CODEWITHSUNDEEP
rindexingdataframe
Nancy
Nancy

Reputation: 4089

Select the last n columns of data frame in R

Is there a way to systematically select the last columns of a data frame? I would like to be able to move the last columns to be the first columns, but maintain the order of the columns when they are moved. I need a way to do this that does not list all the columns using subset(data, select = c(all the columns listed in the new order)) because I will be using many different data frames.

Here's an example where I would like to move the last 2 columns to the front of the data frame. It works, but it's ugly. 

A = rep("A", 5)
B = rep("B", 5)
num1 = c(1:5)
num2 = c(36:40)

mydata2 = data.frame(num1, num2, A, B)

# Move A and B to the front of mydata2
mydata2_move = data.frame(A = mydata2$A, B = mydata2$B, mydata2[,1:    (ncol(mydata2)-2)])

#  A B num1 num2
#1 A B    1   36
#2 A B    2   37
#3 A B    3   38
#4 A B    4   39
#5 A B    5   40

Changing the number of columns in the original data frame causes issues. This works (see below), but the naming gets thrown off. Why do these two examples behave differently? Is there a better way to do this, and to generalize it?

mydata1_move = data.frame(A = mydata1$A, B = mydata1$B, mydata1[,1:   (ncol(mydata1)-2)])

#  A B mydata1...1..ncol.mydata1....2..
#1 A B                                1
#2 A B                                2
#3 A B                                3
#4 A B                                4
#5 A B                                5

Upvotes: 18

Views: 44471

Answers (8)

user3352480
user3352480

Reputation: 31

Using the offset argument in the last_col function, inside select, you can do that.

Below is an example considering the last two columns, and it in a more generic approach.

library(dplyr)

mydata <- mydata %>% select(last_col(offset=c(0,1)), everything())

n <- 2
mydata <- mydata %>% select(last_col(offset=0:(n-1), everything()))

Upvotes: 3

LMc
LMc

Reputation: 18632

relocate() was added with dplyr 1.0.0 to help with this. You can rename and use tidy-selection:

library(dplyr)

mydata2 |> 
  relocate(A:B, .before = 1)

To get the last N columns use the offset argument, which indexes from 0 (0 being the last column):

N <- 2
mydata2 |>
  relocate(last_col(N - 1):last_col(), .before = 1)

Upvotes: 1

Dave Gruenewald
Dave Gruenewald

Reputation: 5689

I know this topic is a little dead, but wanted to chime in with a simple dplyr solution:

library(dplyr)

mydata <- mydata %>%
  select(A, B, everything())

If you are wanting to avoid explicit calls to the last columns, use seq() within last_col(). Let's denote the number of columns we wish to move to the front as n:

mydata <- mydata %>%
  select(
    last_col(seq(n - 1, 0)),
    everything()
  )

Upvotes: 2

Spcogg
Spcogg

Reputation: 303

The problem described doesn't match the title, and existing answers address the moving columns part, doesn't really explain how to select last N columns.

If you wanted to just select the last column in a matrix/data frame without knowing the column name:

mydata2[,ncol(mydata2)]

and if you want last n columns, try

mydata[,(ncol(mydata2)-n-1):ncol(mydata2)]

A little cumbersome, but works. Could write wrapper function if you plan to use it regularly.

Upvotes: 20

Leonardo
Leonardo

Reputation: 85

Another alternative with dplyr:

mydata2 <- select(mydata, 2:ncol(data),1)  
#select any cols from col2 until the last col and place them before col1

Upvotes: 2

jazzurro
jazzurro

Reputation: 23574

You can do a similar thing using the SOfun package, available on GitHub. 

library(SOfun)

foo <- moveMe(colnames(mydata2), "A, B before num1")

mydata2[, foo]

#  A B num1 num2
#1 A B    1   36
#2 A B    2   37
#3 A B    3   38
#4 A B    4   39
#5 A B    5   40

You can move column names like this example from R Help.

x <- names(mtcars)

x
#[1] "mpg"  "cyl"  "disp" "hp"   "drat" "wt"   "qsec" "vs"   "am"   "gear" "carb"

moveMe(x, "hp first; cyl after drat; vs, am, gear before mpg; wt last")
#[1] "hp"   "vs"   "am"   "gear" "mpg"  "disp" "drat" "cyl"  "qsec" "carb" "wt"

Upvotes: 3

Jthorpe
Jthorpe

Reputation: 10167

data frames are just lists, so you can rearrange them as you would any list:

newdata <- c(mydata[colNamesToStart],
             mydata[-which(names(mydata) %in% colNamesToStart)])

Upvotes: 2

jbaums
jbaums

Reputation: 27388

You could use something like this:

move_to_start <- function(x, to_move) {
  x[, c(to_move, setdiff(colnames(x), to_move))]
} 

move_to_start(mydata2, c('A', 'B'))

#   A B num1 num2
# 1 A B    1   36
# 2 A B    2   37
# 3 A B    3   38
# 4 A B    4   39
# 5 A B    5   40

Alternatively, if you want to move the last n columns to the start:

move_to_start <- function(x, n) {
  x[, c(tail(seq_len(ncol(x)), n), seq_len(ncol(x) - n))]
} 

move_to_start(mydata2, 2)

#   A B num1 num2
# 1 A B    1   36
# 2 A B    2   37
# 3 A B    3   38
# 4 A B    4   39
# 5 A B    5   40

Upvotes: 4

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