title: How to efficiently update Eigen sparse matrix entries
body: <p>I have a fixed sparse matrix that I set up before my computations through:</p>
  
  <pre><code>Eigen::SparseMatrix&lt;double, Eigen::ColMajor&gt; A;
  std::vector&lt;Eigen::Triplet&lt;double&gt;&gt; coefficients{
      {0, 0}//, {,},{,},{,}
  };
  A.setFromTriplets(coefficients.begin(), coefficients.end());
  A.makeCompressed();
  </code></pre>
  
  <p>Now, I need to update it with my actual entries. Unfortunately, going through the entries with:</p>
  
  <pre><code>for(...) A.coeffRef(i,j) = new_val(i,j);
  </code></pre>
  
  <p>I get a binary search instead of an efficient constant time access.</p>
  
  <p>I could use <code>valuePtr()</code>, but it seems a little bit "hacky".</p>