Apply function to pandas dataframe row using values in other rows

Question

I have a situation where I have a dataframe row to perform calculations with, and I need to use values in following (potentially preceding) rows to do these calculations (essentially a perfect forecast based on the real data set). I get each row from an earlier df.apply call, so I could pass the whole df along to the downstream objects, but that seems less than ideal based on the complexity of objects in my analysis.

I found one closely related question and answer [1], but the problem is actually fundamentally different in the sense that I do not need the whole df for my calcs, simply the following x number of rows (which might matter for large dfs).

So, for example:

df = pd.DataFrame([100, 200, 300, 400, 500, 600, 700, 800, 900, 1000], 
                  columns=['PRICE'])
horizon = 3

I need to access values in the following 3 (horizon) rows in my row-wise df.apply call. How can I get a naive forecast of the next 3 data points dynamically in my row-wise apply calcs? e.g. for row the first row, where the PRICE is 100, I need to use [200, 300, 400] as a forecast in my calcs.

[1] apply a function to a pandas Dataframe whose returned value is based on other rows

Answer 1

By getting the row's index inside of the df.apply() call using row.name, you can generate the 'forecast' data relative to which row you are currently on. This is effectively a preprocessing step to put the 'forecast' onto the relevant row, or it could be done as part of the initial df.apply() call if the df is available downstream.

df = pd.DataFrame(
    [100, 200, 300, 400, 500, 600, 700, 800, 900, 1000],
    columns=["PRICE"]
)
horizon = 3
    
df["FORECAST"] = df.apply(
    lambda x: [df["PRICE"][x.name + 1 : x.name + horizon + 1]],
    axis=1
)

Results in this:

   PRICE          FORECAST
0    100   [200, 300, 400]
1    200   [300, 400, 500]
2    300   [400, 500, 600]
3    400   [500, 600, 700]
4    500   [600, 700, 800]
5    600   [700, 800, 900]
6    700  [800, 900, 1000]
7    800       [900, 1000]
8    900            [1000]
9   1000                []

Which can be used in your row-wise df.apply() calcs.

EDIT: If you want to strip the index from the resulting 'Forecast':

df["FORECAST"] = df.apply(
    lambda x: [df["PRICE"][x.name + 1 : x.name + horizon + 1].reset_index(drop=True)],
    axis=1
)