title: Implementing custom loss function in keras with condition
body: <p>I need some help with keras loss function. I have been implementing custom loss function on keras with Tensorflow backend. </p>
  
  <p>I have implemented the custom loss function in numpy but it would be great if it could be translated into keras loss function. The loss function takes dataframe and series of user id. The Euclidean distance for same user_id are positive and negative if the user_id are different. The function returns summed up scalar distance of the dataframe.</p>
  
  <pre><code>def custom_loss_numpy (encodings, user_id):
  # user_id: a pandas series of users
  # encodings: a pandas dataframe of encodings
  
      batch_dist = 0
  
      for i in range(len(user_id)):
           first_row = encodings.iloc[i,:].values
           first_user = user_id[i]
  
           for j in range(i+1, len(user_id)):
                second_user = user_id[j]
                second_row = encodings.iloc[j,:].values
  
          # compute distance: if the users are same then Euclidean distance is positive otherwise negative.
              if first_user == second_user:
                  tmp_dist = np.linalg.norm(first_row - second_row)
              else:
                  tmp_dist = -np.linalg.norm(first_row - second_row)
  
              batch_dist += tmp_dist
  
      return batch_dist
  </code></pre>
  
  <p>I have tried to implement into keras loss function. I extracted numpy array from y_true and y_pred tensor objects. </p>
  
  <pre><code>def custom_loss_keras(y_true, y_pred):
      # session of my program
      sess = tf_session.TF_Session().get()
  
      with sess.as_default():
          array_pred = y_pred.eval()
          print(array_pred)
  </code></pre>
  
  <p>But I get the following error. </p>
  
  <pre><code>tensorflow.python.framework.errors_impl.InvalidArgumentError: You must feed a value for placeholder tensor 'dense_1_input' with dtype float and shape [?,102]
   [[Node: dense_1_input = Placeholder[dtype=DT_FLOAT, shape=[?,102], _device="/job:localhost/replica:0/task:0/device:CPU:0"]()]]
  </code></pre>
  
  <p>Any kind of help would be really appreciated. </p>