add calculated column using pandas based on values in other column

Question

I need help with adding Col4 based on data from Col1,2 and Col3. If Col3 has same values for all corresponding values in Col1/Col2, Col4 should read as "YES" otherwise "NO".

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Answer 1

Use GroupBy.transform with count number of unique values and compare by 1 , set new values by numpy.where :

mask = df.groupby(['Col1','Col2'])['Col3'].transform('nunique') == 1
df['Col4'] = np.where(mask, 'yes', 'no')