pandas映射问题

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import pandas as pd
df = pd.DataFrame({'key1' : ['a', 'a', 'b', 'b', 'a'],
'key2' : ['one', 'two', 'one', 'two', 'one'],
'data1' : np.arange(5),
'data2' : np.arange(5,10)})
print(df)
df['data1'] = df['data1'].map(lambda x : "%.3f"%x) #lambda在这里其实是在定义一个简单的函数，一个没有函数名的函数。
df# 将数字保持三位有效数字

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key1 key2
data1
data2
0
a
one
0
5
1
a
two
1
6
2
b
one
2
7
3
b
two
3
8
4
a
one
4
9
key1
object
key2
object
data1
int64
data2
int64
dtype: object

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df = pd.DataFrame({'key1' : ['a', 'a', 'b', 'b', 'a'],
'key2' : ['one', 'two', 'one', 'two', 'one'],
'data1' : np.arange(5),
'data2' : np.arange(5,10)})
print(df)
df['key1'] = df['key1'].map({'a':'c',"b":"d"})
df

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key1 key2
data1
data2
0
a
one
0
5
1
a
two
1
6
2
b
one
2
7
3
b
two
3
8
4
a
one
4
9

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df = pd.DataFrame({'key1' : ['a', 'a', 'b', 'b', 'a'],
'key2' : ['one', 'two', 'one', 'two', 'one'],
'data1' : np.arange(5),
'data2' : np.arange(5,10)})
print(df)
df['key1'] = df['key1'].map({'a':'b',"b":"a"})# 互相映射
df

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key1 key2
data1
data2
0
a
one
0
5
1
a
two
1
6
2
b
one
2
7
3
b
two
3
8
4
a
one
4
9

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df = pd.DataFrame({'key1' : ['a', 'a', 'b', 'b', 'a'],
'key2' : ['one', 'two', 'one', 'two', 'one'],
'data1' : np.arange(5),
'data2' : np.arange(5,10)})
print(df)
df['key1'] = df['key1'].map({'a':'aaaa'})# 互相映射
print(df)##
## 可以看到这里的问题是如果映射的值不是全部的key,那么不完整的key会被映射成Nan

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key1 key2
data1
data2
0
a
one
0
5
1
a
two
1
6
2
b
one
2
7
3
b
two
3
8
4
a
one
4
9
key1 key2
data1
data2
0
aaaa
one
0
5
1
aaaa
two
1
6
2
NaN
one
2
7
3
NaN
two
3
8
4
aaaa
one
4
9

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df = pd.DataFrame({'key1' : ['a', 'a', 'b', 'b', 'a'],
'key2' : ['one', 'two', 'one', 'two', 'one'],
'data1' : np.arange(5),
'data2' : np.arange(5,10)})
print(df)
df['key1'] = df['key1'].map({'a':'aaaa'}).fillna(df['key1'])
print(df)##
## 加入fillna就可以做到粗存在映射的键值保持原来一样

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key1 key2
data1
data2
0
a
one
0
5
1
a
two
1
6
2
b
one
2
7
3
b
two
3
8
4
a
one
4
9
key1 key2
data1
data2
0
aaaa
one
0
5
1
aaaa
two
1
6
2
b
one
2
7
3
b
two
3
8
4
aaaa
one
4
9

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