Adding values in a row to the previous one in R

Question

I would like to perform a simple operation in R that is easily done in excel:

I have a col consisting of 5045 entries called K. I would like to create a second col L where the first value is L1=100+K[1] the second is L2=L1+K[2], the third is L3=L2+K[3] and so on.

Is there a simple way to do this in R? in Excel one just has to pull down the col.

Answer 1

Try something like

L <- 100 + cumsum(K)

Answer 2

One approach is to use cumsum() and cheat a little. For example, given K:

K <- 1:10

and to keep things simple I am adding 1 (not 100) to K[1], we want to produce:

> 1 + K[1]
[1] 2
> (1 + K[1]) + K[2]
[1] 4
> ((1 + K[1]) + K[2]) + K[3]
[1] 7
....

This is a cumulative sum. We need to cheat a little with the constant you want to add to the first element as we only want that to affect that first element, not be added to each element. Hence this is wrong

> L <- cumsum(1 + K) 
> L
 [1]  2  5  9 14 20 27 35 44 54 65

What we actually want is:

> L <- cumsum(c(1, K))[-1]
> L
 [1]  2  4  7 11 16 22 29 37 46 56

Wherein we concatenate the constant to the vector K as the first element and apply cumsum() to that, but drop the first element of the output from cumsum().

This can of course be done in a slightly simpler fashion:

> L <- 1 + cumsum(K)
> L
 [1]  2  4  7 11 16 22 29 37 46 56

i.e. compute the cumusum() and then add on the constant (which I now see is what @gd047 has suggested in their Answer.)