diff --git a/examples/.gitignore b/examples/.gitignore
index 15584ca..2a48b2c 100644
--- a/examples/.gitignore
+++ b/examples/.gitignore
@@ -1 +1,2 @@
 .pyc
+logs/
diff --git a/examples/ppo_cartpole.py b/examples/ppo_cartpole.py
index 6fc986f..418cc52 100755
--- a/examples/ppo_cartpole.py
+++ b/examples/ppo_cartpole.py
@@ -2,6 +2,8 @@
 from __future__ import absolute_import
 
 import tensorflow as tf
+import time
+import numpy as np
 
 # our lib imports here! It's ok to append path in examples
 import sys
@@ -39,7 +41,11 @@ if __name__ == '__main__': # a clean version with only policy net, no value net
 
     clip_param = 0.2
     num_batches = 10
-    batch_size = 512
+    batch_size = 128
+
+    seed = 10
+    np.random.seed(seed)
+    tf.set_random_seed(seed)
 
     # 1. build network with pure tf
     observation = tf.placeholder(tf.float32, shape=(None,) + observation_dim) # network input
@@ -80,9 +86,10 @@ if __name__ == '__main__': # a clean version with only policy net, no value net
         # sync pi and pi_old
         sess.run([tf.assign(theta_old, theta) for (theta_old, theta) in zip(pi_old_var_list, train_var_list)])
 
+        start_time = time.time()
         for i in range(100): # until some stopping criterion met...
             # collect data
-            training_data.collect(num_episodes=120) # YouQiaoben, ShihongSong
+            training_data.collect(num_episodes=20) # YouQiaoben, ShihongSong
 
             # print current return
             print('Epoch {}:'.format(i))
@@ -95,4 +102,6 @@ if __name__ == '__main__': # a clean version with only policy net, no value net
                 sess.run(train_op, feed_dict={observation: data['observations'], action: data['actions'], advantage: data['returns']})
 
