How to add X value to Y rows iterated

Question

If the period is X days, I want the slush to be summed over that time period(the time period is not equal to the length of the dataframe for each slush variable).

I want a new column A to show the Slush of that row + the prior slush within an appropriate period, where in this case, period = row.

I have checked other posts, I don’t believe it is Cumsum as the value is not carried over indefinitely and instead is dropped after its period is over.

Data

Slush	Period
85	1
20	3
40	3
15	1
22	4

Desired Output:

Slush	Period	A
85	1	85
20	3	20
40	3	60
15	1	75
22	4	62

Asked By: Samuel Millington

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Source

Answer 1

You can use numpy for a custom cumsum:

a = np.arange(len(df))

mask = a < (a+df['Period'].to_numpy())[:,None]

df['A'] = (np.triu(mask) * df['Slush'].to_numpy()[:,None]).sum(axis=0)

Output:

   Slush  Period   A
0     85       1  85
1     20       3  20
2     40       3  60
3     15       1  75
4     22       4  62

Intermediates:

np.triu(mask)

array([[ True, False, False, False, False],
       [False,  True,  True,  True, False],
       [False, False,  True,  True,  True],
       [False, False, False,  True, False],
       [False, False, False, False,  True]])

(np.triu(mask) * df['Slush'].to_numpy()[:,None])

array([[85,  0,  0,  0,  0],
       [ 0, 20, 20, 20,  0],
       [ 0,  0, 40, 40, 40],
       [ 0,  0,  0, 15,  0],
       [ 0,  0,  0,  0, 22]])

Answered By: mozway

How to add X value to Y rows iterated

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