# Source code for mars.tensor.indexing.extract

```
#!/usr/bin/env python
# -*- coding: utf-8 -*-
# Copyright 1999-2021 Alibaba Group Holding Ltd.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
from ..datasource import tensor as astensor
[docs]def extract(condition, a):
"""
Return the elements of a tensor that satisfy some condition.
This is equivalent to ``mt.compress(ravel(condition), ravel(arr))``. If
`condition` is boolean ``mt.extract`` is equivalent to ``arr[condition]``.
Note that `place` does the exact opposite of `extract`.
Parameters
----------
condition : array_like
An array whose nonzero or True entries indicate the elements of `arr`
to extract.
a : array_like
Input tensor of the same size as `condition`.
Returns
-------
extract : Tensor
Rank 1 tensor of values from `arr` where `condition` is True.
See Also
--------
take, put, copyto, compress, place
Examples
--------
>>> import mars.tensor as mt
>>> arr = mt.arange(12).reshape((3, 4))
>>> arr.execute()
array([[ 0, 1, 2, 3],
[ 4, 5, 6, 7],
[ 8, 9, 10, 11]])
>>> condition = mt.mod(arr, 3)==0
>>> condition.execute()
array([[ True, False, False, True],
[False, False, True, False],
[False, True, False, False]])
>>> mt.extract(condition, arr).execute()
array([0, 3, 6, 9])
If `condition` is boolean:
>>> arr[condition].execute()
array([0, 3, 6, 9])
"""
condition = astensor(condition, dtype=bool)
return a[condition]
```