Difference between cumsum and sum?

anyone can help me in understanding the difference between cumsum vs sum method. thanks!

Sum is one value, cumsum is list of elements added with all the previous values , say if the elements are 1,2,3 sum is 6 and cumsum (cumulative sum) is 1, 3, 6


With sum, you take a certain number of values and perform a sum to get the total.
Cumsum is the cumulative sum of differences between the values.

Day 1 - 5
Day 2 - 10
Sum = 15

If you perform a cumsum you’ll get a dataframe as follow:
Day 1 - 5
Day 2 - 15 (10 + 5)

So for each row, you’ll get the cumulative total up until that point.

Hope it helps


@heitzsimon @PrajwalPrashanth thanks for both nice answers! it makes sense mathematically in your examples, but I dont see the purpose to use cumulative sum on the number of cases. It gives cumulative sum that makes number way to larger than real sum, isnt it (scary outcome!) ?

No, it will not be larger than original sum. Last value of a cumsum will be same as sum.

Better example is
1, 2, 3, 4

sum = 10
cumsum = 1, 3, 7, 10

Cumsum is needed when something is reported daily and you want day wise standings of cases (current cases on any given day)

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