Swap (selected/subset) data frame columns in R

Question

What is the simplest way that one can swap the order of a selected subset of columns in a data frame in R. The answers I have seen ( Is it possible to swap columns around in a data frame using R? ) use all indices / column names for this. If one has, say, 100 columns and need either: 1) to swap column 99 with column 1, or 2) move column 99 before column 1 (but keeping column 1 now as column 2) the suggested approaches appear cumbersome. Funny there is no small package around for this (Wickham's "reshape" ?) - or can one suggest a simple code ?

Answer 1

If you really want a shortcut for this, you could write a couple of simple functions, such as the following.

To swap the position of two columns:

swapcols <- function(x, col1, col2) {
  if(is.character(col1)) col1 <- match(col1, colnames(x))
  if(is.character(col2)) col2 <- match(col2, colnames(x))
  if(any(is.na(c(col1, col2)))) stop("One or both columns don't exist.")
  i <- seq_len(ncol(x))
  i[col1] <- col2
  i[col2] <- col1
  x[, i]
}

To move a column from one position to another:

movecol <- function(x, col, to.pos) {
  if(is.character(col)) col <- match(col, colnames(x))
  if(is.na(col)) stop("Column doesn't exist.")
  if(to.pos > ncol(x) | to.pos < 1) stop("Invalid position.")
  x[, append(seq_len(ncol(x))[-col], col, to.pos - 1)]
}

And here are examples of each:

(m <- matrix(1:12, ncol=4, dimnames=list(NULL, letters[1:4])))

#      a b c  d
# [1,] 1 4 7 10
# [2,] 2 5 8 11
# [3,] 3 6 9 12

swapcols(m, col1=1, col2=3) # using column indices

#      c b a  d
# [1,] 7 4 1 10
# [2,] 8 5 2 11
# [3,] 9 6 3 12

swapcols(m, 'd', 'a') # or using column names

#       d b c a
# [1,] 10 4 7 1
# [2,] 11 5 8 2
# [3,] 12 6 9 3

movecol(m, col='a', to.pos=2)
#      b a c  d
# [1,] 4 1 7 10
# [2,] 5 2 8 11
# [3,] 6 3 9 12