How I get the value of a variable using the lag position that comes from another variable?-CodePudding

I am trying to get the values of a variable (B) that cames from the lag position given by other variable (A).

The variables are something like this:

#    A  B
# 1: 1 10
# 2: 1 20
# 3: 1 30
# 4: 1 40
# 5: 2 50

I want the output (C) to be like this, the first value woud be zero and the condition start in the second row:

#    A  B C
# 1: 1 10 0
# 2: 1 20 10
# 3: 1 30 20
# 4: 2 40 20
# 5: 2 50 30

I have done it with loops but because it´s a large amount of information is a lot of time to wait. I hope someone could give me an idea.

CodePudding user response：

Here's a way with dplyr:

library(dplyr)
x %>%
  mutate(
    C = c(0, B[(2:n()) - A[-1]])
  )
#    A  B  C
# 1: 1 10  0
# 2: 1 20 10
# 3: 1 30 20
# 4: 2 40 20
# 5: 2 50 30

It translates directly to data.table (with your colons in row names, I thought you might be using that package)

library(data.table)
dt = as.data.table(x)
dt[, C := c(0, B[(2:.N) - A[-1]])]
dt
#    A  B  C
# 1: 1 10  0
# 2: 1 20 10
# 3: 1 30 20
# 4: 2 40 20
# 5: 2 50 30

Using this data:

x = read.table(text ='   A  B
1: 1 10
2: 1 20
3: 1 30
4: 2 40
5: 2 50', header = T)