R中的向量求和

Question

How can I use cumsum to return the index when a threshold is crossed?

v <- c(1,5,7,9,10,14,16,17)
Threshold <- 10

The function would return 3 as the cumulative sum would just be larger than 10, which provides the e index 3 as result.

Answer 1

We could use which

 which(cumsum(v)>Threshold)[1]
 #[1] 3

Or which.max

 which.max(cumsum(v)>Threshold)
 #[1] 3

Or as @nicola commented findInterval is another option. The advantage is it is vectorized and can be used to check multiple Threshold values at once.

 findInterval(Threshold,cumsum(v))+1
 #[1] 3
 findInterval(c(10,49), cumsum(v))+1
 #[1] 3 7