python学习-数据清洗

1、handling missing Data

string_data = pd.Series([‘aardvark‘, ‘artichoke‘, np.nan, ‘avocado‘])
string_data.isnull()
string_data[0] = None
#dropna fillna isnull notnull

from numpy import nan as NA
string_data[string_data.notnull()]

data = pd.DataFrame([[1., 6.5, 3.], [1., NA, NA], [NA, NA, NA], [NA, 6.5, 3.]])
data.dropna(how=‘all‘)
data.dropna(axis=1, how=‘all‘)

df = pd.DataFrame(np.random.randn(7, 3))
df.iloc[:4, 1] = NA
df.iloc[:2, 2] = NA
df.dropna()
df.dropna(thresh=2)

#填充缺失值

df.fillna(0)
df.fillna({1: 0.5, 2: 0})
_ = df.fillna(0, inplace=True)#修改原来对象

df = pd.DataFrame(np.random.randn(6, 3))
df.iloc[2:, 1] = NA
df.iloc[4:, 2] = NA

df.fillna(method=‘ffill‘)
df.fillna(method=‘ffill‘, limit=2)

2、数据转换

#去掉重复值
data = pd.DataFrame({‘k1‘: [‘one‘, ‘two‘] * 3 + [‘two‘],‘k2‘: [1, 1, 2, 3, 3, 4, 4]})
data.duplicated()
data.drop_duplicates()
data[‘v1‘] = range(7)
data.drop_duplicates([‘k1‘])
data.drop_duplicates([‘k1‘, ‘k2‘], keep=‘last‘)

#使用函数和映射转换Map

data = pd.DataFrame({‘food‘: [‘bacon‘, ‘pulled pork‘, ‘bacon‘, ‘Pastrami‘, ‘corned beef‘, ‘Bacon‘,
‘pastrami‘, ‘honey ham‘, ‘nova lox‘], ‘ounces‘: [4, 3, 12, 6, 7.5, 8, 3, 5, 6]})
meat_to_animal = { ‘bacon‘: ‘pig‘, ‘pulled pork‘: ‘pig‘, ‘pastrami‘: ‘cow‘, ‘corned beef‘: ‘cow‘, ‘honey ham‘: ‘pig‘, ‘nova lox‘: ‘salmon‘
}
lowercased = data[‘food‘].str.lower()
data[‘animal‘] = lowercased.map(meat_to_animal)
data[‘food‘].map(lambda x: meat_to_animal[x.lower()])

#Replacing Values
data = pd.Series([1., -999., 2., -999., -1000., 3.])
data.replace(-999, np.nan)
data.replace([-999,-1000],np.nan)
data.replace([-999,-1000],[np.nan,0])
data.replace({-999:np.nan,-1000:0})

#Renaming Axis Indexes
data = pd.DataFrame(np.arange(12).reshape((3, 4)),
                    index=[‘Ohio‘, ‘Colorado‘, ‘New York‘],
                    columns=[‘one‘, ‘two‘, ‘three‘, ‘four‘])
transform = lambda x: x[:4].upper()
data.index.map(transform)
data.index = data.index.map(transform)
data.rename(index=str.title, columns=str.upper)
data.rename(index={‘OHIO‘: ‘INDIANA‘},columns={‘three‘: ‘peekaboo‘})
data.rename(index={‘OHIO‘: ‘INDIANA‘}, inplace=True)

#离散化和分箱
ages = [20, 22, 25, 27, 21, 23, 37, 31, 61, 45, 41, 32]
bins = [18, 25, 35, 60, 100]
cats = pd.cut(ages,bins)
cats.codes
cats.categories
pd.value_counts(cats)
pd.cut(ages,[18,26,36,61,100],right=False)
group_names = [‘Youth‘, ‘YoungAdult‘, ‘MiddleAged‘, ‘Senior‘]
pd.cut(ages,bins,labels=group_names)

data = np.random.rand(20)
pd.cut(data, 4, precision=2)

data = np.random.randn(1000)
 cats = pd.qcut(data, 4)

#Detecting and Filtering Outliers
data = pd.DataFrame(np.random.randn(1000, 4))
data.describe()
col = data[2]
col[np.abs(col)>3]
data[(np.abs(data) > 3).any(1)]
data[np.abs(data) > 3] = np.sign(data) * 3
data.describe()
np.sign(data).head()

#随机排列
df = pd.DataFrame(np.arange(5 * 4).reshape((5, 4)))
sampler = np.random.permutation(5)
df.take(sampler)
df.sample(n=3) #随机取3行

choices = pd.Series([5, 7, -1, 6, 4])
draws = choices.sample(n=10, replace=True)

#Computing Indicator/Dummy Variables
df = pd.DataFrame({‘key‘: [‘b‘, ‘b‘, ‘a‘, ‘c‘, ‘a‘, ‘b‘], ‘data1‘: range(6)})
pd.get_dummies(df[‘key‘])
dummies = pd.get_dummies(df[‘key‘], prefix=‘key‘)
df_with_dummy = df[[‘data1‘]].join(dummies)

np.random.seed(12345)
values = np.random.rand(10)
bins = [0, 0.2, 0.4, 0.6, 0.8, 1]
pd.get_dummies(pd.cut(values, bins))

3、String Object Methods

val = ‘a,b, guido‘
val.split(‘,‘)
pieces = [x.strip() for x in val.split(‘,‘)]
first, second, third = pieces
first + ‘::‘ + second + ‘::‘ + third #等价于下面表达式
‘::‘.join(pieces)

‘guido‘ in val
val.index(‘,‘) #如果不存在会报错
val.find(‘:‘) #
val.count(‘,‘)

val.replace(‘,‘, ‘::‘)
val.replace(‘,‘, ‘‘)

#endswith  startswith rfind strip rstrip lstrip lower upper casefold ljust rjust

4、正则表达式

import re
text = "foo bar\t baz \tqux"
re.split(‘\s+‘, text) #先编译后调用split方法，等价于下面的方法

regex = re.compile(‘\s+‘)
regex.split(text)
regex.findall(text)


text = """Dave  Steve 
Rob 
Ryan 
"""
pattern = r‘[A-Z0-9._%+-][A-Z0-9.-]+\.[A-Z]{2,4}‘
regex = re.compile(pattern, flags=re.IGNORECASE)
regex.findall(text)

m = regex.search(text) #返回第一个匹配类型
text[m.start():m.end()]

print(regex.match(text)) #匹配是否发生在开始位置

print(regex.sub(‘REDACTED‘, text)) #通过替换匹配的值返回一个新值

pattern = r‘([A-Z0-9._%+-]+)@([A-Z0-9.-]+)\.([A-Z]{2,4})‘
regex = re.compile(pattern, flags=re.IGNORECASE)
m = regex.match(‘‘)
regex.findall(text)
print(regex.sub(r‘Username: \1, Domain: \2, Suffix: \3‘, text))

5、Vectorized String Functions in pandas

data = {‘Dave‘: ‘‘, ‘Steve‘: ‘‘,‘Rob‘: ‘‘, ‘Wes‘: np.nan}
data = pd.Series(data)
data.isnull()

data.str.contains(‘gmail‘)
pattern=‘([A-Z0-9._%+-]+)@([A-Z0-9.-]+)\\.([A-Z]{2,4})‘

data.str.findall(pattern, flags=re.IGNORECASE)
matches = data.str.match(pattern, flags=re.IGNORECASE)
matches.str.get(1)