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  <div class="section" id="the-ifelse-operator">
<span id="the-ifelse-operator"></span><h1>The <code class="docutils literal"><span class="pre">IfElse</span></code> Operator<a class="headerlink" href="#the-ifelse-operator" title="永久链接至标题">¶</a></h1>
<p>PaddlePaddle&#8217;s <code class="docutils literal"><span class="pre">IfElse</span></code> operator differs from TensorFlow&#8217;s:</p>
<ul class="simple">
<li>the TensorFlow version takes a scalar boolean value as the condition so that the whole mini-batch goes to either the true or the false branch, whereas</li>
<li>the PaddlePaddle version takes a vector of boolean value as the condition, and instances corresponding to true values go to the true branch, those corresponding to false values go to the false branch.</li>
</ul>
<div class="section" id="example">
<span id="example"></span><h2>Example<a class="headerlink" href="#example" title="永久链接至标题">¶</a></h2>
<p>The following PaddlePaddle program shows the usage of the IfElse operator:</p>
<div class="highlight-python"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">paddle</span> <span class="kn">as</span> <span class="nn">pd</span>

<span class="n">x</span> <span class="o">=</span> <span class="n">minibatch</span><span class="p">([</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">,</span> <span class="mi">30</span><span class="p">])</span> <span class="c1"># shape=[None, 1]</span>
<span class="n">y</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="c1"># shape=[1], value=1</span>
<span class="n">z</span> <span class="o">=</span> <span class="n">minibatch</span><span class="p">([</span><span class="mi">10</span><span class="p">,</span> <span class="mi">20</span><span class="p">,</span> <span class="mi">30</span><span class="p">])</span> <span class="c1"># shape=[None, 1]</span>
<span class="n">cond</span> <span class="o">=</span> <span class="n">larger_than</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="mi">15</span><span class="p">)</span> <span class="c1"># [false, true, true]</span>

<span class="n">ie</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">ifelse</span><span class="p">()</span>
<span class="k">with</span> <span class="n">ie</span><span class="o">.</span><span class="n">true_block</span><span class="p">():</span>
    <span class="n">d</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">layer</span><span class="o">.</span><span class="n">add</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">)</span>
    <span class="n">ie</span><span class="o">.</span><span class="n">output</span><span class="p">(</span><span class="n">d</span><span class="p">,</span> <span class="n">pd</span><span class="o">.</span><span class="n">layer</span><span class="o">.</span><span class="n">softmax</span><span class="p">(</span><span class="n">d</span><span class="p">))</span>
<span class="k">with</span> <span class="n">ie</span><span class="o">.</span><span class="n">false_block</span><span class="p">():</span>
    <span class="n">d</span> <span class="o">=</span> <span class="n">pd</span><span class="o">.</span><span class="n">layer</span><span class="o">.</span><span class="n">fc</span><span class="p">(</span><span class="n">z</span><span class="p">)</span>
    <span class="n">ie</span><span class="o">.</span><span class="n">output</span><span class="p">(</span><span class="n">d</span><span class="p">,</span> <span class="n">d</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span>
<span class="n">o1</span><span class="p">,</span> <span class="n">o2</span> <span class="o">=</span> <span class="n">ie</span><span class="p">(</span><span class="n">cond</span><span class="p">)</span>
</pre></div>
</div>
<p>A challenge to implement the <code class="docutils literal"><span class="pre">IfElse</span></code> operator is to infer those variables to be split, or, say, to identify the variable of the mini-batch or those derived from the mini-batch.</p>
<p>An equivalent C++ program is as follows:</p>
<div class="highlight-c++"><div class="highlight"><pre><span></span><span class="k">namespace</span> <span class="n">pd</span> <span class="o">=</span> <span class="n">paddle</span><span class="p">;</span>

<span class="kt">int</span> <span class="n">x</span> <span class="o">=</span> <span class="mi">10</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">y</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">z</span> <span class="o">=</span> <span class="mi">10</span><span class="p">;</span>
<span class="kt">bool</span> <span class="n">cond</span> <span class="o">=</span> <span class="nb">false</span><span class="p">;</span>
<span class="kt">int</span> <span class="n">o1</span><span class="p">,</span> <span class="n">o2</span><span class="p">;</span>
<span class="k">if</span> <span class="p">(</span><span class="n">cond</span><span class="p">)</span> <span class="p">{</span>
  <span class="kt">int</span> <span class="n">d</span> <span class="o">=</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span><span class="p">;</span>
  <span class="n">o1</span> <span class="o">=</span> <span class="n">z</span><span class="p">;</span>
  <span class="n">o2</span> <span class="o">=</span> <span class="n">pd</span><span class="o">::</span><span class="n">layer</span><span class="o">::</span><span class="n">softmax</span><span class="p">(</span><span class="n">z</span><span class="p">);</span>
<span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
  <span class="kt">int</span> <span class="n">d</span> <span class="o">=</span> <span class="n">pd</span><span class="o">::</span><span class="n">layer</span><span class="o">::</span><span class="n">fc</span><span class="p">(</span><span class="n">z</span><span class="p">);</span>
  <span class="n">o1</span> <span class="o">=</span> <span class="n">d</span><span class="p">;</span>
  <span class="n">o2</span> <span class="o">=</span> <span class="n">d</span><span class="o">+</span><span class="mi">1</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
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