R: Sort columns of a data frame by a vector of column names

Question

I have a data.frame that looks like this:

which has 1000+ columns with similar

Answer 1

Brodie's answer does exactly what you're asking for. However, you imply that your data are large, so I will provide an alternative using "data.table", which has a function called setcolorder that will change the column order by reference.

Here's a reproducible example.

Start with some simple data:

mydf <- data.frame(A = 1:2, B = 3:4, C = 5:6)
matches <- data.frame(X = 1:3, Y = c("C", "A", "B"), Z = 4:6)
mydf
#   A B C
# 1 1 3 5
# 2 2 4 6
matches
#   X Y Z
# 1 1 C 4
# 2 2 A 5
# 3 3 B 6

Provide proof that Brodie's answer works:

out <- mydf[matches$Y]
out
#   C A B
# 1 5 1 3
# 2 6 2 4

Show a more memory efficient way to do the same thing.

library(data.table)
setDT(mydf)
mydf
#    A B C
# 1: 1 3 5
# 2: 2 4 6

setcolorder(mydf, as.character(matches$Y))
mydf
#    C A B
# 1: 5 1 3
# 2: 6 2 4