             # assigning pi to pi_old
-            sess.run([tf.assign(theta_old, theta) for (theta_old, theta) in zip(pi_old_var_list, train_var_list)])
\ No newline at end of file
+            sess.run([tf.assign(theta_old, theta) for (theta_old, theta) in zip(pi_old_var_list, train_var_list)])
+
+            print('Elapsed time: {:.1f} min'.format((time.time() - start_time) / 60))
\ No newline at end of file
diff --git a/examples/ppo_cartpole_gym.py b/examples/ppo_cartpole_gym.py
new file mode 100755
index 0000000..35ac275
--- /dev/null
+++ b/examples/ppo_cartpole_gym.py
@@ -0,0 +1,107 @@
+#!/usr/bin/env python
+from __future__ import absolute_import
+
+import tensorflow as tf
+import gym
+import numpy as np
+import time
+
+# our lib imports here! It's ok to append path in examples
+import sys
+sys.path.append('..')
+from tianshou.core import losses
+from tianshou.data.batch import Batch
+import tianshou.data.advantage_estimation as advantage_estimation
+import tianshou.core.policy.stochastic as policy  # TODO: fix imports as zhusuan so that only need to import to policy
+
+from rllab.envs.box2d.cartpole_env import CartpoleEnv
+from rllab.envs.normalized_env import normalize
+
+
+def policy_net(observation, action_dim, scope=None):
+    """
+    Constructs the policy network. NOT NEEDED IN THE LIBRARY! this is pure tf
+
+    :param observation: Placeholder for the observation. A tensor of shape (bs, x, y, channels)
+    :param action_dim: int. The number of actions.
+    :param scope: str. Specifying the scope of the variables.
+    """
+    # with tf.variable_scope(scope):
+    net = tf.layers.dense(observation, 32, activation=tf.nn.tanh)
+    net = tf.layers.dense(net, 32, activation=tf.nn.tanh)
+
+    act_logits = tf.layers.dense(net, action_dim, activation=None)
+
+    return act_logits
+
+
+if __name__ == '__main__': # a clean version with only policy net, no value net
+    env = gym.make('CartPole-v0')
+    observation_dim = env.observation_space.shape
+    action_dim = env.action_space.n
+
+    clip_param = 0.2
+    num_batches = 10
+    batch_size = 512
+
+    seed = 10
+    np.random.seed(seed)
+    tf.set_random_seed(seed)
+
+    # 1. build network with pure tf
+    observation = tf.placeholder(tf.float32, shape=(None,) + observation_dim) # network input
+
+    with tf.variable_scope('pi'):
+        action_logits = policy_net(observation, action_dim, 'pi')
+        train_var_list = tf.get_collection(tf.GraphKeys.TRAINABLE_VARIABLES) # TODO: better management of TRAINABLE_VARIABLES
+    with tf.variable_scope('pi_old'):
+        action_logits_old = policy_net(observation, action_dim, 'pi_old')
+        pi_old_var_list = tf.get_collection(tf.GraphKeys.TRAINABLE_VARIABLES, 'pi_old')
+
+    # 2. build losses, optimizers
+    pi = policy.OnehotCategorical(action_logits, observation_placeholder=observation) # YongRen: policy.Gaussian (could reference the policy in TRPO paper, my code is adapted from zhusuan.distributions) policy.DQN etc.
+    # for continuous action space, you may need to change an environment to run
+    pi_old = policy.OnehotCategorical(action_logits_old, observation_placeholder=observation)
+
+    action = tf.placeholder(dtype=tf.int32, shape=(None,)) # batch of integer actions
+    advantage = tf.placeholder(dtype=tf.float32, shape=(None,)) # advantage values used in the Gradients
+
+    ppo_loss_clip = losses.ppo_clip(action, advantage, clip_param, pi, pi_old) # TongzhengRen: losses.vpg ... management of placeholders and feed_dict
+
+    total_loss = ppo_loss_clip
+    optimizer = tf.train.AdamOptimizer(1e-4)
+    train_op = optimizer.minimize(total_loss, var_list=train_var_list)
+
+    # 3. define data collection
+    training_data = Batch(env, pi, advantage_estimation.full_return) # YouQiaoben: finish and polish Batch, advantage_estimation.gae_lambda as in PPO paper
+                                                             # ShihongSong: Replay(), see dqn_example.py
+    # maybe a dict to manage the elements to be collected
+
+    # 4. start training
+    config = tf.ConfigProto()
+    config.gpu_options.allow_growth = True
+    with tf.Session(config=config) as sess:
+        sess.run(tf.global_variables_initializer())
+        # sync pi and pi_old
+        sess.run([tf.assign(theta_old, theta) for (theta_old, theta) in zip(pi_old_var_list, train_var_list)])
+
+        start_time = time.time()
+        for i in range(100): # until some stopping criterion met...
+            # collect data
+            training_data.collect(num_episodes=50) # YouQiaoben, ShihongSong
+
+            # print current return
+            print('Epoch {}:'.format(i))
+            training_data.statistics()
+
+            # update network
+            for _ in range(num_batches):
+                data = training_data.next_batch(batch_size)  # YouQiaoben, ShihongSong
+                # TODO: auto managing of the placeholders? or add this to params of data.Batch
+                sess.run(train_op, feed_dict={observation: data['observations'], action: data['actions'],
+                                              advantage: data['returns']})
+
+            # assigning pi to pi_old
+            sess.run([tf.assign(theta_old, theta) for (theta_old, theta) in zip(pi_old_var_list, train_var_list)])
+
+            print('Elapsed time: {:.1f} min'.format((time.time() - start_time) / 60))
\ No newline at end of file
diff --git a/tianshou/core/policy/dqn.py b/tianshou/core/policy/dqn.py
index 8533549..2f6db5a 100644
--- a/tianshou/core/policy/dqn.py
+++ b/tianshou/core/policy/dqn.py
@@ -54,7 +54,7 @@ class DQNOld(QValuePolicy):
         return the action (int) to be executed.
         no exploration when exploration=None.
         """
-        # TODO: ensure thread safety
+        # TODO: ensure thread safety, tf.multinomial to init
         sess = tf.get_default_session()
         sampled_action = sess.run(tf.multinomial(self.logits, num_samples=1),
                                   feed_dict={self._observation_placeholder: observation[None]})
diff --git a/tianshou/core/policy/stochastic.py b/tianshou/core/policy/stochastic.py
index e2c2dea..cda204d 100644
--- a/tianshou/core/policy/stochastic.py
+++ b/tianshou/core/policy/stochastic.py
@@ -35,6 +35,7 @@ class OnehotCategorical(StochasticPolicy):
 
