<p>In <a href="https://github.com/mlpack/mlpack/pull/391#discussion_r23537713">src/mlpack/core/math/lin_alg.cpp</a>:</p>
<pre style='color:#555'>> + else
> + output(idx++) = M_SQRT2 * input(i, j);
> + }
> + }
> +}
> +
> +void mlpack::math::Svec(const arma::sp_mat& input, arma::sp_vec& output)
> +{
> + const size_t n = input.n_rows;
> + const size_t n2bar = n * (n + 1) / 2;
> +
> + output.zeros(n2bar, 1);
> +
> + size_t idx = 0;
> +
> + for (auto it = input.begin(); it != input.end(); ++it)
</pre>
<p>If you use <code>row_col_iterator</code> (which, of course, isn't documented on the website... I'll have to make a change and submit it...), which exists for both <code>mat</code> and <code>sp_mat</code>, it will iterate over all points in <code>mat</code> and all nonzero points in <code>sp_mat</code>. Using that, you could condense <code>Svec</code> into one templated function, and the speed should be equal to either implementation here.</p>
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