dataframe - Multiply previous row value by constant R

Question

Have a simple R question but cannot seem to find an answer:

I have a data frame like this:

assumption_val year
1.2            2015
0              2016
0              2017
0              2018
0              2019

I want to grow each value as 20% greater than compared to the previous year, to output something like this:

assumption_val year
1.2            2015
1.44           2016
1.73           2017
2.07           2018
2.49           2019

How can I reference the previous row and multiply it by 1.2 to achieve this?

Thanks!

Answer 1

You are looking for cumprod:

cumprod(rep(1.2, 5))

Like its better known friend, cumsum, it accumulates past results, but it performs a multiplication rather than addition.

df <- data.frame(assumption_val=cumprod(rep(1.2, 5)), 
                 years=2015:2019)

A nice generalization of these functions is Reduce. For example, here is Reduce performing this calculation. You can replace the "*" with "+" and have cumsum.

Reduce("*", rep(1.2, 5), accumulate = T)

A nice feature of this method is that you can adjust the growth rate in each period. For instance if you wanted to start at 1.5 rather than 1.2, you would simply adjust your growth vector to c(1.5, rep(1.2, 4)) to calculate the new growth as follows:

cumprod(c(1.5, rep(1.2, 4)))