How to access the last element in a Pandas series?

问题

Let us consider the following data frame:

import pandas as pd

d = {'col1': [1, 2, 3], 'col2': [3, 4, 5]}
df=pd.DataFrame(data=d)

If I want to access the first element in pandas series df['col1'], I can simply go df['col1'][0].

But how can I access the last element in this series? I have tried df['col1'][-1] which returns the following error:

KeyError: -1L

I know that I could go for something like df['col1'][len(df)-1] but why is reverse indexing impossible here?

回答1:

For select last value need Series.iloc or Series.iat, because df['col1'] return Series:

print (df['col1'].iloc[-1])
3
print (df['col1'].iat[-1])
3

Or convert Series to numpy array and select last:

print (df['col1'].values[-1])
3

Or use DataFrame.iloc or DataFrame.iat - but is necessary position of column by Index.get_loc:

print (df.iloc[-1, df.columns.get_loc('col1')])
3
print (df.iat[-1, df.columns.get_loc('col1')])
3

Or is possible use last value of index (necessary not duplicated) and select by DataFrame.loc:

print (df.loc[df.index[-1], 'col1'])
3

回答2:

You can also use tail:

print(df['col1'].tail(1).item())

Output:

3

来源：https://stackoverflow.com/questions/56288949/how-to-access-the-last-element-in-a-pandas-series