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<h1 class="post-title">Five Minutes Neural Network</h1>
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<h1 id="develop-a-neural-network-with-mxnet-in-five-minutes">Develop a Neural Network with MXNet in Five Minutes</h1>
<p>This tutorial is designed for new users of the <code>mxnet</code> package for R. It shows how to construct a neural network to do regression in 5 minutes. It shows how to perform classification and regression tasks, respectively. The data we use is in the <code>mlbench</code> package. Instructions to install R and MXNet&#39;s R package in different environments can be found <a href="/get_started?version=master&platform=linux&language=r&environ=pip&processor=cpu">here</a>.</p>
<h2 id="classification">Classification</h2>
<div class="highlight"><pre><code class="language-" data-lang=""> ## Loading required package: mlbench
</code></pre></div><div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="o">!</span><span class="n">require</span><span class="p">(</span><span class="n">mlbench</span><span class="p">))</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="n">install.packages</span><span class="p">(</span><span class="s1">'mlbench'</span><span class="p">)</span><span class="w">
</span><span class="p">}</span><span class="w">
</span></code></pre></div><div class="highlight"><pre><code class="language-" data-lang=""> ## Loading required package: mxnet
</code></pre></div><div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">require</span><span class="p">(</span><span class="n">mxnet</span><span class="p">)</span><span class="w">
</span></code></pre></div><div class="highlight"><pre><code class="language-" data-lang=""> ## Loading required datasets
</code></pre></div><div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">data</span><span class="p">(</span><span class="n">Sonar</span><span class="p">,</span><span class="w"> </span><span class="n">package</span><span class="o">=</span><span class="s2">"mlbench"</span><span class="p">)</span><span class="w">
</span><span class="n">Sonar</span><span class="p">[,</span><span class="m">61</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nf">as.numeric</span><span class="p">(</span><span class="n">Sonar</span><span class="p">[,</span><span class="m">61</span><span class="p">])</span><span class="m">-1</span><span class="w">
</span><span class="n">train.ind</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nf">c</span><span class="p">(</span><span class="m">1</span><span class="o">:</span><span class="m">50</span><span class="p">,</span><span class="w"> </span><span class="m">100</span><span class="o">:</span><span class="m">150</span><span class="p">)</span><span class="w">
</span><span class="n">train.x</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">data.matrix</span><span class="p">(</span><span class="n">Sonar</span><span class="p">[</span><span class="n">train.ind</span><span class="p">,</span><span class="w"> </span><span class="m">1</span><span class="o">:</span><span class="m">60</span><span class="p">])</span><span class="w">
</span><span class="n">train.y</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Sonar</span><span class="p">[</span><span class="n">train.ind</span><span class="p">,</span><span class="w"> </span><span class="m">61</span><span class="p">]</span><span class="w">
</span><span class="n">test.x</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">data.matrix</span><span class="p">(</span><span class="n">Sonar</span><span class="p">[</span><span class="o">-</span><span class="n">train.ind</span><span class="p">,</span><span class="w"> </span><span class="m">1</span><span class="o">:</span><span class="m">60</span><span class="p">])</span><span class="w">
</span><span class="n">test.y</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Sonar</span><span class="p">[</span><span class="o">-</span><span class="n">train.ind</span><span class="p">,</span><span class="w"> </span><span class="m">61</span><span class="p">]</span><span class="w">
</span></code></pre></div>
<p>We are going to use a multi-layer perceptron as our classifier. In <code>mxnet</code>, we have a function called <code>mx.mlp</code> for building a general multi-layer neural network to do classification or regression.</p>
<p><code>mx.mlp</code> requires the following parameters:</p>
<ul>
<li>Training data and label</li>
<li>Number of hidden nodes in each hidden layer</li>
<li>Number of nodes in the output layer</li>
<li>Type of the activation</li>
<li>Type of the output loss</li>
<li>The device to train (GPU or CPU)</li>
<li>Other parameters for <code>mx.model.FeedForward.create</code></li>
</ul>
<p>The following code shows an example usage of <code>mx.mlp</code>:</p>
<div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">mx.set.seed</span><span class="p">(</span><span class="m">0</span><span class="p">)</span><span class="w">
</span><span class="n">model</span><span class="w"> </span><span class="o">&lt;-</span><span class="w"> </span><span class="n">mx.mlp</span><span class="p">(</span><span class="n">train.x</span><span class="p">,</span><span class="w"> </span><span class="n">train.y</span><span class="p">,</span><span class="w"> </span><span class="n">hidden_node</span><span class="o">=</span><span class="m">10</span><span class="p">,</span><span class="w"> </span><span class="n">out_node</span><span class="o">=</span><span class="m">2</span><span class="p">,</span><span class="w"> </span><span class="n">out_activation</span><span class="o">=</span><span class="s2">"softmax"</span><span class="p">,</span><span class="w">
</span><span class="n">num.round</span><span class="o">=</span><span class="m">20</span><span class="p">,</span><span class="w"> </span><span class="n">array.batch.size</span><span class="o">=</span><span class="m">15</span><span class="p">,</span><span class="w"> </span><span class="n">learning.rate</span><span class="o">=</span><span class="m">0.07</span><span class="p">,</span><span class="w"> </span><span class="n">momentum</span><span class="o">=</span><span class="m">0.9</span><span class="p">,</span><span class="w">
</span><span class="n">eval.metric</span><span class="o">=</span><span class="n">mx.metric.accuracy</span><span class="p">)</span><span class="w">
</span></code></pre></div><div class="highlight"><pre><code class="language-" data-lang=""> ## Auto detect layout of input matrix, use rowmajor..
