Pandas- Adding a specific amount of weeks to a date-CodePudding

I want to add a new column in a df which adds a week to a previous date.

What I have :

start_date         num_of_weeks      

01/5/2019.        1

01/12/2019.       2
01/12/2019.       2

01/19/2019.       3. 
01/19/2019.       3
01/19/2019.       3

For the num_of_weeks corresponding to each start_date I want to add one week ( 7 days) to that date.

So If num_of_weeks = 3 then


week1 = start_date   7
week2 = week1  7
week3 = week2   7

** Desired Output = **



start_date         num_of_weeks          weeks

01/5/2019.        1                      01/12/2019

01/12/2019.       2                      01/19/2019
01/12/2019.       2                      01/26/2019

01/19/2019.       3.                     01/26/2019
01/19/2019.       3.                     02/04/2019
01/19/2019.       3                      02/11/2019

CodePudding user response：

First, we'll group by "start_date" and create some kind of "subindex" to assign a number to each row by using pd.Series.rank. This new value will show us how many weeks to add to each row:

>> df["extra_weeks"] = df.groupby("start_date").rank("first", ascending=False)
>> df
  start_date  num_of_weeks  extra_weeks
0 2019-01-05             1          1.0
1 2019-01-12             2          1.0
2 2019-01-12             2          2.0
3 2019-01-19             3          1.0
4 2019-01-19             3          2.0
5 2019-01-19             3          3.0

Now, you could use pd.to_timedelta on the "extra_weeks" column and add that result to your "start_date" column:

>> df["weeks"] = df["start_date"]   pd.to_timedelta(df["extra_weeks"], unit="w")
>> df
  start_date  num_of_weeks  extra_weeks      weeks
0 2019-01-05             1          1.0 2019-01-12
1 2019-01-12             2          1.0 2019-01-19
2 2019-01-12             2          2.0 2019-01-26
3 2019-01-19             3          1.0 2019-01-26
4 2019-01-19             3          2.0 2019-02-02
5 2019-01-19             3          3.0 2019-02-09

CodePudding user response：

This is a good opportunity to leverage the dateutil library. Specifically you'll be interested in the relativedelta function.