Select the last n columns of the data frame in R

Question

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 as the first columns, but maintain the order of the columns when moving them. I need a way to do this that does not list all columns using a subset (data, select = c (all columns listed in new order)), because I will use a lot of different data frames.

Here is an example where I would like to move the last 2 columns to the front of the data frame. It works, but it is 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)]) # AB num1 num2 #1 AB 1 36 #2 AB 2 37 #3 AB 3 38 #4 AB 4 39 #5 AB 5 40 

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

 mydata1_move = data.frame(A = mydata1$A, B = mydata1$B, mydata1[,1: (ncol(mydata1)-2)]) # AB mydata1...1..ncol.mydata1....2.. #1 AB 1 #2 AB 2 #3 AB 3 #4 AB 4 #5 AB 5 

Answer 1

You can 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')) # AB num1 num2 # 1 AB 1 36 # 2 AB 2 37 # 3 AB 3 38 # 4 AB 4 39 # 5 AB 5 40 

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

 move_to_start <- function(x, n) { x[, c(tail(seq_len(ncol(x)), n), seq_len(ncol(x) - n))] } move_to_start(mydata2, 2) # AB num1 num2 # 1 AB 1 36 # 2 AB 2 37 # 3 AB 3 38 # 4 AB 4 39 # 5 AB 5 40 

Answer 2

The described problem does not correspond to the name, and the existing answers concern part of the moving columns, in fact they do not explain how to select the last N columns.

If you want to simply select the last n columns in the matrix / data frame without knowing the column names:

 mydata2[,ncol(mydata2)] 

and if you want to use the last n columns try

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

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

Answer 3

data frames are only lists, so you can change them like any list:

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

Answer 4

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] # AB num1 num2 #1 AB 1 36 #2 AB 2 37 #3 AB 3 38 #4 AB 4 39 #5 AB 5 40 

You can move column names, for example, in this example from the 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" 

Answer 5

I know this topic is a bit dead, but wanted to listen with a simple dplyr solution:

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

I hope this helps any future visitors in this matter.