setFromTriplets(tripletList.begin(), tripletList.end()); //create the matrix
This requires C++11 and is untested.
davidhigh's answer addresses the sparse requirement of your question. However, I don't think that your python code actually produces a sparse matrix, but rather a dense matrix with mostly zeros. A similarly elegant version for Eigen can be
MatrixXd mat;
mat2 = (MatrixXd::Random(5,5).array() > 0.3).cast<double>() * MatrixXd::Random(5,5).array();
Note that this uses the standard C++ rand()
, so may not be sufficiently "random", depending on your needs. You can also replace MatrixXd
with MatrixXf
if you prefer float
s over double
s (change the cast<...>()
as well).
davidhigh's answer has O(rows*cols)
complexity and can be impractical and take too long for large matrices. Here's an adapted version that has only O(nnz)
complexity. p
is the desired sparsity. You may adjust the range of valdis
if the value in your matrix needs to be in other ranges.
typedef Eigen::SparseMatrix<double, Eigen::RowMajor> SpMat;
SpMat getRandomSpMat(size_t rows, size_t cols, double p) {
typedef Eigen::Triplet<double> T;
std::random_device rd; //Will be used to obtain a seed for the random number engine
std::mt19937 gen(rd()); //Standard mersenne_twister_engine seeded with rd()
std::uniform_real_distribution<> valdis(0, 1.0);
std::uniform_int_distribution<> rowdis(0, rows-1);
std::uniform_int_distribution<> coldis(0, cols-1);
std::vector<Eigen::Triplet<double> > tripletList;
size_t nnz = (size_t) (rows * (cols * p));
std::set<size_t> nnz_pos;
for (size_t i = 0; i < nnz; ++i) {
auto r = rowdis(gen);
auto c = coldis(gen);
size_t pos = r * cols + c;
while (nnz_pos.find(pos) != nnz_pos.end()) {
r = rowdis(gen);
c = coldis(gen);
pos = r * cols + c;
}
nnz_pos.insert(pos);
tripletList.push_back(T(r, c, valdis(gen)));
}
SpMat mat(rows,cols);
mat.setFromTriplets(tripletList.begin(), tripletList.end()); //create the matrix
return mat;
}
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