Let's say I have the following data.table
in R
:
library(data.table)
DT = data.table(x=rep(c("b","a","c"),each=3), y=c(1,3,6), v=1:9)
I want to order it by two columns (say columns x
and v
). I used this:
DT[order(x,v)] # sorts first by x then by v (both in ascending order)
But now, I want to sort it by x
(in decreasing order) and have the following code:
DT[order(-x)] #Error in -x : invalid argument to unary operator
Therefore, I think this error is due to the fact that class(DT$x)=character
. Could you give me any suggestion in order to solve this issue?
I know I can use DT[order(x,decreasing=TRUE)]
, but I want to know the syntax to sort by several columns using both ways (some decreasing, some increasing) at the same time.
Note that if you use DT[order(-y,v)]
the result is ok, but if you use DT[order(-x,v)]
there is an error. So, my question is: how to solve this error?
This question is related to
r
string
sorting
data.table
key
You can only use -
on the numeric entries, so you can use decreasing and negate the ones you want in increasing order:
DT[order(x,-v,decreasing=TRUE),]
x y v
[1,] c 1 7
[2,] c 3 8
[3,] c 6 9
[4,] b 1 1
[5,] b 3 2
[6,] b 6 3
[7,] a 1 4
[8,] a 3 5
[9,] a 6 6
DT[order(-x)]
works as expected. I have data.table version 1.9.4. Maybe this was fixed in a recent version.
Also, I suggest the setorder(DT, -x)
syntax in keeping with the set* commands like setnames
, setkey
Source: Stackoverflow.com