update one column twice in a data.table efficient in R

Question

I have a data table that looks like this:

DT <- data.table(Zeit = c(0.024, 0.4, 0.05),
                 Gier = c(1, 2, 3),
                 GierVZ = c(1, 0, 1),
                 Quer = c(2, 4, 6))

Now I want to update and add some columns to this data table. But I am not able to update Gier twice because it would create a duplicate and get an error.

DT[, ':='(Zeit   = round(Zeit, digits = 2),
          Gier   = replace(Gier, Gier == 163.83, NA),
          GierVZ = factor(GierVZ, levels = c(0, 1), labels = c("positiv", "negativ")),
          Quer   = Quer * 9.81,
          Gier   = ifelse(GierVZ == "negativ", Gier * -1, Gier))]

How can I avoid this in general and still create some readable fast code? I am sure there is an easy answer to this. But I am kind of a newbie to data tables and I think (at least at the moment) it is not that intuitive like dplyr, but it is much faster for my big data.

Answer 1

You could evaluate Gier in curly braces:

DT[, ':='(Zeit   = round(Zeit, digits = 2),
          Gier   = {Gier[Gier == 163.83] <- NA; ifelse(GierVZ, -Gier, Gier)},
          GierVZ = factor(GierVZ, levels = c(0, 1), labels = c("positiv", "negativ")),
          Quer   = Quer * 9.81)]

Answer 2

This approach has roughly the same level of readability to me & accomplishes your goal:

DT[ , `:=`(
  Zeit = round(Zeit, digits=2L),
  GierVZ = factor(GierVZ, levels = c(0, 1), labels = c("positiv", "negativ")),
  Quer   = Quer * 9.81
)]
DT[Gier == 163.83, Gier := NA]
DT[ , Gier := fifelse(GierVZ == "negativ", Gier * -1, Gier))]

Alternatively, in the development version of data.table (Installation instructions), you could benefit from fcase:

DT[ , `:=`(
  Zeit   = round(Zeit, digits=2L),
  GierVZ = factor(GierVZ, levels = c(0, 1), labels = c("positiv", "negativ")),
  Quer   = Quer * 9.81
  Gier   = fcase(
      Gier == 163.83    , NA_real_, 
    GierVZ == 'negative',    -Gier, 
    GierVZ == 'positiv' ,     Gier
  )
)]

It would be easier if you could skip writing out the last GierVZ=='positiv' condition; this is a feature request in progress.