Running sum on a column conditional on value

Question

I have a vector of binary variables which state whether a product is on promotion in the period. I'm trying to work out how to calculate the duration of each promotion and the duration between promotions.

promo.flag = c(1,1,0,1,0,0,1,1,1,0,1,1,0))

So in other words: if promo.flag is same as previous period then running.total + 1, else running.total is reset to 1

I've tried playing with apply functions and cumsum but can't manage to get the conditional reset of running total working :-(

The output I need is:

promo.flag =  c(1,1,0,1,0,0,1,1,1,0,1,1,0)
rolling.sum = c(1,2,1,1,1,2,1,2,3,1,1,2,0)

Can anybody shed any light on how to achieve this in R?

Answer 1

It sounds like you need run length encoding (via the rle command in base R).

unlist(sapply(rle(promo.flag)$lengths,seq))

Gives you a vector 1 2 1 1 1 2 1 2 3 1 1 2 1. Not sure what you're going for with the zero at the end, but I assume it's a terminal condition and easy to change after the fact.

This works because rle() returns a list of two, one of which is named lengths and contains a compact sequence of how many times each is repeated. Then seq when fed a single integer gives you a sequence from 1 to that number. Then apply repeatedly calls seq with the single numbers in rle()$lengths, generating a list of the mini sequences. unlist then turns that list into a vector.