熊猫组由自定义功能

Question

这应该很简单.我想要的是能够按函数的结果进行分组,就像在SQL中你可以按表达式分组:

SELECT substr(name, 1) as letter, COUNT(*) as count
FROM table
GROUP BY substr(name, 1)

这将计算名称列以字母表的每个字母开头的行数.

我想在python中做同样的事情,所以我假设我可以将一个函数传递给groupby.但是,这只会将索引列(第一列)传递给函数,例如0,1或2.我想要的是名称列:

import pandas

# Return the first letter
def first_letter(row):

    # row is 0, then 1, then 2 etc.
    return row.name[0]

#Generate a data set of words
test = pandas.DataFrame({'name': ["benevolent", "hidden", "absurdity", "anonymous", "furious", "antidemocratic", "honeydew"]})

#              name
# 0      benevolent
# 1          hidden
# 2       absurdity
# 3       anonymous
# 4         furious
# 5  antidemocratic
# 6        honeydew

test.groupby(first_letter)

我在这做错了什么.除了行索引之外的其他东西如何组？

Answer 1

为第一个字母创建一个新列:

def first_letter(row):
    return row[0]

test['first'] = test['name'].apply(first_letter)

并将其分组:

group = test.groupby('first')

用它:

>>> group.count()

     name
first      
a         3
b         1
f         1
h         2