[英]Pandas - merge two DataFrames with Identical Column Names and combine information of two DataFrames in one cell
[英]Pandas: how to merge two dataframes on a column by keeping the information of the first one?
我有两个数据df1
和df2
。 df1
包含人的年龄信息,而df2
包含人的性别信息。 并非所有人都在df1
或df2
df1
Name Age
0 Tom 34
1 Sara 18
2 Eva 44
3 Jack 27
4 Laura 30
df2
Name Sex
0 Tom M
1 Paul M
2 Eva F
3 Jack M
4 Michelle F
如果我在df2
中没有此信息,我想在df1
中获取人们的性别信息并设置NaN
。 我试图做df1 = pd.merge(df1, df2, on = 'Name', how = 'outer')
但我在df2
中保留了一些我不想要的人的信息。
df1
Name Age Sex
0 Tom 34 M
1 Sara 18 NaN
2 Eva 44 F
3 Jack 27 M
4 Laura 30 NaN
Sample
:
df1 = pd.DataFrame({'Name': ['Tom', 'Sara', 'Eva', 'Jack', 'Laura'],
'Age': [34, 18, 44, 27, 30]})
#print (df1)
df3 = df1.copy()
df2 = pd.DataFrame({'Name': ['Tom', 'Paul', 'Eva', 'Jack', 'Michelle'],
'Sex': ['M', 'M', 'F', 'M', 'F']})
#print (df2)
df1['Sex'] = df1['Name'].map(df2.set_index('Name')['Sex'])
print (df1)
Name Age Sex
0 Tom 34 M
1 Sara 18 NaN
2 Eva 44 F
3 Jack 27 M
4 Laura 30 NaN
与左连接merge
的替代解决方案:
df = df3.merge(df2[['Name','Sex']], on='Name', how='left')
print (df)
Name Age Sex
0 Tom 34 M
1 Sara 18 NaN
2 Eva 44 F
3 Jack 27 M
4 Laura 30 NaN
如果需要多列映射(例如Year
和Code
)需要与左连接merge
:
df1 = pd.DataFrame({'Name': ['Tom', 'Sara', 'Eva', 'Jack', 'Laura'],
'Year':[2000,2003,2003,2004,2007],
'Code':[1,2,3,4,4],
'Age': [34, 18, 44, 27, 30]})
print (df1)
Name Year Code Age
0 Tom 2000 1 34
1 Sara 2003 2 18
2 Eva 2003 3 44
3 Jack 2004 4 27
4 Laura 2007 4 30
df2 = pd.DataFrame({'Name': ['Tom', 'Paul', 'Eva', 'Jack', 'Michelle'],
'Sex': ['M', 'M', 'F', 'M', 'F'],
'Year':[2001,2003,2003,2004,2007],
'Code':[1,2,3,5,3],
'Val':[21,34,23,44,67]})
print (df2)
Name Sex Year Code Val
0 Tom M 2001 1 21
1 Paul M 2003 2 34
2 Eva F 2003 3 23
3 Jack M 2004 5 44
4 Michelle F 2007 3 67
#merge by all columns
df = df1.merge(df2, on=['Year','Code'], how='left')
print (df)
Name_x Year Code Age Name_y Sex Val
0 Tom 2000 1 34 NaN NaN NaN
1 Sara 2003 2 18 Paul M 34.0
2 Eva 2003 3 44 Eva F 23.0
3 Jack 2004 4 27 NaN NaN NaN
4 Laura 2007 4 30 NaN NaN NaN
#specified columns - columns for join (Year, Code) need always + appended columns (Val)
df = df1.merge(df2[['Year','Code', 'Val']], on=['Year','Code'], how='left')
print (df)
Name Year Code Age Val
0 Tom 2000 1 34 NaN
1 Sara 2003 2 18 34.0
2 Eva 2003 3 44 23.0
3 Jack 2004 4 27 NaN
4 Laura 2007 4 30 NaN
如果map
出错,则表示连接列重复,此处为Name
:
df1 = pd.DataFrame({'Name': ['Tom', 'Sara', 'Eva', 'Jack', 'Laura'],
'Age': [34, 18, 44, 27, 30]})
print (df1)
Name Age
0 Tom 34
1 Sara 18
2 Eva 44
3 Jack 27
4 Laura 30
df3, df4 = df1.copy(), df1.copy()
df2 = pd.DataFrame({'Name': ['Tom', 'Tom', 'Eva', 'Jack', 'Michelle'],
'Val': [1,2,3,4,5]})
print (df2)
Name Val
0 Tom 1 <-duplicated name Tom
1 Tom 2 <-duplicated name Tom
2 Eva 3
3 Jack 4
4 Michelle 5
s = df2.set_index('Name')['Val']
df1['New'] = df1['Name'].map(s)
print (df1)
InvalidIndexError:重新索引仅对具有唯一值的索引对象有效
解决方案通过DataFrame.drop_duplicates
删除重复项,或使用 map by dict
进行最后一次重复匹配:
#default keep first value
s = df2.drop_duplicates('Name').set_index('Name')['Val']
print (s)
Name
Tom 1
Eva 3
Jack 4
Michelle 5
Name: Val, dtype: int64
df1['New'] = df1['Name'].map(s)
print (df1)
Name Age New
0 Tom 34 1.0
1 Sara 18 NaN
2 Eva 44 3.0
3 Jack 27 4.0
4 Laura 30 NaN
#add parameter for keep last value
s = df2.drop_duplicates('Name', keep='last').set_index('Name')['Val']
print (s)
Name
Tom 2
Eva 3
Jack 4
Michelle 5
Name: Val, dtype: int64
df3['New'] = df3['Name'].map(s)
print (df3)
Name Age New
0 Tom 34 2.0
1 Sara 18 NaN
2 Eva 44 3.0
3 Jack 27 4.0
4 Laura 30 NaN
#map by dictionary
d = dict(zip(df2['Name'], df2['Val']))
print (d)
{'Tom': 2, 'Eva': 3, 'Jack': 4, 'Michelle': 5}
df4['New'] = df4['Name'].map(d)
print (df4)
Name Age New
0 Tom 34 2.0
1 Sara 18 NaN
2 Eva 44 3.0
3 Jack 27 4.0
4 Laura 30 NaN
您还可以使用join
方法:
df1.set_index("Name").join(df2.set_index("Name"), how="left")
编辑:添加set_index("Name")
从数据框创建字典的@jezrael 答案的简单补充。
它可能会有所帮助..
