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    <li>Design for TensorArray</li>
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  <div class="section" id="design-for-tensorarray">
<span id="design-for-tensorarray"></span><h1>Design for TensorArray<a class="headerlink" href="#design-for-tensorarray" title="Permalink to this headline"></a></h1>
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<p>This design doc presents the necessity of a new C++ class <code class="docutils literal"><span class="pre">TensorArray</span></code>.
In addition to the very simple C++ implementation</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">TensorArray</span> <span class="p">{</span>
 <span class="k">public</span><span class="o">:</span>
  <span class="k">explicit</span> <span class="n">TensorArray</span><span class="p">(</span><span class="k">const</span> <span class="n">LoDTensor</span><span class="o">&amp;</span><span class="p">);</span>
  <span class="k">explicit</span> <span class="nf">TensorArray</span><span class="p">(</span><span class="kt">size_t</span> <span class="n">size</span><span class="p">);</span>

 <span class="k">private</span><span class="o">:</span>
  <span class="n">vector</span><span class="o">&lt;</span><span class="n">LoDTensor</span><span class="o">&gt;</span> <span class="n">values_</span><span class="p">;</span>
<span class="p">};</span>
</pre></div>
</div>
<p>We also need to expose it to PaddlePaddle&#8217;s Python API,
because users would want to use it with our very flexible operators <code class="docutils literal"><span class="pre">WhileLoop</span></code>.
An example for a RNN based on dynamic operators is</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="nb">input</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">data</span><span class="p">(</span><span class="o">...</span><span class="p">)</span>
<span class="n">num_steps</span> <span class="o">=</span> <span class="n">Var</span><span class="p">(</span><span class="mi">12</span><span class="p">)</span>

<span class="n">TensorArray</span> <span class="n">states</span><span class="p">(</span><span class="n">size</span><span class="o">=</span><span class="n">num_steps</span><span class="p">)</span>
<span class="n">TensorArray</span> <span class="n">step_inputs</span><span class="p">(</span><span class="n">unstack_from</span><span class="o">=</span><span class="nb">input</span><span class="p">)</span>
<span class="n">TensorArray</span> <span class="n">step_outputs</span><span class="p">(</span><span class="n">size</span><span class="o">=</span><span class="n">num_steps</span><span class="p">)</span>

<span class="n">W</span> <span class="o">=</span> <span class="n">Tensor</span><span class="p">(</span><span class="o">...</span><span class="p">)</span>
<span class="n">U</span> <span class="o">=</span> <span class="n">Tensor</span><span class="p">(</span><span class="o">...</span><span class="p">)</span>
<span class="n">default_state</span> <span class="o">=</span> <span class="n">some_op</span><span class="p">()</span>

<span class="n">step</span> <span class="o">=</span> <span class="n">Var</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>

<span class="n">wloop</span> <span class="o">=</span> <span class="n">paddle</span><span class="o">.</span><span class="n">create_whileloop</span><span class="p">(</span><span class="n">loop_vars</span><span class="o">=</span><span class="p">[</span><span class="n">step</span><span class="p">])</span>
<span class="k">with</span> <span class="n">wloop</span><span class="o">.</span><span class="n">frame</span><span class="p">():</span>
    <span class="n">wloop</span><span class="o">.</span><span class="n">break_if</span><span class="p">(</span><span class="n">pd</span><span class="o">.</span><span class="n">equal</span><span class="p">(</span><span class="n">step</span><span class="p">,</span> <span class="n">num_steps</span><span class="p">)</span>
    <span class="n">pre_state</span> <span class="o">=</span> <span class="n">states</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="n">step</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="n">default_state</span><span class="p">)</span>
    <span class="n">step_input</span> <span class="o">=</span> <span class="n">step_inputs</span><span class="o">.</span><span class="n">read</span><span class="p">(</span><span class="n">step</span><span class="p">)</span>
    <span class="n">state</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">sigmoid</span><span class="p">(</span><span class="n">pd</span><span class="o">.</span><span class="n">matmul</span><span class="p">(</span><span class="n">U</span><span class="p">,</span> <span class="n">pre_state</span><span class="p">)</span> <span class="o">+</span> <span class="n">pd</span><span class="o">.</span><span class="n">matmul</span><span class="p">(</span><span class="n">W</span><span class="p">,</span> <span class="n">step_input</span><span class="p">))</span>
    <span class="n">states</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">step</span><span class="p">,</span> <span class="n">state</span><span class="p">)</span>
    <span class="n">step_outputs</span><span class="o">.</span><span class="n">write</span><span class="p">(</span><span class="n">step</span><span class="p">,</span> <span class="n">state</span><span class="p">)</span> <span class="c1"># output state</span>
    <span class="n">step</span><span class="o">.</span><span class="n">update</span><span class="p">(</span><span class="n">state</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span>

