How would you mutate Explorer Series data like an Enum reduce

"Hello everyone,

I’m currently diving into Elixir’s data libraries, exploring ways to apply stock market technical analysis logic using Explorer and data frames.

During my attempt to calculate the RSI (Relative Strength Index) for a stock, I encountered a requirement to compute series elements based on a formula that involves the previous row in a data frame. I initially tried using the shift function, but it appears not to track changes effectively, possibly due to lazy evaluation.

Let me simplify the problem. Imagine having a list of 1’s, and you want to update each element by adding it to the previous one.

%{a: [1, 1, 1, 1, 1]}
|> DF.new()
|> DF.mutate(c: S.add(a, S.shift(a, 1)))

What I expect is [1, 2, 3, 4, 5].

I’m wondering if there’s a more straightforward approach, perhaps involving a reduction function, where I can easily track the updated values. It’s possible I’m making it more complex than necessary. "

Here is the actual livebook file I’m testing this out.

Javascript equivalent code that I’m trying out in elixir dataframe.

Screenshot from 2024-02-04 19-12-28698×819 61.4 KB