As I know, the standard way to use dcast function on object with class data.table is to specify variables (which will become column names) and values (which will become values of these columns).

I have a data.table with three columns - id, variable and value:

library(data.table) dt <- data.table(id = c(1, 2, 1), variable = c("var_1", "var_1", "var_2"), value = c(100, 200, 300)) dt #> id variable value #> 1: 1 var_1 100 #> 2: 2 var_1 200 #> 3: 1 var_2 300

And I want this output provided by dcast:

dt_wide <- dcast(dt, id ~ variable, value.var = "value") dt_wide #> id var_1 var_2 #> 1: 1 100 300 #> 2: 2 200 NA

But my question is - can I do this without specyfing variable? I.e. can I use dcast and get the output as above, having object as below?

dt[, variable := NULL] dt #> id value #> 1: 1 100 #> 2: 2 200 #> 3: 1 300 # dcast(dt)? Result: data.table(id = c(1, 2), V1 = c(100, 200), V2 = c(300, NA)) #> id V1 V2 #> 1: 1 100 300 #> 2: 2 200 NA

I can imagine this is theoretically possible, algorithm could look like this:

  1. Starting from top, take the first value from each id and put it into newly created column (choose name automatically).
  2. Take the second value for each id - if nothing for some id, put NA.
  3. And so on, until all values are taken.

I'm asking because I have the data only with id and value columns and want to perform this without additional computing (i.e. adding new column).

In my case dcast is really fast and I have found that addind new column is more computationaly expensive than performing dcast - so I would like to avoid this. Although maybe dcast is so fast because of use this variable column :)

0

1 Answer

With dcast, we can create formula on the fly with an expression created with paste and rowid

library(data.table) dcast(dt, id ~ paste0('var_', rowid(id)))

-output

 id var_1 var_2 1: 1 100 300 2: 2 200 NA