<span class="n">output</span> <span class="o">=</span> <span class="n">step_outputs</span><span class="o">.</span><span class="n">stack</span><span class="p">()</span>
</pre></div>
</div>
<div class="section" id="background">
<span id="background"></span><h2>Background<a class="headerlink" href="#background" title="Permalink to this headline"></a></h2>
<p>Steps are one of the core concepts of RNN. In each time step of RNN, there should be several input segments, states, and output segments; all these components act like arrays, for example, call <code class="docutils literal"><span class="pre">states[step_id]</span></code> will get the state in <code class="docutils literal"><span class="pre">step_id</span></code>th time step.</p>
<p>An RNN can be implemented with the following pseudocode</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="n">Array</span> <span class="n">states</span><span class="p">;</span>
<span class="n">Array</span> <span class="n">input_segments</span><span class="p">;</span>
<span class="n">Array</span> <span class="n">output_segments</span><span class="p">;</span>
<span class="n">Parameter</span> <span class="n">W</span><span class="p">,</span> <span class="n">U</span><span class="p">;</span>

<span class="n">step</span> <span class="o">=</span> <span class="mi">1</span>
<span class="n">seq_len</span> <span class="o">=</span> <span class="mi">12</span>
<span class="n">while_loop</span> <span class="p">{</span>
   <span class="k">if</span> <span class="p">(</span><span class="n">step</span> <span class="o">==</span> <span class="n">seq_len</span><span class="p">)</span> <span class="k">break</span><span class="p">;</span>
    <span class="n">states</span><span class="p">[</span><span class="n">step</span><span class="p">]</span> <span class="o">=</span> <span class="n">sigmoid</span><span class="p">(</span><span class="n">W</span> <span class="o">*</span> <span class="n">states</span><span class="p">[</span><span class="n">step</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span> <span class="o">+</span> <span class="n">U</span> <span class="o">*</span> <span class="n">input_segments</span><span class="p">[</span><span class="n">step</span><span class="p">]);</span>
    <span class="n">output_segments</span><span class="p">[</span><span class="n">step</span><span class="p">]</span> <span class="o">=</span> <span class="n">states</span><span class="p">[</span><span class="n">step</span><span class="p">]</span> <span class="c1">// take state as output</span>
   <span class="n">step</span><span class="o">++</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>According to the <a class="reference external" href="https://github.com/PaddlePaddle/Paddle/issues/4561">RNN roadmap</a>, there are several different RNNs that PaddlePaddle will eventually support.</p>
<p>Currently, the basic RNN implementation supported by PaddlePaddle is the <code class="docutils literal"><span class="pre">recurrent_op</span></code> which takes tensors as input and splits them into <code class="docutils literal"><span class="pre">input_segments</span></code>.</p>
<p>Since a tensor cannot store variable-length sequences directly, PaddlePaddle implements the tensor with level of details (<code class="docutils literal"><span class="pre">LoDTensor</span></code> for short).
Segmenting the <code class="docutils literal"><span class="pre">LoDTensor</span></code> is much more complicated than splitting a tensor, that makes it necessary to refactor the <code class="docutils literal"><span class="pre">recurrent_op</span></code> with <code class="docutils literal"><span class="pre">LoDTensor</span></code> segmenting support.</p>
<p>As the next step in RNN support, <code class="docutils literal"><span class="pre">dynamic_recurrent_op</span></code> should be introduced to handle inputs with variable-length sequences.</p>
<p>The implementation is similar to <code class="docutils literal"><span class="pre">recurrent_op</span></code>.
The key difference is the way <strong>the original input <code class="docutils literal"><span class="pre">LoDTensors</span></code> and outupts are split to get the <code class="docutils literal"><span class="pre">input_segments</span></code> and the <code class="docutils literal"><span class="pre">output_segments</span></code>.</strong></p>
<p>Though it can&#8217;t be built over <code class="docutils literal"><span class="pre">recurrent_op</span></code> or <code class="docutils literal"><span class="pre">dynamic_recurrent_op</span></code> directly,
the logic behind splitting a tensor or a LoD tensor into <code class="docutils literal"><span class="pre">input_segments</span></code> remains the same.</p>
</div>
<div class="section" id="why-tensorarray">
<span id="why-tensorarray"></span><h2>Why <code class="docutils literal"><span class="pre">TensorArray</span></code><a class="headerlink" href="#why-tensorarray" title="Permalink to this headline"></a></h2>
<p>The logic behind splitting the inputs to segments, states and outputs is similar and can be shared in a seperate module.</p>
<p>The array of <code class="docutils literal"><span class="pre">states</span></code>, <code class="docutils literal"><span class="pre">input_segments</span></code> and <code class="docutils literal"><span class="pre">output_segments</span></code> would be exposed to users when writing a dynamic RNN model similar to the above pseudo codes.</p>
<p>So there should be an array-like container, which can store the segments of a tensor or LoD tensor.</p>
<p><strong>This container can store an array of tensors and provides several methods to split a tensor or a LoD tensor</strong> .
This is where the notion of <code class="docutils literal"><span class="pre">TensorArray</span></code> comes from.</p>
</div>
<div class="section" id="introduce-tensorarray-to-uniform-all-the-three-rnns">
<span id="introduce-tensorarray-to-uniform-all-the-three-rnns"></span><h2>Introduce TensorArray to uniform all the three RNNs<a class="headerlink" href="#introduce-tensorarray-to-uniform-all-the-three-rnns" title="Permalink to this headline"></a></h2>
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<p>TensorArray as a new concept is borrowed from TensorFlow,
it is meant to be used with dynamic iteration primitives such as <code class="docutils literal"><span class="pre">while_loop</span></code> and <code class="docutils literal"><span class="pre">map_fn</span></code>.</p>
<p>This concept can be used to support our new design of dynamic operations, and help to refactor some existing variant-sentence-related layers,
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such as <code class="docutils literal"><span class="pre">recurrent_op</span></code>, <code class="docutils literal"><span class="pre">RecurrentGradientMachine</span></code>.</p>
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<p>In <a class="reference external" href="https://github.com/PaddlePaddle/Paddle/pull/4401">our design for dynamic RNN</a>,
<code class="docutils literal"><span class="pre">TensorArray</span></code> is used to segment inputs and store states in all time steps.
By providing some methods similar to a C++ array,
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the definition of some state-based dynamic models such as RNN can be more natural and highly flexible.</p>
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</div>
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<div class="section" id="dynamic-operations-on-tensorarray">
<span id="dynamic-operations-on-tensorarray"></span><h2>Dynamic-operations on TensorArray<a class="headerlink" href="#dynamic-operations-on-tensorarray" title="Permalink to this headline"></a></h2>
<p><code class="docutils literal"><span class="pre">TensorArray</span></code> will be used directly when defining dynamic models, so some operators listed below should be implemented</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="c1"># several helper operators for TensorArray</span>
<span class="k">def</span> <span class="nf">tensor_array_stack</span><span class="p">(</span><span class="n">ta</span><span class="p">,</span> <span class="n">tensor</span><span class="p">):</span>
    <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">    get a tensor array `ta`, return a packed `tensor`.</span>
<span class="sd">    &#39;&#39;&#39;</span>
    <span class="k">pass</span>

