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

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?

>Solution :

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)