How to use values from a previous row and column

Question

I am trying to create a new variable which is a function of previous rows and columns. I have found the lag() function in dplyr but it can't accomplish exactly what I would like.

library(dplyr)
x = data.frame(replicate(2, sample(1:3,10,rep=TRUE)))

   X1 X2
1   1  3
2   2  3
3   2  2
4   1  3
5   2  3
6   2  1
7   3  2
8   1  1
9   1  3
10  2  2

x = mutate(x, new_col = # if x2==1, then the value of x1 in the previous row,
                        # if x2!=1, then 0))

My best attempt:

foo = function(x){
    if (x==1){
        return(lag(X1))
    }else{
        return(0)
}

x = mutate(x, new_col=foo(X1))

Answer 1

We can use ifelse

x %>% 
  mutate(newcol = ifelse(X2==1, lag(X1), 0))

Answer 2

In base R, you can use

x$newcol <- (x$X2 == 1) * c(NA, tail(x$X1, -1))

(x$X2 == 1) ensures 0s for all elements of X2 not equal to 1, and the multiple of the two terms will return the lagged values of X1 when X2 == 1.