Find the index of the last true occurrence in a column by row

Question

I have the following table format:

id	bool
1	true
2	true
3	false
4	false
5	false
6	true

I’d like it so that I could get another column with the index of the last true occurrence in the bool column by row. If it’s true in it’s own row then return it’s own id. It doesn’t sound too hard using a for loop but I want it in a clean pandas format. I.e in this example I would get:

column = [1,2,2,2,2,6]

Asked By: 43zombiegit

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Source

Answer 1

IIUC, you can mask and ffill:

df['new'] = df['id'].where(df['bool']).ffill(downcast='infer')

output:

   id    bool  new
0    1   True    1
1    2   True    2
2    3  False    2
3    4  False    2
4    5  False    2
5    6   True    6

Answered By: mozway

Answer 2

In your case do

df['new'] = df['id'].mul(df['bool']).cummax()
Out[344]: 
0    1
1    2
2    2
3    2
4    2
5    6
dtype: int64

Answered By: BENY

Answer 3

df1.assign(col1=np.where(df1.bool2,df1.id,pd.NA)).fillna(method='pad')

  id  bool2  col1
0   1   True     1
1   2   True     2
2   3  False     2
3   4  False     2
4   5  False     2
5   6   True     6

Answered By: G.G

Find the index of the last true occurrence in a column by row

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