One-Hot-Encoder

Encode a collection of categorical features using a 1-of-K encoding scheme. Input columns to the one-hot-encoder must by of type int, string, dict, or list. The transformed output is a column of type dictionary (max_categories per column dimension sparse vector) where the key corresponds to the index of the categorical variable and the value is 1.

The behaviour of the one-hot-encoder for each input data column type is as follows. (see transform() for examples of the same).

• string : The key in the output dictionary is the string category and the value is 1.

• int : Behave similar to string columns.

• list : Each value in the list is treated like an individual string. Hence, a list of categorical variables can be used to represent a feature where all categories in the list are simultaneously hot.

• dict : They key of the dictionary is treated as a namespace and the value is treated as a sub-category in the namespace. The categorical variable being encoded in this case is a combination of the namespace and the sub-category.

You can specify the number of categories with the parameter max_categories.

Introductory Example

# Create data.
sf = graphlab.SFrame({'a': [1,2,3], 'b' : [2,3,4]})

# Create a one-hot encoder.
from graphlab.toolkits.feature_engineering import OneHotEncoder
encoder = graphlab.feature_engineering.create(sf, OneHotEncoder())

# Transform the data.
transformed_sf = encoder.transform(sf)

Columns:
    encoded_features    dict

Rows: 5

Data:
+--------------------+
|  encoded_features  |
+--------------------+
| {0: 1, 1: 1, 2: 1} |
| {2: 1, 3: 1, 4: 1} |
| {5: 1, 6: 1, 7: 1} |
| {2: 1, 3: 1, 4: 1} |
| {0: 1, 3: 1, 6: 1} |
+--------------------+
[5 rows x 1 columns]

# Save the transformer.
encoder.save('save-path')

# Return the indices in the encoding.
encoder['feature_encoding']

Columns:
    feature    str
    category   str
    index      int

Rows: 4

Data:
+---------+----------+-------+
| feature | category | index |
+---------+----------+-------+
|    a    |    2     |   0   |
|    a    |    3     |   1   |
|    b    |    2     |   2   |
|    b    |    3     |   3   |
+---------+----------+-------+
[4 rows x 3 columns]

Fitting and transforming

Once a OneHotEncoder object is constructed, it must first be fitted and then the transform function can be called to generate encoded features.

# String/Integer columns
# ----------------------------------------------------------------------
from graphlab.toolkits.feature_engineering import OneHotEncoder
sf = graphlab.SFrame({'a' : [1,2,3,2,3], 'b' : [2,3,4,2,3]})

# Create a OneHotEncoder
encoder = graphlab.feature_engineering.create(sf, OneHotEncoder())

# Fit and transform on the same data.
transformed_sf = encoder.fit_transform(sf)

Columns:
        encoded_features  dict

Rows: 5

Data:
+------------------+
| encoded_features |
+------------------+
|   {0: 1, 3: 1}   |
|   {1: 1, 4: 1}   |
|   {2: 1, 5: 1}   |
|   {1: 1, 3: 1}   |
|   {2: 1, 4: 1}   |
+------------------+
[5 rows x 1 columns]

# Lists can be used to encode sets of categories for each example.
# ----------------------------------------------------------------------
from graphlab.toolkits.feature_engineering import OneHotEncoder
sf = graphlab.SFrame({'categories': [['cat', 'mammal'],
                                         ['dog', 'mammal'],
                                         ['human', 'mammal'],
                                         ['seahawk', 'bird'],
                                         ['wasp', 'insect']]})

# Construct and fit.
encoder = graphlab.feature_engineering.create(sf, OneHotEncoder())

# Transform the data
transformed_sf = encoder.transform(sf)

Columns:
    encoded_features    dict

Rows: 5

Data:
+------------------+
| encoded_features |
+------------------+
|   {0: 1, 1: 1}   |
|   {0: 1, 2: 1}   |
|   {0: 1, 3: 1}   |
|   {4: 1, 6: 1}   |
|   {5: 1, 7: 1}   |
+------------------+
[5 rows x 1 columns]

# Dictionaries can be used for name spaces & sub-categories.
# ----------------------------------------------------------------------
from graphlab.toolkits.feature_engineering import OneHotEncoder
sf = graphlab.SFrame({'attributes':
                [{'height':'tall', 'age': 'senior', 'weight': 'thin'},
                 {'height':'short', 'age': 'child', 'weight': 'thin'},
                 {'height':'giant', 'age': 'adult', 'weight': 'fat'},
                 {'height':'short', 'age': 'child', 'weight': 'thin'},
                 {'height':'tall', 'age': 'child', 'weight': 'fat'}]})

# Construct and fit.
encoder = graphlab.feature_engineering.create(sf, OneHotEncoder())

# Transform the data
transformed_sf = encoder.transform(sf)

Columns:
    encoded_features    dict

Rows: 5

Data:
+--------------------+
|  encoded_features  |
+--------------------+
| {0: 1, 1: 1, 2: 1} |
| {2: 1, 3: 1, 4: 1} |
| {5: 1, 6: 1, 7: 1} |
| {2: 1, 3: 1, 4: 1} |
| {0: 1, 3: 1, 6: 1} |
+--------------------+
[5 rows x 1 columns]