Change Column Value if Value Exists In Another Column

Question

Given the following pandas df -

Holding Account	Account Type	Column A	Column B
Rupert 06 (23938996)	Holding Account	1825973	1702598
Rupert 07 (23938996)	Holding Account	1697870	1825973
-	-	-	-
Caroline 06 (0131465)	Holding Account	11112222	5435450
Caroline 07 (0131465)	Holding Account	7896545	11112222

I am trying to find a way to do the following -

• Step 1 - For the entire df, search for instances of Column B values appearing in Column A
  (example: Column B of Caroline 07 == Column A of Caroline 06)
• Step 2 - Rows that meet the above criteria should have there Column B value changed to the Column B value of the row who they matches
  (example: Caroline 7 Column B value will change from 11112222 to 5435450

This means the pandas df will now look as follows -

Holding Account	Account Type	Column A	Column B
Rupert 06 (23938996)	Holding Account	1825973	1702598
Rupert 07 (23938996)	Holding Account	1697870	1702598
-	-	-	-
Caroline 06 (0131465)	Holding Account	11112222	5435450
Caroline 07 (0131465)	Holding Account	7896545	5435450

Does anyone have some suggestions about how I might achieve such a result?

Useful info:

• Dypes = object

Answer 1

please try this:

import numpy as np
df['Column B'] = np.where(df['Column B'].isin(df['Column A'].values),df['Column B'].shift(),df['Column B'])