<span class="k">def</span> <span class="nf">tensor_array_unstack</span><span class="p">(</span><span class="n">tensor</span><span class="p">,</span> <span class="n">ta</span><span class="p">):</span>
    <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">    get a `tensor`, unstack it and get a tensor array `ta`.</span>
<span class="sd">    &#39;&#39;&#39;</span>
    <span class="k">pass</span>

<span class="k">def</span> <span class="nf">tensor_array_write</span><span class="p">(</span><span class="n">ta</span><span class="p">,</span> <span class="n">index</span><span class="p">,</span> <span class="n">tensor</span><span class="p">,</span> <span class="n">data_shared</span><span class="p">):</span>
    <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">    get a `tensor` and a scalar tensor `index`, write `tensor` into index-th</span>
<span class="sd">    value of the tensor array `ta`.</span>
<span class="sd">    `data_shared` is an attribute that specifies whether to copy or reference the tensors.</span>
<span class="sd">    &#39;&#39;&#39;</span>
    <span class="k">pass</span>

<span class="k">def</span> <span class="nf">tensor_array_read</span><span class="p">(</span><span class="n">ta</span><span class="p">,</span> <span class="n">index</span><span class="p">,</span> <span class="n">tensor</span><span class="p">):</span>
    <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">    get a tensor array `ta`, a scalar tensor `index`, read the index-th value of</span>
<span class="sd">    `ta` and return as the `tensor`.</span>
<span class="sd">    &#39;&#39;&#39;</span>
    <span class="k">pass</span>

