A subset of columns with data.table in R

Question

I am trying to fit a data set by selecting some columns from a data table. However, my code does not work with some options.

Here is an example data.table

library(data.table) DT <- data.table( ID = 1:50, Capacity = sample(100:1000, size = 50, replace = F), Code = sample(LETTERS[1:4], 50, replace = T), State = rep(c("Alabama","Indiana","Texas","Nevada"), 50)) 

Here is the code for the working subset:

 DT[,1:2] 

And here is a piece of code that doesn't work. Note that this works with the data framework, but not with data.table.

 DT[,seq(1:2)] 

I need something in the lines of the second format, because I am a subset based on the output of grep () and give the same result as the second format. What am I doing wrong?

Thanks!

Answer 1

In recent versions of data.table, numbers can be used in j to indicate columns. This behavior includes formats such as DT[,1:2] to indicate the numerical range of columns. (Note that this syntax does not work in older versions of data.table).

So why does DT[,1:2] , but DT[,seq(1:2)] does not? The answer is incorrect in the code for data.table:::[.data.table , which includes the lines:

  if (!missing(j)) { jsub = replace_dot_alias(substitute(j)) root = if (is.call(jsub)) as.character(jsub[[1L]])[1L] else "" if (root == ":" || (root %chin% c("-", "!") && is.call(jsub[[2L]]) && jsub[[2L]][[1L]] == "(" && is.call(jsub[[2L]][[2L]]) && jsub[[2L]][[2L]][[1L]] == ":") || (!length(all.vars(jsub)) && root %chin% c("", "c", "paste", "paste0", "-", "!") && missing(by))) { with = FALSE } 

We see here that data.table automatically sets the with = FALSE parameter for you when it detects the use of a function : in j . It does not have the same functionality as for seq , so we need to specify with = FALSE if we want to use the seq syntax.

 DT[,seq(1:2), with=FALSE] 

Answer 2

The lesson I learned is to use list instead of c :

  DT[ ,list(ID,Capacity)] #--------------------------- ID Capacity 1: 1 483 2: 2 703 3: 3 924 4: 4 267 5: 5 588 --- 196: 46 761 197: 47 584 198: 48 402 199: 49 416 200: 50 130 

This allows you to ignore these annoying quotes, and also moves you in the direction of viewing the argument j as an evaluated expression with the environment of the datatable itself.

To "get" named columns by number, use the mget function and the names function. R 'names' are language elements, i.e. Data objects in the search path from the current environment. Data column names are not actually R names . Thus, you need a function that takes the value of the character and forces the interpreter to consider it a fully qualified name . Datatable - [- function syntax for element j treats column names as language objects, not character values, like [.data.frame -function:

 DT[ ,mget(names(DT)[c(1,2)])] ID Capacity 1: 1 483 2: 2 703 3: 3 924 4: 4 267 5: 5 588 --- 196: 46 761 197: 47 584 198: 48 402 199: 49 416 200: 50 130 

Answer 3

The main problem here is that the columns in data.table are reference objects, so you cannot use the same syntax as data.frame. those. without quoted names or numbers

therefore, DT[,c("ID", "Capacity")] will not work for the same reason that DT[,seq(1:2)] will not work.

However, adding ,with=FALSE causes data.table to refer as data.frame will

therefore, DT[,c("ID", "Capacity"), with=FALSE] and DT[,seq(1:2), with=FALSE] now gives you what you want.

  ID Capacity 1: 1 913 2: 2 602 3: 3 861 4: 4 967 5: 5 374 --- 196: 46 163 197: 47 254 198: 48 390 199: 49 853 200: 50 486 

EDIT: as directed by @Rich Scriven