<p><b>@rcurtin</b> commented on this pull request.</p>

<hr>

<p>In <a href="https://github.com/mlpack/mlpack/pull/792#pullrequestreview-2062154">src/mlpack/core/optimizers/gradient_descent/gradient_descent_impl.hpp</a>:</p>
<pre style='color:#555'>&gt; +    stepSize(stepSize),
+    maxIterations(maxIterations),
+    tolerance(tolerance)
+{ /* Nothing to do. */ }
+
+//! Optimize the function (minimize).
+template&lt;typename FunctionType&gt;
+double GradientDescent&lt;FunctionType&gt;::Optimize(
+    arma::mat&amp; iterate)
+{
+  // To keep track of where we are and how things are going.
+  double overallObjective = function.Evaluate(iterate);
+  double lastObjective = DBL_MAX;
+
+  // Now iterate!
+  arma::vec gradient(iterate.n_cols);
</pre>
<p>This should be <code>arma::mat</code> not <code>arma::vec</code>, and you should use both the number of rows and columns to initialize it.  Otherwise it will fail if we use an objective function that uses a 2-d matrix instead of a 1-d vector.</p>

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