<span class="k">def</span> <span class="nf">tensor_array_size</span><span class="p">(</span><span class="n">ta</span><span class="p">,</span> <span class="n">tensor</span><span class="p">):</span>
    <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">    get a tensor array `ta`, return the size of `ta` and return as the scalar `tensor`.</span>
<span class="sd">    &#39;&#39;&#39;</span>
    <span class="k">pass</span>
</pre></div>
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</div>
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<p>It is trivial for users to use so many low-level operators, so some helper methods should be proposed in python wrapper to make <code class="docutils literal"><span class="pre">TensorArray</span></code> easier to use,
for example</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">TensorArray</span><span class="p">:</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">name</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">desc</span> <span class="o">=</span> <span class="n">TensorArrayDesc</span><span class="p">()</span>

    <span class="k">def</span> <span class="nf">stack</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="o">=</span><span class="bp">None</span><span class="p">):</span>
        <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">        Pack the values in a `TensorArray` into a tensor with rank one higher</span>
<span class="sd">        than each tensor in `values`.</span>
<span class="sd">        `stack` can be used to split tensor into time steps for RNN or whileloop.</span>

<span class="sd">        @name: str</span>
<span class="sd">            the name of the variable to output.</span>
<span class="sd">        &#39;&#39;&#39;</span>
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        <span class="n">tensor</span> <span class="o">=</span> <span class="n">Var</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
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        <span class="n">tensor_array_stack</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">tensor</span><span class="p">)</span>
        <span class="k">return</span> <span class="n">tensor</span>

    <span class="k">def</span> <span class="nf">unstack</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">input</span><span class="p">):</span>
        <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">        Unpacks the given dimension of a rank-`R` tensor into rank-`(R-1)` tensors.</span>
<span class="sd">        `unstack` can be used to concatenate all the time steps for RNN or whileloop.</span>

<span class="sd">        @input: str</span>
<span class="sd">            the name of input tensor</span>
<span class="sd">        &#39;&#39;&#39;</span>
        <span class="n">tensor_array_unstack</span><span class="p">(</span><span class="n">tensor</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">write</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">index</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">data_shared</span><span class="o">=</span><span class="bp">True</span><span class="p">):</span>
        <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">        Write value into index of the TensorArray.</span>
<span class="sd">        If `data_shared` is set to True, than the index-th value in TensorArray will</span>
<span class="sd">        be shared with the tensor passed in.</span>

<span class="sd">        @index: str</span>
<span class="sd">            name of a scalar tensor</span>
<span class="sd">        @value: str</span>
<span class="sd">            name of a tensor</span>
<span class="sd">        @data_shared: bool</span>
<span class="sd">        &#39;&#39;&#39;</span>
        <span class="n">tensor_array_write</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">index</span><span class="p">,</span> <span class="n">value</span><span class="p">,</span> <span class="n">data_shared</span><span class="p">)</span>

    <span class="k">def</span> <span class="nf">read</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">index</span><span class="p">,</span> <span class="n">output</span><span class="p">):</span>
        <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">        Read the value at location `index` in the `TensorArray`.</span>

<span class="sd">        @index: str</span>
<span class="sd">            name of a scalar tensor</span>
<span class="sd">        @output:</span>
<span class="sd">            name of a output variable</span>
<span class="sd">        &#39;&#39;&#39;</span>
        <span class="n">tensor_array_read</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">index</span><span class="p">,</span> <span class="n">output</span><span class="p">)</span>


    <span class="k">def</span> <span class="nf">size</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">output</span><span class="p">):</span>
        <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">        Return the number of values.</span>

<span class="sd">        @output: str</span>
<span class="sd">            name of a scalar tensor</span>
<span class="sd">        &#39;&#39;&#39;</span>
        <span class="n">tensor_array_size</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">,</span> <span class="n">output</span><span class="p">)</span>
</pre></div>
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</div>
</div>
<div class="section" id="lodtensor-related-supports">
<span id="lodtensor-related-supports"></span><h2>LoDTensor-related Supports<a class="headerlink" href="#lodtensor-related-supports" title="Permalink to this headline"></a></h2>
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<p>The <code class="docutils literal"><span class="pre">RecurrentGradientMachine</span></code> in Paddle serves as a flexible RNN layer; it takes varience-length sequences as input, and output sequences too.</p>
<p>Since each step of RNN can only take a tensor-represented batch of data as input,
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some preprocess should be taken on the inputs such as sorting the sentences by their length in descending order and cut each word and pack to new batches.</p>
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<p>Such cut-like operations can be embedded into <code class="docutils literal"><span class="pre">TensorArray</span></code> as general methods called <code class="docutils literal"><span class="pre">unpack</span></code> and <code class="docutils literal"><span class="pre">pack</span></code>,
these two operations are similar to <code class="docutils literal"><span class="pre">stack</span></code> and <code class="docutils literal"><span class="pre">unstack</span></code> except that they operate on variable-length sequences formated as a LoD tensor rather than a tensor.</p>
<p>Some definitions are like</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">unpack</span><span class="p">(</span><span class="n">level</span><span class="p">):</span>
    <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">    Split LodTensor in some `level` and generate batches, if set `sort_by_length`,</span>
<span class="sd">    will sort by length.</span>

