sklearn特征抽取

特征抽取sklearn.feature_extraction 模塊提供了從原始數據如文本，圖像等眾抽取能夠被機器學習算法直接處理的特征向量。

1.特征抽取方法之 Loading Features from Dicts

measurements=[
    {'city':'Dubai','temperature':33.},
    {'city':'London','temperature':12.},
    {'city':'San Fransisco','temperature':18.},
]

from sklearn.feature_extraction import DictVectorizer
vec=DictVectorizer()
print(vec.fit_transform(measurements).toarray())
print(vec.get_feature_names())

#[[  1.   0.   0.  33.]
 #[  0.   1.   0.  12.]
 #[  0.   0.   1.  18.]]

#['city=Dubai', 'city=London', 'city=San Fransisco', 'temperature']

2.特征抽取方法之 Features hashing

3.特征抽取方法之 Text Feature Extraction

詞袋模型 the bag of words represenatation

#詞袋模型
from sklearn.feature_extraction.text import CountVectorizer
#查看默認的參數
vectorizer=CountVectorizer(min_df=1)
print(vectorizer)

"""
CountVectorizer(analyzer='word', binary=False, decode_error='strict',
        dtype=<class 'numpy.int64'>, encoding='utf-8', input='content',
        lowercase=True, max_df=1.0, max_features=None, min_df=1,
        ngram_range=(1, 1), preprocessor=None, stop_words=None,
        strip_accents=None, token_pattern='(?u)\\b\\w\\w+\\b',
        tokenizer=None, vocabulary=None)

"""

corpus=["this is the first document.",
        "this is the second second document.",
        "and the third one.",
        "Is this the first document?"]
x=vectorizer.fit_transform(corpus)
print(x)

"""
(0, 1)    1
  (0, 2)    1
  (0, 6)    1
  (0, 3)    1
  (0, 8)    1
  (1, 5)    2
  (1, 1)    1
  (1, 6)    1
  (1, 3)    1
  (1, 8)    1
  (2, 4)    1
  (2, 7)    1
  (2, 0)    1
  (2, 6)    1
  (3, 1)    1
  (3, 2)    1
  (3, 6)    1
  (3, 3)    1
  (3, 8)    1
"""

 默認是可以識別的字符串至少為2個字符

analyze=vectorizer.build_analyzer()
print(analyze("this is a document to anzlyze.")==
　　　　(["this","is","document","to","anzlyze"]))

#True

在fit階段被analyser發現的每一個詞語都會被分配一個獨特的整形索引，該索引對應於特征向量矩陣中的一列

print(vectorizer.get_feature_names()==(
    ["and","document","first","is","one","second","the","third","this"]
))
#True
print(x.toarray())
"""
[[0 1 1 1 0 0 1 0 1]
 [0 1 0 1 0 2 1 0 1]
 [1 0 0 0 1 0 1 1 0]
 [0 1 1 1 0 0 1 0 1]]
"""

獲取屬性

print(vectorizer.vocabulary_.get('document'))
#1

對於一些沒有出現過的字或者字符，則會顯示為0

vectorizer.transform(["somthing completely new."]).toarray()
"""
[[0 1 1 1 0 0 1 0 1]
 [0 1 0 1 0 2 1 0 1]
 [1 0 0 0 1 0 1 1 0]
 [0 1 1 1 0 0 1 0 1]]
"""

在上邊的語料庫中，第一個和最后一個單詞是一模一樣的，只是順序不一樣，他們會被編碼成相同的特征向量，所以詞袋表示法會丟失了單詞順序的前后相關性信息，為了保持某些局部的順序性，可以抽取2個詞和一個詞    

bigram_vectorizer=CountVectorizer(ngram_range=(1,2),token_pattern=r"\b\w+\b",min_df=1)
analyze=bigram_vectorizer.build_analyzer()
print(analyze("Bi-grams are cool!")==(['Bi','grams','are','cool','Bi grams',
                                 'grams are','are cool']))

#True
x_2=bigram_vectorizer.fit_transform(corpus).toarray()
print(x_2)

"""
[[0 0 1 1 1 1 1 0 0 0 0 0 1 1 0 0 0 0 1 1 0]
 [0 0 1 0 0 1 1 0 0 2 1 1 1 0 1 0 0 0 1 1 0]
 [1 1 0 0 0 0 0 0 1 0 0 0 1 0 0 1 1 1 0 0 0]
 [0 0 1 1 1 1 0 1 0 0 0 0 1 1 0 0 0 0 1 0 1]]
"""