## Start training with 1 devices
## [1] Train-accuracy=0.488888888888889
## [2] Train-accuracy=0.514285714285714
## [3] Train-accuracy=0.514285714285714
## [4] Train-accuracy=0.514285714285714
## [5] Train-accuracy=0.514285714285714
## [6] Train-accuracy=0.523809523809524
## [7] Train-accuracy=0.619047619047619
## [8] Train-accuracy=0.695238095238095
## [9] Train-accuracy=0.695238095238095
## [10] Train-accuracy=0.761904761904762
## [11] Train-accuracy=0.828571428571429
## [12] Train-accuracy=0.771428571428571
## [13] Train-accuracy=0.742857142857143
## [14] Train-accuracy=0.733333333333333
## [15] Train-accuracy=0.771428571428571
## [16] Train-accuracy=0.847619047619048
## [17] Train-accuracy=0.857142857142857
## [18] Train-accuracy=0.838095238095238
## [19] Train-accuracy=0.838095238095238
## [20] Train-accuracy=0.838095238095238
</code></pre></div>
<p>Note that <code>mx.set.seed</code> controls the random process in <code>mxnet</code>. You can see the accuracy in each round during training. It&#39;s also easy to make predictions and evaluate.</p>
<p>To get an idea of what is happening, view the computation graph from R:</p>
<div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">graph.viz</span><span class="p">(</span><span class="n">model</span><span class="o">$</span><span class="n">symbol</span><span class="p">)</span><span class="w">
</span></code></pre></div>
<p><a href="https://github.com/apache/mxnet"><img src="https://raw.githubusercontent.com/dmlc/web-data/master/mxnet/knitr/graph.computation.png"></a></p>
<div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">preds</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">predict</span><span class="p">(</span><span class="n">model</span><span class="p">,</span><span class="w"> </span><span class="n">test.x</span><span class="p">)</span><span class="w">
</span></code></pre></div><div class="highlight"><pre><code class="language-" data-lang=""> ## Auto detect layout of input matrix, use rowmajor.
</code></pre></div><div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">pred.label</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">max.col</span><span class="p">(</span><span class="n">t</span><span class="p">(</span><span class="n">preds</span><span class="p">))</span><span class="m">-1</span><span class="w">
</span><span class="n">table</span><span class="p">(</span><span class="n">pred.label</span><span class="p">,</span><span class="w"> </span><span class="n">test.y</span><span class="p">)</span><span class="w">
</span></code></pre></div><div class="highlight"><pre><code class="language-" data-lang=""> ## test.y
## pred.label 0 1
## 0 24 14
## 1 36 33
</code></pre></div>
<p>Note for that for multi-class predictions, mxnet outputs <code>nclass</code> x <code>nexamples</code>, with each row corresponding to the probability of the class.</p>
<h2 id="regression">Regression</h2>
<p>Again, let us preprocess the data:</p>
<div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">data</span><span class="p">(</span><span class="n">BostonHousing</span><span class="p">,</span><span class="w"> </span><span class="n">package</span><span class="o">=</span><span class="s2">"mlbench"</span><span class="p">)</span><span class="w">
</span><span class="n">train.ind</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">seq</span><span class="p">(</span><span class="m">1</span><span class="p">,</span><span class="w"> </span><span class="m">506</span><span class="p">,</span><span class="w"> </span><span class="m">3</span><span class="p">)</span><span class="w">
</span><span class="n">train.x</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">data.matrix</span><span class="p">(</span><span class="n">BostonHousing</span><span class="p">[</span><span class="n">train.ind</span><span class="p">,</span><span class="w"> </span><span class="m">-14</span><span class="p">])</span><span class="w">
</span><span class="n">train.y</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">BostonHousing</span><span class="p">[</span><span class="n">train.ind</span><span class="p">,</span><span class="w"> </span><span class="m">14</span><span class="p">]</span><span class="w">
