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

score 84 · Accepted Answer · answered Feb 14 '14 at 14:11

84

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

data[,ncol(data)]

answered Feb 14 '14 at 14:11

Troy

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This appears to not work for data tables, only data frames. Am I missing something, why would this be? I'm running R 3.6.2 and data table 1.12.8. – jeromeResearch Nov 24 '20 at 00:43
@jeromeResearch for data.table, use `x[, ncol(x), with = FALSE]` or use ".." `ix <- ncol(x); x[, ..ix ]`. – zx8754 Nov 05 '21 at 13:17

score 15 · Answer 2 · answered Feb 14 '14 at 14:16

15

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]

answered Feb 14 '14 at 14:16

James

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Besides being a clever solution, you answer solved another problem that I had. +1 – DomingoR May 18 '17 at 20:38
This creates a dataframe when using the extracted vector (from a dataframe) in a mathematical operation. Not ideal. – luchonacho Nov 04 '21 at 20:20

score 6 · Answer 3 · answered Feb 14 '14 at 14:13

6

To refer to last column:

colnames(data)[ncol(data)]

answered Feb 14 '14 at 14:13

cianius

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zx8754 · Answer 4 · 2016-04-04T07:43:19.593

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

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

edited Apr 04 '16 at 07:43

answered Feb 14 '14 at 14:16

zx8754

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If you know the last name, then `train[,'last']` works; zx's solution is more flexible. – Carl Witthoft Feb 14 '14 at 15:58

score 5 · Answer 5 · edited Aug 21 '18 at 20:18

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

edited Aug 21 '18 at 20:18

MS Berends

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answered Aug 21 '18 at 19:32

BMLopes

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score 4 · Answer 6 · answered Apr 20 '20 at 10:52

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

https://stackoverflow.com/a/44353413

answered Apr 20 '20 at 10:52

tlask

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score 3 · Answer 7 · answered Feb 14 '14 at 15:20

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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.

answered Feb 14 '14 at 15:20

BrodieG

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ah yea that `list(data.frame)` to get the column names.. well i'm not using `R` for general purpose programming so just memorize these "interesting" things – WestCoastProjects Sep 16 '18 at 23:26

score 1 · Answer 8 · edited Apr 17 '22 at 12:55

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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)]))

edited Apr 17 '22 at 12:55

Martin Gal

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answered Apr 16 '22 at 17:04

Dre Day

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Refer to the last column in R

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