# OneHotEncoder¶

One-hot encoding.

This transformer will encode every feature it is provided with. If a list or set is provided, this transformer will encode every entry in the list/set. You can apply it to a subset of features by composing it with compose.Select or compose.SelectType.

## Parameters¶

• sparse – defaults to False

Whether or not 0s should be made explicit or not.

## Examples¶

Let us first create an example dataset.

>>> from pprint import pprint
>>> import random
>>> import string

>>> random.seed(42)
>>> alphabet = list(string.ascii_lowercase)
>>> X = [
...     {
...         'c1': random.choice(alphabet),
...         'c2': random.choice(alphabet),
...     }
...     for _ in range(4)
... ]
>>> pprint(X)
[{'c1': 'u', 'c2': 'd'},
{'c1': 'a', 'c2': 'x'},
{'c1': 'i', 'c2': 'h'},
{'c1': 'h', 'c2': 'e'}]


We can now apply one-hot encoding. All the provided are one-hot encoded, there is therefore no need to specify which features to encode.

>>> from river import preprocessing

>>> oh = preprocessing.OneHotEncoder(sparse=True)
>>> for x in X:
...     oh = oh.learn_one(x)
...     pprint(oh.transform_one(x))
{'c1_u': 1, 'c2_d': 1}
{'c1_a': 1, 'c2_x': 1}
{'c1_i': 1, 'c2_h': 1}
{'c1_h': 1, 'c2_e': 1}


The sparse parameter can be set to False in order to include the values that are not present in the output.

>>> oh = preprocessing.OneHotEncoder(sparse=False)
>>> for x in X[:2]:
...     oh = oh.learn_one(x)
...     pprint(oh.transform_one(x))
{'c1_u': 1, 'c2_d': 1}
{'c1_a': 1, 'c1_u': 0, 'c2_d': 0, 'c2_x': 1}


A subset of the features can be one-hot encoded by using an instance of compose.Select.

>>> from river import compose

>>> pp = compose.Select('c1') | preprocessing.OneHotEncoder()

>>> for x in X:
...     pp = pp.learn_one(x)
...     pprint(pp.transform_one(x))
{'c1_u': 1}
{'c1_a': 1, 'c1_u': 0}
{'c1_a': 0, 'c1_i': 1, 'c1_u': 0}
{'c1_a': 0, 'c1_h': 1, 'c1_i': 0, 'c1_u': 0}


You can preserve the c2 feature by using a union:

>>> pp = compose.Select('c1') | preprocessing.OneHotEncoder()
>>> pp += compose.Select('c2')

>>> for x in X:
...     pp = pp.learn_one(x)
...     pprint(pp.transform_one(x))
{'c1_u': 1, 'c2': 'd'}
{'c1_a': 1, 'c1_u': 0, 'c2': 'x'}
{'c1_a': 0, 'c1_i': 1, 'c1_u': 0, 'c2': 'h'}
{'c1_a': 0, 'c1_h': 1, 'c1_i': 0, 'c1_u': 0, 'c2': 'e'}


Similar to the above examples, we can also pass values as a list. This will one-hot encode all of the entries individually.

>>> X = [{'c1': ['u', 'a'], 'c2': ['d']},
...     {'c1': ['a', 'b'], 'c2': ['x']},
...     {'c1': ['i'], 'c2': ['h', 'z']},
...     {'c1': ['h', 'b'], 'c2': ['e']}]

>>> oh = preprocessing.OneHotEncoder(sparse=True)
>>> for x in X:
...     oh = oh.learn_one(x)
...     pprint(oh.transform_one(x))
{'c1_a': 1, 'c1_u': 1, 'c2_d': 1}
{'c1_a': 1, 'c1_b': 1, 'c2_x': 1}
{'c1_i': 1, 'c2_h': 1, 'c2_z': 1}
{'c1_b': 1, 'c1_h': 1, 'c2_e': 1}


Processing mini-batches is also possible.

>>> from pprint import pprint
>>> import random
>>> import string

>>> random.seed(42)
>>> alphabet = list(string.ascii_lowercase)
>>> X = pd.DataFrame(
...     {
...         'c1': random.choice(alphabet),
...         'c2': random.choice(alphabet),
...     }
...     for _ in range(4)
... )
>>> X
c1 c2
0  u  d
1  a  x
2  i  h
3  h  e

>>> oh = preprocessing.OneHotEncoder(sparse=True)
>>> oh = oh.learn_many(X)

>>> df = oh.transform_many(X)
>>> df.loc[:, sorted(df.columns)]
c1_a  c1_h  c1_i  c1_u  c2_d  c2_e  c2_h  c2_x
0     0     0     0     1     1     0     0     0
1     1     0     0     0     0     0     0     1
2     0     0     1     0     0     0     1     0
3     0     1     0     0     0     1     0     0


Keep in mind that ability for sparse transformations is limited in mini-batch case, which might affect speed/memory footprint of your training loop.

Here's a non-sparse example:

>>> oh = preprocessing.OneHotEncoder(sparse=False)
>>> X_init = pd.DataFrame([{'c1': "Oranges", 'c2': "Apples"}])
>>> oh = oh.learn_many(X_init)
>>> oh = oh.learn_many(X)

>>> df = oh.transform_many(X)
>>> df.loc[:, sorted(df.columns)]
c1_Oranges  c1_a  c1_h  c1_i  c1_u  c2_Apples  c2_d  c2_e  c2_h  c2_x
0           0     0     0     0     1          0     1     0     0     0
1           0     1     0     0     0          0     0     0     0     1
2           0     0     0     1     0          0     0     0     1     0
3           0     0     1     0     0          0     0     1     0     0


## Methods¶

clone

Return a fresh estimator with the same parameters.

The clone has the same parameters but has not been updated with any data. This works by looking at the parameters from the class signature. Each parameter is either - recursively cloned if it's a River classes. - deep-copied via copy.deepcopy if not. If the calling object is stochastic (i.e. it accepts a seed parameter) and has not been seeded, then the clone will not be idempotent. Indeed, this method's purpose if simply to return a new instance with the same input parameters.

learn_many

Update with a mini-batch of features.

A lot of transformers don't actually have to do anything during the learn_many step because they are stateless. For this reason the default behavior of this function is to do nothing. Transformers that however do something during the learn_many can override this method.

Parameters

• X (pandas.core.frame.DataFrame)

Returns

Transformer: self

learn_one

Update with a set of features x.

A lot of transformers don't actually have to do anything during the learn_one step because they are stateless. For this reason the default behavior of this function is to do nothing. Transformers that however do something during the learn_one can override this method.

Parameters

• x (dict)

Returns

Transformer: self

transform_many

Transform a mini-batch of features.

Parameters

• X (pandas.core.frame.DataFrame)

Returns

DataFrame: A new DataFrame.

transform_one

Transform a set of features x.

Parameters

• x (dict)
• y – defaults to None

Returns

dict: The transformed values.