Split a data.table at position

Question

I want to calculate the mean of a column in a data.table way by using row numbers or a vector with position.

Here is a sample data and a postion vector:

x <- data.table(a = c(1,2,3,4,5,6,7,8))
pos <- c(3,5)

I tried:

x[mean(a), by = pos]

So i want the mean from row 1:2, 3:4 and 5:8.

Answer 1

Here is an option:

x[, mean(a), cumsum(replace(rep(0, nrow(x)), pos, 1L))]

output:

   cumsum  V1
1:      0 1.5
2:      1 3.5
3:      2 6.5

Answer 2

You can use findInterval / cut to create groups based on pos :

library(data.table)
x[, mean(a), findInterval(a, pos)]

#   findInterval  V1
#1:            0 1.5
#2:            1 3.5
#3:            2 6.5