</span><span class="n">test.x</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">data.matrix</span><span class="p">(</span><span class="n">BostonHousing</span><span class="p">[</span><span class="o">-</span><span class="n">train.ind</span><span class="p">,</span><span class="w"> </span><span class="m">-14</span><span class="p">])</span><span class="w">
</span><span class="n">test.y</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">BostonHousing</span><span class="p">[</span><span class="o">-</span><span class="n">train.ind</span><span class="p">,</span><span class="w"> </span><span class="m">14</span><span class="p">]</span><span class="w">
</span></code></pre></div>
<p>Although we can use <code>mx.mlp</code> again to do regression by changing the <code>out_activation</code>, this time we are going to introduce a flexible way to configure neural networks in <code>mxnet</code>. Configuration is done by the &quot;Symbol&quot; system in <code>mxnet</code>. The Symbol system takes care of the links among nodes, activation, dropout ratio, etc. Configure a multi-layer neural network as follows:</p>
<div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="c1"># Define the input data</span><span class="w">
</span><span class="n">data</span><span class="w"> </span><span class="o">&lt;-</span><span class="w"> </span><span class="n">mx.symbol.Variable</span><span class="p">(</span><span class="s2">"data"</span><span class="p">)</span><span class="w">
</span><span class="c1"># A fully connected hidden layer</span><span class="w">
</span><span class="c1"># data: input source</span><span class="w">
</span><span class="c1"># num_hidden: number of neurons in this hidden layer</span><span class="w">
</span><span class="n">fc1</span><span class="w"> </span><span class="o">&lt;-</span><span class="w"> </span><span class="n">mx.symbol.FullyConnected</span><span class="p">(</span><span class="n">data</span><span class="p">,</span><span class="w"> </span><span class="n">num_hidden</span><span class="o">=</span><span class="m">1</span><span class="p">)</span><span class="w">
</span><span class="c1"># Use linear regression for the output layer</span><span class="w">
</span><span class="n">lro</span><span class="w"> </span><span class="o">&lt;-</span><span class="w"> </span><span class="n">mx.symbol.LinearRegressionOutput</span><span class="p">(</span><span class="n">fc1</span><span class="p">)</span><span class="w">
</span></code></pre></div>
<p>What matters for a regression task is mainly the last function. It enables the new network to optimize for squared loss. Now let&#39;s train on this simple data set. In this configuration, we dropped the hidden layer so that the input layer is directly connected to the output layer.</p>
<p>Next, make prediction with this structure and other parameters with <code>mx.model.FeedForward.create</code>:</p>
<div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">mx.set.seed</span><span class="p">(</span><span class="m">0</span><span class="p">)</span><span class="w">
</span><span class="n">model</span><span class="w"> </span><span class="o">&lt;-</span><span class="w"> </span><span class="n">mx.model.FeedForward.create</span><span class="p">(</span><span class="n">lro</span><span class="p">,</span><span class="w"> </span><span class="n">X</span><span class="o">=</span><span class="n">train.x</span><span class="p">,</span><span class="w"> </span><span class="n">y</span><span class="o">=</span><span class="n">train.y</span><span class="p">,</span><span class="w">
</span><span class="n">ctx</span><span class="o">=</span><span class="n">mx.cpu</span><span class="p">(),</span><span class="w"> </span><span class="n">num.round</span><span class="o">=</span><span class="m">50</span><span class="p">,</span><span class="w"> </span><span class="n">array.batch.size</span><span class="o">=</span><span class="m">20</span><span class="p">,</span><span class="w">
</span><span class="n">learning.rate</span><span class="o">=</span><span class="m">2e-6</span><span class="p">,</span><span class="w"> </span><span class="n">momentum</span><span class="o">=</span><span class="m">0.9</span><span class="p">,</span><span class="w"> </span><span class="n">eval.metric</span><span class="o">=</span><span class="n">mx.metric.rmse</span><span class="p">)</span><span class="w">
</span></code></pre></div><div class="highlight"><pre><code class="language-" data-lang=""> ## Auto detect layout of input matrix, use rowmajor.
