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

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.