Pandas conditionally copy values from one column to another row

Question

I have this Dataframe:

I would like to copy the value of the Date column to the New_Date column, but not only to the same exact row, I want to every row that has the same User_ID value.

So, it will be:

I tried groupby and then copy, but groupby made all values become lists and other columns with same user_id can have different values in different rows and then it messes up many things.

I tried also:

df['New_Date'] = df.apply(lambda x: x['Date'] if x['User_ID'] == x['User_ID'] else x['New_Date'], axis=1)

But it only copied values to the same row and left the other two empty.

And this:

if (df['User_ID'] == df['User_ID']):
    df['New_Date'] = np.where(df['New_Date'] == '', df['Date'], df['New_Date'])

None accomplished my intention.

Help is appreciated, Thanks!

rhug123 · Accepted Answer · 2022-08-12 00:29:57Z

1

try this:

df['New_Date'] = df.groupby('User_Id')['Date'].transform('first')

answered Aug 12, 2022 at 0:29

rhug123

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Comments

Nathaniel · Accepted Answer · 2022-08-12 00:10:40Z

0

If I'm understanding you correctly, just copy the Date column and then use .fillna() with ffill=True. If you post your data as text I can provide example code.

answered Aug 12, 2022 at 0:10

Nathaniel

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2 Comments

dNm Over a year ago

Thanks, but that would not do what I want. Several issues… sometimes the first row is empty and the 2nd row is the one containing value, and other times it is the opposite. and also what about the condition of having the same User_ID.

Paul H Over a year ago

@dNm your example should demonstrate the full extent of the issue. Your question should have code that creates a dataframe of expected input and another dataframe of expected output. From the sounds of it, the moved goal post involves a group/apply. Should be easy.

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