title: tensorflow how to get prediction
body: <p>I want to compute predictions, using the following code:</p>
  
  <pre class="lang-py prettyprint-override"><code>import tensorflow as tf
  
  x = tf.placeholder("float", [None, n_input])
  y = tf.placeholder("float", [None, n_classes])
  
  pred = multilayer_perceptron(x, weights, biases)
  
  
  cost = tf.reduce_mean(tf.nn.softmax_cross_entropy_with_logits(pred, y))
  optimizer = tf.train.AdamOptimizer(learning_rate=learning_rate).minimize(cost)
  
  # Initializing the variables
  
  ##trn.txt start
  
  ##tst.txt end
  with tf.Session() as sess:
      sess.run(init)
  
      # Training cycle
      for epoch in range(training_epochs):
          avg_cost = 0.
          total_batch = int(num_lines_trn/batch_size)
          # Loop over all batches
          for i in range(total_batch):
              batch_x, batch_y = bat_x[i*batch_size:(i+1)*batch_size],bat_y[i*batch_size:(i+1)*batch_size]#mnist.train.next_batch(batch_size)
              # Run optimization op (backprop) and cost op (to get loss value)
              _, c = sess.run([optimizer, cost], feed_dict={x: batch_x,
                                                            y: batch_y})
              # Compute average loss
              avg_cost += c / total_batch
  
      correct_prediction = tf.equal(tf.argmax(pred, 1), tf.argmax(y, 1))
  
      accuracy = tf.reduce_mean(tf.cast(correct_prediction, "float"))
  
      print(sess.run(accuracy, feed_dict={x: tst_x, y: tst_y}))
      print(sess.run(accuracy, feed_dict={x: tst_x}))
  </code></pre>
  
  <p>The line </p>
  
  <pre><code>print(sess.run(accuracy, feed_dict={x: tst_x, y: tst_y}))
  </code></pre>
  
  <p>returns <code>0.80353</code> which is the accuracy for the batch.</p>
  
  <p>However I want to get prediction result. so I added:</p>
  
  <pre><code>print(sess.run(accuracy, feed_dict={x: tst_x}))
  </code></pre>
  
  <p>But this line returns an error:</p>
  
  <blockquote>
    <p>You must feed a value for placeholder tensor 'Placeholder_7' with
    dtype float</p>
  </blockquote>
  
  <p>How can I solve this problem?</p>