# sequence_expand¶

paddle.static.nn. sequence_expand ( x, y, ref_level=- 1, name=None ) [source]

Sequence Expand Layer. This layer will expand the input variable `x` according to specified level `ref_level` lod of `y`. Please note that the lod level of `x` is at most 1. If the lod level of `x` is 1, than the size of lod of `x` must be equal to the length of `ref_level` lod of `y`. If the lod level of `x` is 0, then the first dim of `x` should be equal to the size of `ref_level` of `y`. The rank of x is at least 2. When rank of `x` is greater than 2, then it would be viewed as a 2-D tensor.

Note

Please note that the input `x` should be Tensor or Tensor, and input `y` must be Tensor.

Following examples will explain how sequence_expand works:

```Case 1

Consider 2 sequences [a][b] and [c][d], now we want to expand them to [a][b], [a][b], [c][d] and [c][d].
Sequence [a][b] expand twice and [c][d] expands twice, so the lod which according to is [2, 2].

Input x is a 1-level Tensor:
x.lod  = [[2,        2]]    #lod based on length may be easier to understand
x.data = [[a], [b], [c], [d]]
x.dims = [4, 1]

input y is a Tensor:
y.lod = [[2,    2],    #the 0th level lod, according to this level
[3, 3, 1, 1]] #the 1st level lod, it has nothing to do with this level

ref_level: 0

then output is a 1-level Tensor out:
out.lod =  [[2,        2,        2,        2]]    #lod based on offset
out.data = [[a], [b], [a], [b], [c], [d], [c], [d]]
out.dims = [8, 1]

Case 2

Consider 3 sequences [a], [b], [c], now we want to expand them to [a][a], [c][c][c].
It's obvious that the lod info of expanded sequences is [2, 0, 3].

x is a Tensor:
x.data = [[a], [b], [c]]
x.dims = [3, 1]

y is a Tensor:
y.lod = [[2, 0, 3]]

ref_level: -1

then output is a 1-level Tensor:
out.data = [[a], [a], [c], [c], [c]]
out.dims = [5, 1]
```
Parameters
• x (Tensor) – The input variable which is a Tensor or Tensor, with the dims `[M, K]`. The lod level is at most 1. The data type should be float32, float64, int32 or int64.

• y (Tensor) – The input variable which is a Tensor, the lod level is at least 1.

• ref_level (int) – Lod level of `y` to be referred by `x`. If set to -1, refer the last level of lod.

• name (str, optional) – For detailed information, please refer to Name. Usually name is no need to set and None by default.

Returns

Tensor, The expanded variable which is a Tensor, with dims `[N, K]`. `N` depends on the lod info of `x` and `y`. The data type is same as input.

Examples

```>>> import paddle
>>> from paddle import base
>>> paddle.enable_static()
>>> import numpy as np

>>> x = paddle.static.data(name='x', shape=[4, 1], dtype='float32')
>>> y = paddle.static.data(name='y', shape=[8, 1],
...             dtype='float32', lod_level=1)
>>> out = paddle.static.nn.sequence_expand(x=x, y=y, ref_level=0)

>>> exe = paddle.static.Executor(base.CPUPlace())
>>> place = paddle.CPUPlace()

>>> np_data = np.array([[1], [2], [3], [4]]).astype('float32')
>>> x_lod_tensor = base.create_lod_tensor(np_data, [[2, 2]], place)
>>> print(x_lod_tensor)
- lod: {{0, 2, 4}}
- place: Place(cpu)
- shape: [4, 1]
- layout: NCHW
- dtype: float32
- data: [1 2 3 4]

>>> np_data = np.array([[1], [2], [3], [4], [5], [6], [7], [8]]).astype('float32')
>>> y_lod_tensor = base.create_lod_tensor(np_data, [[2, 2], [3,3,1,1]], place)
>>> print(y_lod_tensor)
- lod: {{0, 2, 4}{0, 3, 6, 7, 8}}
- place: Place(cpu)
- shape: [8, 1]
- layout: NCHW
- dtype: float32
- data: [1 2 3 4 5 6 7 8]

>>> out_main = exe.run(base.default_main_program(),
...                 feed={'x': x_lod_tensor, 'y': y_lod_tensor},
...                 fetch_list=[out], return_numpy=False)
>>> print(out_main[0])
- lod: {{0, 2, 4, 6, 8}}
- place: Place(cpu)
- shape: [8, 1]
- layout: NCHW
- dtype: float32
- data: [1 2 1 2 3 4 3 4]
```