Populating a new column based the value of the prior value of the newly created column

Question

I have an existing dataframe with 2 columns. I want to create a third column (s) based on these specifications:

Current value of s = prior value of s + [prior value of s * current value of X2]

I have managed to get some calculations as such which populates s based on other two columns as a test. Of course this does not do calculations on its own prior value.

library(dplyr)
set.seed(123)
a <- data.frame(replicate(2,sample(1:10,10,rep = TRUE)))
a %>% mutate(s = lag(X1, default = 1) +lag(X1, default = 1) * X2)

   X1 X2   s
1   3 10  11
2   8  5  18
3   5  7  64
4   9  6  35
5  10  2  27
6   1  9 100
7   6  3   4
8   9  1  12
9   6  4  45
10  5 10  66

Ideally I would need something like this :

library(dplyr)
set.seed(123)
a <- data.frame(replicate(2,sample(1:10,10,rep = TRUE)))
a %>% mutate(s = lag(s, default = 1) +lag(s, default = 1) * X2)

   X1 X2   s
1   3 10  11
2   8  5  66
3   5  7  528
4   9  6  3696

But this causes an issue as s is not defined.

Any advise is much appreciated.

Regards, Aksel

Answer 1

Not a bad time to use a for loop then

a$s <- 0
for (i in 1:nrow(a)) {
  if (i == 1) 
    a$s[i] = a$X2[i] + 1
  else
    a$s[i] = a$s[i-1] + (a$s[i-1] * a$X2[i])
}


a
#   X1 X2        s
#1   3 10       11
#2   8  5       66
#3   5  7      528
#4   9  6     3696
#5  10  2    11088
#6   1  9   110880
#7   6  3   443520
#8   9  1   887040
#9   6  4  4435200
#10  5 10 48787200