R: Insert a column at a specific place in R data table

Question

I have following data table

      user_id user_system_days event total_pics 
[1,] 21196680             0     0         124   
[2,] 21197699             0     0           4   
[3,] 21197861             0     0          14   
[4,] 21198820             1     0          21   
[5,] 21199601             0     0           3   
[6,] 21203847             0     0           1   

Based on a value in column event I want to insert a column to the data table, My approach was by writing a function, and use that function to return a value.

opposite_event  <- function(x) {
  y <- 1
  if (x == 1) 
    y <- 0
  return (y)
}

I called function as follows:

dt[,op_event:=opposite_event(dt$event)]

This actually added the column but values were wrong:

      user_id user_system_days event total_pics op_event
[1,] 21196680             0     0         124      0
[2,] 21197699             0     0           4      0
[3,] 21197861             0     0          14      0
[4,] 21198820             1     0          21      0
[5,] 21199601             0     0           3      0
[6,] 21203847             0     0           1      0

Based on my function if value of the dt$event is 1 then return value should be 0 and if dt$event is 0 then return value should 1 Plus I wasn't sure how to insert the column into a specific location like right after event column.

Answer 1

if is not vectorized, Replace it with y <- ifelse(x == 0, 1, 0).

You don't really need a custom function in my opinion:

dt[,op_event := ifelse(event == 0, 1, 0)]