获取最后一个满足条件的可用值

试试这个:

mapper = df.drop_duplicates('Number 1', keep='last').set_index('Number 1')['Analysis 1']
df['Analysis 2'] = df['Number 2'].map(mapper)
df

输出:

Date (DD/MM/AA)  Number 1  Analysis 1  Number 2  Analysis 2
0      2021-01-01        80          51        80          51
1      2021-02-01        81          52        80          51
2      2021-03-01        83          53        81          52
3      2021-04-01        82          54        81          52
4      2021-05-01        83          55        81          52
5      2021-06-01        84          56        82          54