<p>In <a href="https://github.com/mlpack/mlpack/pull/374#discussion-diff-22743409">src/mlpack/methods/matrix_completion/matrix_completion.hpp</a>:</p>
<pre style='color:#555'>&gt; +   * Construct a matrix completion problem, specifying the maximum rank of the
&gt; +   * solution.
&gt; +   *
&gt; +   * @param m Number of rows of original matrix.
&gt; +   * @param n Number of columns of original matrix.
&gt; +   * @param indices Matrix containing the indices of the known entries (must be
&gt; +   *    [2 x p]).
&gt; +   * @param values Vector containing the values of the known entries (must be
&gt; +   *    length p).
&gt; +   * @param r Maximum rank of solution.
&gt; +   */
&gt; +  MatrixCompletion(const size_t m,
&gt; +                   const size_t n,
&gt; +                   const arma::umat&amp; indices,
&gt; +                   const arma::vec&amp; values,
&gt; +                   const size_t r);
</pre>
<p>You could use <code>arma::sp_mat</code> instead of <code>arma::umat</code> and <code>arma::vec</code>.  I know that there are a few other classes that request sparse matrices in <code>umat</code>/<code>vec</code> format, so maybe it is best to provide both, if possible.</p>

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