fix concepts.rst with regard to new buffer behavior (#316)

fix #315

docs/tutorials/concepts.rst

@@ -77,10 +77,10 @@ The following code snippet illustrates its usage, including:
 ::
 
     >>> import pickle, numpy as np
-    >>> from tianshou.data import ReplayBuffer
+    >>> from tianshou.data import Batch, ReplayBuffer
     >>> buf = ReplayBuffer(size=20)
     >>> for i in range(3):
-    ...     buf.add(obs=i, act=i, rew=i, done=0, obs_next=i + 1, info={})
+    ...     buf.add(Batch(obs=i, act=i, rew=i, done=0, obs_next=i + 1, info={}))
 
     >>> buf.obs
     # since we set size = 20, len(buf.obs) == 20.
@@ -96,7 +96,7 @@ The following code snippet illustrates its usage, including:
     >>> buf2 = ReplayBuffer(size=10)
     >>> for i in range(15):
     ...     done = i % 4 == 0
-    ...     buf2.add(obs=i, act=i, rew=i, done=done, obs_next=i + 1, info={})
+    ...     buf2.add(Batch(obs=i, act=i, rew=i, done=done, obs_next=i + 1, info={}))
     >>> len(buf2)
     10
     >>> buf2.obs
@@ -147,25 +147,26 @@ The following code snippet illustrates its usage, including:
     >>> buf = ReplayBuffer(size=9, stack_num=4, ignore_obs_next=True)
     >>> for i in range(16):
     ...     done = i % 5 == 0
-    ...     ep_len, ep_rew = buf.add(obs={'id': i}, act=i, rew=i,
+    ...     ptr, ep_rew, ep_len, ep_idx = buf.add(
-    ...                              done=done, obs_next={'id': i + 1})
+    ...         Batch(obs={'id': i}, act=i, rew=i,
+    ...               done=done, obs_next={'id': i + 1}))
     ...     print(i, ep_len, ep_rew)
-    0 1 0.0
+    0 [1] [0.]
-    1 0 0.0
+    1 [0] [0.]
-    2 0 0.0
+    2 [0] [0.]
-    3 0 0.0
+    3 [0] [0.]
-    4 0 0.0
+    4 [0] [0.]
-    5 5 15.0
+    5 [5] [15.]
-    6 0 0.0
+    6 [0] [0.]
-    7 0 0.0
+    7 [0] [0.]
-    8 0 0.0
+    8 [0] [0.]
-    9 0 0.0
+    9 [0] [0.]
-    10 5 40.0
+    10 [5] [40.]
-    11 0 0.0
+    11 [0] [0.]
-    12 0 0.0
+    12 [0] [0.]
-    13 0 0.0
+    13 [0] [0.]
-    14 0 0.0
+    14 [0] [0.]
-    15 5 65.0
+    15 [5] [65.]
     >>> print(buf)  # you can see obs_next is not saved in buf
     ReplayBuffer(
         obs: Batch(
@@ -175,8 +176,6 @@ The following code snippet illustrates its usage, including:
         rew: array([ 9., 10., 11., 12., 13., 14., 15.,  7.,  8.]),
         done: array([False, True, False, False, False, False, True, False,
                      False]),
-        info: Batch(),
-        policy: Batch(),
     )
     >>> index = np.arange(len(buf))
     >>> print(buf.get(index, 'obs').id)
@@ -194,16 +193,21 @@ The following code snippet illustrates its usage, including:
     >>> abs(buf.get(index, 'obs')['id'] - buf[index].obs.id).sum().sum()
     0
     >>> # we can get obs_next through __getitem__, even if it doesn't exist
+    >>> # however, [:] will select the item according to timestamp,
+    >>> # that equals to index == [7, 8, 0, 1, 2, 3, 4, 5, 6]
     >>> print(buf[:].obs_next.id)
-    [[ 7  8  9 10]
+    [[ 7  7  7  8]
+     [ 7  7  8  9]
+     [ 7  8  9 10]
      [ 7  8  9 10]
      [11 11 11 12]
      [11 11 12 13]
      [11 12 13 14]
      [12 13 14 15]
-     [12 13 14 15]
+     [12 13 14 15]]
-     [ 7  7  7  8]
+    >>> full_index = np.array([7, 8, 0, 1, 2, 3, 4, 5, 6])
-     [ 7  7  8  9]]
+    >>> np.allclose(buf[:].obs_next.id, buf[full_index].obs_next.id)
+    True
 
 .. raw:: html