## Start training with 1 devices
## [1] Train-rmse=16.063282524034
## [2] Train-rmse=12.2792375712573
## [3] Train-rmse=11.1984634005885
## [4] Train-rmse=10.2645236892904
## [5] Train-rmse=9.49711005504284
## [6] Train-rmse=9.07733734175182
## [7] Train-rmse=9.07884450847991
## [8] Train-rmse=9.10463850277417
## [9] Train-rmse=9.03977049028532
## [10] Train-rmse=8.96870685004475
## [11] Train-rmse=8.93113287361574
## [12] Train-rmse=8.89937257821847
## [13] Train-rmse=8.87182096922953
## [14] Train-rmse=8.84476075083586
## [15] Train-rmse=8.81464673014974
## [16] Train-rmse=8.78672567900196
## [17] Train-rmse=8.76265872846474
## [18] Train-rmse=8.73946101419974
## [19] Train-rmse=8.71651926303267
## [20] Train-rmse=8.69457600919277
## [21] Train-rmse=8.67354928674563
## [22] Train-rmse=8.65328755392436
## [23] Train-rmse=8.63378039680078
## [24] Train-rmse=8.61488162586984
## [25] Train-rmse=8.5965105183022
## [26] Train-rmse=8.57868133563275
## [27] Train-rmse=8.56135851937663
## [28] Train-rmse=8.5444819772098
## [29] Train-rmse=8.52802114610432
## [30] Train-rmse=8.5119504512622
## [31] Train-rmse=8.49624261719241
## [32] Train-rmse=8.48087453238701
## [33] Train-rmse=8.46582689119887
## [34] Train-rmse=8.45107881002491
## [35] Train-rmse=8.43661331401712
## [36] Train-rmse=8.42241575909639
## [37] Train-rmse=8.40847217331365
## [38] Train-rmse=8.39476931796395
## [39] Train-rmse=8.38129658373974
## [40] Train-rmse=8.36804269059018
## [41] Train-rmse=8.35499817678397
## [42] Train-rmse=8.34215505742154
## [43] Train-rmse=8.32950441908131
## [44] Train-rmse=8.31703985777311
## [45] Train-rmse=8.30475363906755
## [46] Train-rmse=8.29264031506106
## [47] Train-rmse=8.28069372820073
## [48] Train-rmse=8.26890902770415
## [49] Train-rmse=8.25728089053853
## [50] Train-rmse=8.24580511500735
</code></pre></div>
<p>It&#39;s also easy to make a prediction and evaluate it:</p>
<div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">preds</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">predict</span><span class="p">(</span><span class="n">model</span><span class="p">,</span><span class="w"> </span><span class="n">test.x</span><span class="p">)</span><span class="w">
</span></code></pre></div><div class="highlight"><pre><code class="language-" data-lang=""> ## Auto detect layout of input matrix, use rowmajor..
</code></pre></div><div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="nf">sqrt</span><span class="p">(</span><span class="n">mean</span><span class="p">((</span><span class="n">preds</span><span class="o">-</span><span class="n">test.y</span><span class="p">)</span><span class="o">^</span><span class="m">2</span><span class="p">))</span><span class="w">
</span></code></pre></div><div class="highlight"><pre><code class="language-" data-lang=""> ## [1] 7.800502
</code></pre></div>
<p>Currently, we have four predefined metrics: &quot;accuracy&quot;, &quot;rmse&quot;, &quot;mae&quot;, and &quot;rmsle&quot;. MXNet provides the interface for defining your own metrics:</p>
<div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">demo.metric.mae</span><span class="w"> </span><span class="o">&lt;-</span><span class="w"> </span><span class="n">mx.metric.custom</span><span class="p">(</span><span class="s2">"mae"</span><span class="p">,</span><span class="w"> </span><span class="k">function</span><span class="p">(</span><span class="n">label</span><span class="p">,</span><span class="w"> </span><span class="n">pred</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w">
</span><span class="n">pred</span><span class="w"> </span><span class="o">&lt;-</span><span class="w"> </span><span class="n">mx.nd.reshape</span><span class="p">(</span><span class="n">pred</span><span class="p">,</span><span class="w"> </span><span class="n">shape</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="m">0</span><span class="p">)</span><span class="w">
</span><span class="n">res</span><span class="w"> </span><span class="o">&lt;-</span><span class="w"> </span><span class="n">mx.nd.mean</span><span class="p">(</span><span class="n">mx.nd.abs</span><span class="p">(</span><span class="n">label</span><span class="o">-</span><span class="n">pred</span><span class="p">))</span><span class="w">