     def __init__(self, logits, observation_placeholder, dtype=None, group_ndims=0, **kwargs):
         self._logits = tf.convert_to_tensor(logits)
+        self._action = tf.multinomial(self.logits, num_samples=1)
 
         if dtype is None:
             dtype = tf.int32
@@ -65,7 +66,7 @@ class OnehotCategorical(StochasticPolicy):
         # TODO: this may be ugly. also maybe huge problem when parallel
         sess = tf.get_default_session()
         # observation[None] adds one dimension at the beginning
-        sampled_action = sess.run(tf.multinomial(self.logits, num_samples=1),
+        sampled_action = sess.run(self._action,
                                            feed_dict={self._observation_placeholder: observation[None]})
 
         sampled_action = sampled_action[0, 0]
@@ -103,6 +104,9 @@ class Normal(StochasticPolicy):
         self._logstd = tf.convert_to_tensor(logstd, dtype = tf.float32)
         self._std = tf.exp(self._logstd)
 
+        shape = tf.broadcast_dynamic_shape(tf.shape(self._mean), tf.shape(self._std))
+        self._action = tf.random_normal(tf.concat([[1], shape], 0), dtype = tf.float32) * self._std + self._mean
+
         super(Normal, self).__init__(
             act_dtype = tf.float32,
             param_dtype = tf.float32,
@@ -126,14 +130,9 @@ class Normal(StochasticPolicy):
     def _act(self, observation):
         # TODO: getting session like this maybe ugly. also maybe huge problem when parallel
         sess = tf.get_default_session()
-        mean, std = self._mean, self._std
-        shape = tf.broadcast_dynamic_shape(tf.shape(self._mean),\
-                                           tf.shape(self._std))
-
 
         # observation[None] adds one dimension at the beginning
-        sampled_action = sess.run(tf.random_normal(tf.concat([[1], shape], 0),
-                                  dtype = tf.float32) * std + mean,
+        sampled_action = sess.run(self._action,
                                   feed_dict={self._observation_placeholder: observation[None]})
         sampled_action = sampled_action[0, 0]
         return sampled_action
diff --git a/tianshou/data/batch.py b/tianshou/data/batch.py
index 4d7b1f2..a2a8dde 100644
--- a/tianshou/data/batch.py
+++ b/tianshou/data/batch.py
@@ -14,19 +14,20 @@ class Batch(object):
         self._advantage_estimation_function = advantage_estimation_function
         self._is_first_collect = True
 
+    def collect(self, num_timesteps=0, num_episodes=0,
+                apply_function=True):  # specify how many data to collect here, or fix it in __init__()
+        assert sum(
+            [num_timesteps > 0, num_episodes > 0]) == 1, "One and only one collection number specification permitted!"
 
-    def collect(self, num_timesteps=0, num_episodes=0, apply_function=True): # specify how many data to collect here, or fix it in __init__()
-        assert sum([num_timesteps > 0, num_episodes > 0]) == 1, "One and only one collection number specification permitted!"
-
-        if num_timesteps > 0: # YouQiaoben: finish this implementation, the following code are just from openai/baselines
+        if num_timesteps > 0:  # YouQiaoben: finish this implementation, the following code are just from openai/baselines
             t = 0
-            ac = self.env.action_space.sample() # not used, just so we have the datatype
-            new = True # marks if we're on first timestep of an episode
+            ac = self.env.action_space.sample()  # not used, just so we have the datatype
+            new = True  # marks if we're on first timestep of an episode
             if self.is_first_collect:
                 ob = self.env.reset()
                 self.is_first_collect = False
             else:
-                ob = self.raw_data['observations'][0] # last observation!
+                ob = self.raw_data['observations'][0]  # last observation!
 
