There should be one and only one value in column B for every value in column A - Pandas

Question

I have a data frame as shown in image:

enter image description here

I want an output similar to the Value Column.
What it means is for every value in column A, there can only be one and only one value in column B.
Even if value in column A repeats, the value in column B should also repeat.
Please help

Instead of a screenshot of data, could you replace it with the code that creates the actual data frame? — user1717828, Sep 20 '21 at 20:25
Seems like you need to group your data by column A and take the column B. Then you can apply some methods, like median. In this case, method *first* can do the trick. But it would be nice if you provided some data. — user12628549, Sep 20 '21 at 21:01
This is not a great-expectation related question. I would suggest removing that tag. — MariaMadalina, Nov 16 '21 at 17:53

score 0 · Accepted Answer · answered Sep 20 '21 at 20:53

Use transform to set the first 'B' value encountered for each 'A' value to each row then test if this first value is equal to 'B' value.

df['V'] = df['B'] == df.groupby('A')['B'].transform('first')

Output:

>>> df
   A  B      V
0  1  a   True
1  1  a   True
2  2  a   True
3  3  b   True
4  1  a   True
5  2  b  False
6  3  c  False
7  1  d  False

Details:

>>> df.groupby('A')['B'].transform('first')
0    a
1    a
2    a
3    b
4    a
5    a
6    b
7    a
Name: B, dtype: object

There should be one and only one value in column B for every value in column A - Pandas

1 Answers1