Select the last n columns of data frame in R

问题

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

回答1:

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

回答2:

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 n columns in a matrix/data frame without knowing the column names:

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.

回答3:

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"

回答4:

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)])

回答5:

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())

Hopefully that helps out any future visitors to this question.

回答6:

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())

来源：https://stackoverflow.com/questions/28017141/select-the-last-n-columns-of-data-frame-in-r

标签

indexing

dataframe