</span><span class="nf">return</span><span class="p">(</span><span class="n">res</span><span class="p">)</span><span class="w">
</span><span class="p">})</span><span class="w">
</span></code></pre></div>
<p>This is an example of the mean absolute error metric. Simply plug it into the training function:</p>
<div class="highlight"><pre><code class="language-r" data-lang="r"><span class="w"> </span><span class="n">mx.set.seed</span><span class="p">(</span><span class="m">0</span><span class="p">)</span><span class="w">
</span><span class="n">model</span><span class="w"> </span><span class="o">&lt;-</span><span class="w"> </span><span class="n">mx.model.FeedForward.create</span><span class="p">(</span><span class="n">lro</span><span class="p">,</span><span class="w"> </span><span class="n">X</span><span class="o">=</span><span class="n">train.x</span><span class="p">,</span><span class="w"> </span><span class="n">y</span><span class="o">=</span><span class="n">train.y</span><span class="p">,</span><span class="w">
</span><span class="n">ctx</span><span class="o">=</span><span class="n">mx.cpu</span><span class="p">(),</span><span class="w"> </span><span class="n">num.round</span><span class="o">=</span><span class="m">50</span><span class="p">,</span><span class="w"> </span><span class="n">array.batch.size</span><span class="o">=</span><span class="m">20</span><span class="p">,</span><span class="w">
</span><span class="n">learning.rate</span><span class="o">=</span><span class="m">2e-6</span><span class="p">,</span><span class="w"> </span><span class="n">momentum</span><span class="o">=</span><span class="m">0.9</span><span class="p">,</span><span class="w"> </span><span class="n">eval.metric</span><span class="o">=</span><span class="n">demo.metric.mae</span><span class="p">)</span><span class="w">
</span></code></pre></div><div class="highlight"><pre><code class="language-" data-lang=""> ## Auto detect layout of input matrix, use rowmajor.
## Start training with 1 devices
## [1] Train-mae=14.953625731998
## [2] Train-mae=11.4802955521478
## [3] Train-mae=8.50700579749213
## [4] Train-mae=7.30591265360514
## [5] Train-mae=7.38049803839789
## [6] Train-mae=7.36036252975464
## [7] Train-mae=7.06519222259521
## [8] Train-mae=6.9962231847975
## [9] Train-mae=6.96296903822157
## [10] Train-mae=6.9046172036065
## [11] Train-mae=6.87867620256212
## [12] Train-mae=6.85872554779053
## [13] Train-mae=6.81936407089233
## [14] Train-mae=6.79135354359945
## [15] Train-mae=6.77438741260105
## [16] Train-mae=6.75365140702989
## [17] Train-mae=6.73369296391805
## [18] Train-mae=6.71600982877943
## [19] Train-mae=6.69932826360067
## [20] Train-mae=6.6852519777086
## [21] Train-mae=6.67343420452542
## [22] Train-mae=6.66315894656711
## [23] Train-mae=6.65314838621351
## [24] Train-mae=6.64388704299927
## [25] Train-mae=6.63480265935262
## [26] Train-mae=6.62583245171441
## [27] Train-mae=6.61697626113892
## [28] Train-mae=6.60842116673787
## [29] Train-mae=6.60040124257406
## [30] Train-mae=6.59264140658908
## [31] Train-mae=6.58551020092434
## [32] Train-mae=6.57864215638902
## [33] Train-mae=6.57178926467896
## [34] Train-mae=6.56495311525133
## [35] Train-mae=6.55813185373942
## [36] Train-mae=6.5513252152337
## [37] Train-mae=6.54453214009603
## [38] Train-mae=6.53775374094645
## [39] Train-mae=6.53098879920112
## [40] Train-mae=6.52423816257053
## [41] Train-mae=6.51764053768582
## [42] Train-mae=6.51121346155802
## [43] Train-mae=6.5047902001275
## [44] Train-mae=6.49837123023139
## [45] Train-mae=6.49216641320123
## [46] Train-mae=6.48598252402412
## [47] Train-mae=6.4798010720147
## [48] Train-mae=6.47362396452162
## [49] Train-mae=6.46745183732775
## [50] Train-mae=6.46128723356459
</code></pre></div>
<p>Congratulations! You&#39;ve learned the basics for using MXNet in R. To learn how to use MXNet&#39;s advanced features, see the other tutorials.</p>
<h2 id="next-steps">Next Steps</h2>
<ul>
<li><a href="https://mxnet.io/tutorials/r/classifyRealImageWithPretrainedModel.html">Classify Real-World Images with Pre-trained Model</a></li>
<li><a href="https://mxnet.io/tutorials/r/mnistCompetition.html">Handwritten Digits Classification Competition</a></li>
<li><a href="https://mxnet.io/tutorials/r/charRnnModel.html">Character Language Model using RNN</a></li>
</ul>
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