median of panda datetime64 column

问题:

Is there a way to compute and return in datetime format the median of a datetime column? I want to calculate the median of a column in python which is in datetime64[ns] format. Below is a sample to the column:

df['date'].head()

0   2017-05-08 13:25:13.342
1   2017-05-08 16:37:45.545
2   2017-01-12 11:08:04.021
3   2016-12-01 09:06:29.912
4   2016-06-08 03:16:40.422

Name: recency, dtype: datetime64[ns]

My aim is to have the median in same datetime format as the date column above:

Tried converting to np.array:

median_ = np.median(np.array(df['date']))

But that throws the error:

TypeError: ufunc add cannot use operands with types dtype('<M8[ns]') and dtype('<M8[ns]')

Converting to int64 and then calculating the median and attempt to the return format to datetime does not work

df['date'].astype('int64').median().astype('datetime64[ns]')

回答1:

How about just taking the middle value?

dates = list(df.sort('date')['date'])
print dates[len(dates)//2]

If the table is sorted you can even skip a line.

回答2:

You can also try quantile(0.5) with some conversions, which is not quite the same as the median if the length of the data frame is even, but this might suffice:

df['date'].astype('datetime64[ns]').quantile(.5)

回答3:

You are close, the median() return a float so convert it to be an int first:

import math

median = math.floor(df['date'].astype('int64').median())

Then convert the int represent the date into datetime64:

result = np.datetime64(median, "ns") #unit: nanosecond