Python:
df1 = pd.DataFrame({'Name': ['Tom', 'Sara', 'Eva', 'Jack', 'Laura'],
'Age': [34, 18, 44, 27, 30]})
df2 = pd.DataFrame({'Name': ['Tom', 'Paul', 'Eva', 'Paul', 'Jack', 'Michelle', 'Tom'],
'Something': ['M', 'M', 'F', 'M', 'A', 'F', 'B']})
df1_dict = pd.Series(df1.Age.values, index=df1.Name).to_dict()
df2['Age'] = df2['Name'].map(df1_dict)
print(df2)
输出:
Name Something Age
0 Tom M 34.0
1 Paul M NaN
2 Eva F 44.0
3 Paul M NaN
4 Jack A 27.0
5 Michelle F NaN
6 Tom B 34.0
尚未提及重新索引,但它非常快,并且可以根据需要自动填充缺失值。
DataFrame.reindex
使用公共键 ( Name
) 作为映射数据帧 ( df2
) 的索引:
如果df2
的索引已经是Name
,只需直接reindex
:
df2['Sex'].reindex(df1['Name'])
否则事先set_index
:
df2.set_index('Name')['Sex'].reindex(df1['Name'])
请注意,当分配到现有数据帧时,重新索引的索引将未对齐,因此仅分配数组值:
df1['Sex'] = df2.set_index('Name')['Sex'].reindex(df1['Name']).array
# Name Age Sex
# 0 Tom 34 M
# 1 Sara 18 NaN
# 2 Eva 44 F
# 3 Jack 27 M
# 4 Laura 30 NaN
我还注意到一个常见的假设,即重新索引很慢,但实际上很快(est):
reindex
支持自动填充缺失值:
fill_value
:静态替换method
:给定单调索引的算法替换( ffill
、 bfill
或nearest
) 例如,用不想说(PNS) 填充空的Sex
值:
df2.set_index('Name')['Sex'].reindex(df1['Name'], fill_value='PNS')
# Name Age Sex
# 0 Tom 34 M
# 1 Sara 18 PNS
# 2 Eva 44 F
# 3 Jack 27 M
# 4 Laura 30 PNS
使用fill_value
重新索引比链接fillna
更快:
映射数据框 ( df2
) 不能有重复的键,因此drop_duplicates
如果适用:
df2.drop_duplicates('Name').set_index('Name')['Sex'].reindex(df1['Name'])
时序数据:
''' Note: This is python code in a js snippet, so "run code snippet" will not work. The snippet is just to avoid cluttering the main post with supplemental code. ''' df1 = pd.DataFrame({'Name': np.arange(n), 'Age': np.random.randint(100, size=n)}).sample(frac=1).reset_index(drop=True) df2 = pd.DataFrame({'Name': np.arange(n) + int(n * 0.5), 'Sex': np.random.choice(list('MF'), size=n)}).sample(frac=1).reset_index(drop=True) def reindex_(df1, df2): df1['Sex'] = df2.set_index('Name')['Sex'].reindex(df1['Name']).array return df1 def map_(df1, df2): df1['Sex'] = df1['Name'].map(df2.set_index('Name')['Sex']) return df1 def dict_(df1, df2): df1['Sex'] = df1['Name'].map(dict(zip(df2['Name'], df2['Sex']))) return df1 def merge_(df1, df2): return df1.merge(df2[['Name', 'Sex']], left_on='Name', right_on='Name', how='left') def join_(df1, df2): return df1.set_index('Name').join(df2.set_index('Name'), how='left').reset_index() reindex_fill_value_ = lambda df1, df2: df2.set_index('Name')['Sex'].reindex(df1['Name'], fill_value='PNTS') reindex_fillna_ = lambda df1, df2: df2.set_index('Name')['Sex'].reindex(df1['Name']).fillna('PNTS') map_fillna_ = lambda df1, df2: df1['Name'].map(df2.set_index('Name')['Sex']).fillna('PNTS')
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