Refer to the last column in R

Question

I am trying to do some manipulation on the last column in a generic way.

I found here on the forums this nice piece of code that returns the name of the last columns:

tail(names(train),1) #returns [1] "last"

I still can't figure out how to reference directly to my dataset's last columns as:

data$last

Answer 1

just use ncol() to get the index of the last col

data[,ncol(data)]

Answer 2

Take the first element of the reversed vector of column names:

rev(names(mtcars))[1]
[1] "carb"

Similarly, to get the last column, you can use

rev(mtcars)[1]

Answer 3

To refer to last column:

colnames(data)[ncol(data)]

Answer 4

I prefer @Troy's solution, here is another way:

train[, tail(colnames(train), 1)]

Answer 5

Troy's answer is simpler, and can be adapted to refer to "n" elements before the last column, using the ":" operator.

If you want to refer to the last threee columns, you could write:

data[,ncol(data)] # refers to the last column
data[,(ncol(data)-2):ncol(data)] # refers to the three last columns

Answer 6

Function last_col() from tidyselect package may help. See also answer here

https://stackoverflow.com/a/44353413

Answer 7

You can use tail, but you have to coerce to list:

tail(as.list(mtcars), 1)

This will return a vector with the contents of the column. If you want to preserve the structure, you can use:

utils:::tail.default(mtcars, 1)

so that tail treats the input like a list. The only reason really to use this approach over Troy's are if you want more than just the last column (i.e. last N), where it becomes a lot easier to do it this way.

Answer 8

Here's an example of indexing just the last column name. Reference the names(df1[,ncol(df1)]):

df1 <- df1 %>% 
  add_column(new1 = NA, new2 = NA, new3 = NA, .after = names(df1[,ncol(df1)]))