To find the last occurrence of particular value and replace certain values occurs after the particul-CodePudding

I am new to coding. I want a logic that works on csv file. Lets say I am having a dataframe with a column name "label" and I wanted to have an output look like column "expected label".

print (df)
   label
0      A
1      B
2      C
3      A
4      B
5      C
6      A
7      B
8      C
9      B
10     C

I need to find the last occurrence of the value "A" and replace other values (in this case "B" & "C" with "H") occurs only after the last occurrence. This is my code and don't know how to proceed after that.

last=df['labels']
max(loc for loc, val in enumerate(last) if val == 'A')

This gets me the last positional occurrence of "A". What I need is how to replace values after finding the last occurrence like I mentioned in the "expected label" column. I really appreciate the help.

CodePudding user response：

Use Series.cummax with swapped order in iloc[::-1] for False for all values after last A and set values in Series.where:

df['Expected'] = df['label'].where(df['label'].eq('A').iloc[::-1].cummax(), 
                                   df['label'].replace(to_replace =["B", "C"], value ="H"))
print (df)
   label Expected
0      A        A
1      B        B
2      C        C
3      A        A
4      B        B
5      C        C
6      A        A
7      B        H
8      C        H
9      B        H
10     E        E

Or:

df['Expected'] = df['label'].where(df['label'].eq('A').iloc[::-1].cummax() | 
                                   ~df['label'].isin(['B','C']), 'H')
print (df)