Perform arithmetic operation with variables at t and t-1 (R)-CodePudding

I want to perform the following mathematical operation between two variables at different time periods of the same dataframe:

lag_var = var1(t)/var2(t-1)

> df

var1   var2    lag_var
1       10       NA
3       12       0.3
1       16       0.083
6       19       0.375
16       7       0.84

Any ideas?

CodePudding user response：

This can be done easiest in dplyr or in base R by:

df <- read.table(text = "var1   var2
1       10       
3       12       
1       16       
6       19       
16       7", header = TRUE)

# dplyr
df$lag_dplyr <-  df$var1 / dplyr::lag(df$var2)

# Base R
df$lag_base <- sapply(1:nrow(df), function(x) {
  l <- df$var1 / df$var2[x - 1]
  l[x]})

Output:

# var1 var2  lag_dplyr   lag_base
# 1    1   10         NA         NA
# 2    3   12 0.30000000 0.30000000
# 3    1   16 0.08333333 0.08333333
# 4    6   19 0.37500000 0.37500000
# 5   16    7 0.84210526 0.84210526

CodePudding user response：

You can achieve that via the lag() function and mutate, e.g.

library(dplyr)     
df %>%  mutate(lag_var = var1 / dplyr::lag(var2))

Data:

df <- read.table(text = "var1   var2    lag_var
1       10       NA
3       12       0.3
1       16       0.083
6       19       0.375
16       7       0.84", h = T)