<span class="sd">    Returns:</span>
<span class="sd">        - a new `TensorArray`, whose values are LodTensors and represents batches</span>
<span class="sd">          of data.</span>
<span class="sd">        - an int32 Tensor, which stores the map from the new batch&#39;s indices to</span>
<span class="sd">          original LoDTensor</span>
<span class="sd">    &#39;&#39;&#39;</span>
    <span class="k">pass</span>

<span class="k">def</span> <span class="nf">pack</span><span class="p">(</span><span class="n">level</span><span class="p">,</span> <span class="n">indices_map</span><span class="p">):</span>
    <span class="sd">&#39;&#39;&#39;</span>
<span class="sd">    Recover the original LoD-arranged LoDTensor with the values in a `TensorArray`</span>
<span class="sd">    and `level` and `indices_map`.</span>
<span class="sd">    &#39;&#39;&#39;</span>
    <span class="k">pass</span>
</pre></div>
</div>
<p>With these two methods, a varience-length sentence supported RNN can be implemented like</p>
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<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="c1">// input is the varient-length data</span>
<span class="n">LodTensor</span> <span class="nf">sentence_input</span><span class="p">(</span><span class="n">xxx</span><span class="p">);</span>
<span class="n">TensorArray</span> <span class="n">ta</span><span class="p">;</span>
<span class="n">Tensor</span> <span class="n">indice_map</span><span class="p">;</span>
<span class="n">Tensor</span> <span class="n">boot_state</span> <span class="o">=</span> <span class="n">xxx</span><span class="p">;</span> <span class="c1">// to initialize rnn&#39;s first state</span>
<span class="n">TensorArray</span><span class="o">::</span><span class="n">unpack</span><span class="p">(</span><span class="n">input</span><span class="p">,</span> <span class="mi">1</span><span class="cm">/*level*/</span><span class="p">,</span> <span class="nb">true</span><span class="cm">/*sort_by_length*/</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">ta</span><span class="p">,</span> <span class="o">&amp;</span><span class="n">indice_map</span><span class="p">);</span>
<span class="n">TessorArray</span> <span class="n">step_outputs</span><span class="p">;</span>
<span class="n">TensorArray</span> <span class="n">states</span><span class="p">;</span>

<span class="k">for</span> <span class="p">(</span><span class="kt">int</span> <span class="n">step</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">step</span> <span class="o">=</span> <span class="n">ta</span><span class="p">.</span><span class="n">size</span><span class="p">();</span> <span class="n">step</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span>
  <span class="k">auto</span> <span class="n">state</span> <span class="o">=</span> <span class="n">states</span><span class="p">.</span><span class="n">read</span><span class="p">(</span><span class="n">step</span><span class="p">);</span>
  <span class="c1">// rnnstep is a function which acts like a step of RNN</span>
  <span class="k">auto</span> <span class="n">step_input</span> <span class="o">=</span> <span class="n">ta</span><span class="p">.</span><span class="n">read</span><span class="p">(</span><span class="n">step</span><span class="p">);</span>
  <span class="k">auto</span> <span class="n">step_output</span> <span class="o">=</span> <span class="n">rnnstep</span><span class="p">(</span><span class="n">step_input</span><span class="p">,</span> <span class="n">state</span><span class="p">);</span>
  <span class="n">step_outputs</span><span class="p">.</span><span class="n">write</span><span class="p">(</span><span class="n">step_output</span><span class="p">,</span> <span class="nb">true</span><span class="cm">/*data_shared*/</span><span class="p">);</span>
<span class="p">}</span>

<span class="c1">// rnn_output is the final output of an rnn</span>
<span class="n">LoDTensor</span> <span class="n">rnn_output</span> <span class="o">=</span> <span class="n">ta</span><span class="p">.</span><span class="n">pack</span><span class="p">(</span><span class="n">ta</span><span class="p">,</span> <span class="n">indice_map</span><span class="p">);</span>
</pre></div>
</div>
<p>the code above shows that by embedding the LoDTensor-related preprocess operations into <code class="docutils literal"><span class="pre">TensorArray</span></code>,
the implementation of a RNN that supports varient-length sentences is far more concise than <code class="docutils literal"><span class="pre">RecurrentGradientMachine</span></code> because the latter mixes all the codes together, hard to read and extend.</p>
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