Cumulative sum at intervals

Question

Consider this dataframe:

dfgg
Out[305]: 
                   Parts_needed   output
Year Month PartId              
2018 1     L27849            72    72
     2     L27849            75   147
     3     L27849           101   248
     4     L27849           103   351
     5     L27849            77
     6     L27849           120
     7     L27849            59
     8     L27849            79
     9     L27849            28
     10    L27849            64
     11    L27849           511
     12    L27849            34
2019 1     L27849            49
     2     L27849            68
     3     L27849            75
     4     L27849            45
     5     L27849            84
     6     L27849            42
     7     L27849            40
     8     L27849            52
     9     L27849           106
     10    L27849            75
     11    L27849           176
     12    L27849            58  2193
2020 1     L27849           135  2328
     2     L27849            45  2301
     3     L27849            21  2247
     4     L27849            35
     5     L27849            17
     6     L27849            39
                        ...
2025 7     L27849            94
     8     L27849            13
     9     L27849            94
     10    L27849            65
     11    L27849           141
     12    L27849            34
2026 1     L27849            22
     2     L27849           132
     3     L27849            49
     4     L27849            33
     5     L27849            48
     6     L27849            53
     7     L27849           103
     8     L27849           122
     9     L27849           171
     10    L27849           182
     11    L27849            68
     12    L27849            23
2027 1     L27849            44
     2     L27849            21
     3     L27849            52
     4     L27849            53
     5     L27849            57
     6     L27849           187
     7     L27849            69
     8     L27849            97
     9     L27849            31
     10    L27849            29
     11    L27849            33
     12    L27849            8

In this dataframe, I need to obtain cumulative sum of Parts_needed at intervals of 2 years. For eg: for 1-2018, 72 will keep getting added to the following rows 75,101,103.. upto 1-2020 135. Similarly, at 2-2018, 75 will keep getting added to the following rows 101,103.. upto 2-2020 45. For the last 2 years however, the cumulative sum will be for whatever rows are remaining. I'm not being able to set a range with np.cumsum() Can somebody help me please?

edit: I have edited, to include the expected output. For 2-2020, the output is 2328+45-72 (since 72 has been added for 2 years) For 3-2020, the output is 2301+21-75 (since 75 has been added for 2 years) and so on.

What is expected output? Can you add it to question, what numbers? — jezrael, Mar 23 '18 at 09:02

Dan · Accepted Answer · 2018-03-23T09:47:25.313

Basically you want a running total if the beginning was zero padded. You can do that with convolution. Here is a simple numpy example which you should be able to adapt to your pandas use case:

import numpy as np
a = np.array([10,20,3,4,5,6,7])
width = 4
kernel = np.ones(width)
np.convolve(a,kernel)

returning

array([10., 30., 33., 37., 32., 18., 22., 18., 13.,  7.])

As you can see this is a cumulative sum up until 37 in the output (or a[3]) and after that it's a sum of a rolling 4 element window.

This will work for you assuming you always have 24 rows for each 2 year period.

Here is a pandas example using only 2 months per year (so width is 4 instead of 24):

>>> import numpy as np
>>> import pandas as pd
>>> df = pd.DataFrame({'year':[18,18,19,19,20,20,21,21],'month':[1,2,1,2,1,2,1,2],'parts':[230,5,2,12,66,32,1,2]})
>>> df
   month  parts  year
0      1    230    18
1      2      5    18
2      1      2    19
3      2     12    19
4      1     66    20
5      2     32    20
6      1      1    21
7      2      2    21
>>> width = 4
>>> kernel = np.ones(width)
>>> # Drop the last elements as you don't want the window to roll passed the end
>>> np.convolve(df['parts'],kernel)[:-width+1]
array([230., 235., 237., 249.,  85., 112., 111., 101.])

Now you just assign that last array to a new column of your DataFrame

Cumulative sum at intervals

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