             # Initialize history arrays
             observations = np.array([ob for _ in range(num_timesteps)])
@@ -76,9 +77,11 @@ class Batch(object):
             rewards = []
             episode_start_flags = []
 
-            t_count = 0
+            # t_count = 0
 
             for _ in range(num_episodes):
+                t_count = 0
+
                 ob = self._env.reset()
                 observations.append(ob)
                 episode_start_flags.append(True)
@@ -92,7 +95,7 @@ class Batch(object):
 
                     t_count += 1
                     if t_count >= 100:  # force episode stop, just to test if memory still grows
-                        done = True
+                        break
 
                     if done:  # end of episode, discard s_T
                         break
@@ -110,8 +113,9 @@ class Batch(object):
             del rewards
             del episode_start_flags
 
-            self.raw_data = {'observations': self.observations, 'actions': self.actions, 'rewards': self.rewards, 'episode_start_flags': self.episode_start_flags}
-        
+            self.raw_data = {'observations': self.observations, 'actions': self.actions, 'rewards': self.rewards,
+                             'episode_start_flags': self.episode_start_flags}
+
             self._is_first_collect = False
 
         if apply_function:
@@ -133,6 +137,7 @@ class Batch(object):
 
         return current_batch
 
+    # TODO: this will definitely be refactored with a proper logger
     def statistics(self):
         """
         compute the statistics of the current sampled paths
@@ -143,16 +148,21 @@ class Batch(object):
         num_timesteps = rewards.shape[0]
 
         returns = []
+        episode_lengths = []
         max_return = 0
+        num_episodes = 1
         episode_start_idx = 0
         for i in range(1, num_timesteps):
             if episode_start_flags[i] or (
                     i == num_timesteps - 1):  # found the start of next episode or the end of all episodes
+                if episode_start_flags[i]:
+                    num_episodes += 1
                 if i < rewards.shape[0] - 1:
                     t = i - 1
                 else:
                     t = i
                 Gt = 0
+                episode_lengths.append(t - episode_start_idx)
                 while t >= episode_start_idx:
                     Gt += rewards[t]
                     t -= 1
@@ -163,5 +173,8 @@ class Batch(object):
                 episode_start_idx = i
 
         print('AverageReturn: {}'.format(np.mean(returns)))
-        print('StdReturn:   : {}'.format(np.std(returns)))
-        print('MaxReturn    : {}'.format(max_return))
\ No newline at end of file
+        print('StdReturn    : {}'.format(np.std(returns)))
+        print('NumEpisodes  : {}'.format(num_episodes))
+        print('MinMaxReturns: {}..., {}'.format(np.sort(returns)[:3], np.sort(returns)[-3:]))
+        print('AverageLength: {}'.format(np.mean(episode_lengths)))
+        print('MinMaxLengths: {}..., {}'.format(np.sort(episode_lengths)[:3], np.sort(episode_lengths)[-3:]))
diff --git a/tianshou/data/replay_buffer/binary_heap.py b/tianshou/data/replay_buffer/binary_heap.py
index e2b1474..2deac14 100644
--- a/tianshou/data/replay_buffer/binary_heap.py
+++ b/tianshou/data/replay_buffer/binary_heap.py
@@ -7,7 +7,7 @@
 import sys
 import math
 
-import utility
+from . import utility
 
 
 class BinaryHeap(object):
diff --git a/tianshou/data/replay_buffer/rank_based.py b/tianshou/data/replay_buffer/rank_based.py
index b71ca68..0a6641f 100644
--- a/tianshou/data/replay_buffer/rank_based.py
+++ b/tianshou/data/replay_buffer/rank_based.py
@@ -154,6 +154,7 @@ class RankBasedExperience(ReplayBuffer):
         target = list()
 
         sess = tf.get_default_session()
+        # TODO: pre-build the thing in sess.run
         current_datas, current_wis, current_indexs = self.sample({'global_step': sess.run(tf.train.get_global_step())})
 
         for i in range(0, batch_size):