Python：在计数条件下删除行

Question

我在过滤pandas数据框时遇到问题。

city 
NYC 
NYC 
NYC 
NYC 
SYD 
SYD 
SEL 
SEL
...

df.city.value_counts()

我想删除频率小于4的城市行数，例如SYD和SEL。

如果没有逐个城市手动删除它们，会有什么办法呢？

Answer 1

您可以使用过滤器

df.groupby('city').filter(lambda x : len(x)>3)
Out[1743]: 
  city
0  NYC
1  NYC
2  NYC
3  NYC

Answer 2

这是使用pd.Series.value_counts的一种方法。

counts = df['city'].value_counts()

res = df[~df['city'].isin(counts[counts < 5].index)]

Answer 3

我认为您正在寻找value_counts()

# Import the great and powerful pandas
import pandas as pd

# Create some example data
df = pd.DataFrame({
    'city': ['NYC', 'NYC', 'SYD', 'NYC', 'SEL', 'NYC', 'NYC']
})

# Get the count of each value
value_counts = df['city'].value_counts()

# Select the values where the count is less than 3 (or 5 if you like)
to_remove = value_counts[value_counts <= 3].index

# Keep rows where the city column is not in to_remove
df = df[~df.city.isin(to_remove)]

Answer 4

另一种解决方案：

threshold=3
df['Count'] = df.groupby('City')['City'].transform(pd.Series.value_counts)
df=df[df['Count']>=threshold]
df.drop(['Count'], axis = 1, inplace = True)
print(df)

  City
0  NYC
1  NYC
2  NYC
3  NYC