r - Swapping values between two columns using data.table -

i have been breaking head on translating this question data.table solution. (to keep simple i'll use same data set)
when v2 == "b want swap columns between v1 <-> v3.

dt <- data.table(v1=c(1,2,4), v2=c("a","a","b"), v3=c(2,3,1)) #v1 v2 v3 #1:  1   2 #2:  2   3 #3:  4  b  1 

the code below working solution data.frame, because of amount of frustration has given me because using data.table without realising i'm determined find solution data.table.

dt <- data.table(v1=c(1,2,4), v2=c("a","a","b"), v3=c(2,3,1)) df <- as.data.frame(dt) df[df$v2 == "b", c("v1", "v3")] <- df[df$v2 == "b", c("v3", "v1")]  #  v1 v2 v3 #1  1   2 #2  2   3 #3  1  b  4 

i have tried writing lapply function looping through target swapping list, tried narrow down problem replace 1 value, attempted call column names in different ways without success.
closest attempt i've managed get:

> dt[dt$v2 == "b", c("v1", "v3")] <- dt[dt$v2 == "b", c(v3, v1)] #warning messages: #1: in `[<-.data.table`(`*tmp*`, dt$v2 == "b", c("v1", "v3"), value = c(1,  : #  supplied 2 items assigned 1 items of column 'v1' (1 unused) #2: in `[<-.data.table`(`*tmp*`, dt$v2 == "b", c("v1", "v3"), value = c(1,  : #  supplied 2 items assigned 1 items of column 'v3' (1 unused) 

how can data.table solution?

we can try

dt[v2=="b", c("v3", "v1") := .(v1, v3)]