Hello I have a matrix of size rows x cols. I also have a vector of size rows. The vector contains index values that corresponds to rows in the matrix. I would like to re-arrange/sort the contents of the matrix according to the entries in the vector. Can this be done efficiently in R and possibly in place? Or would it be better to allocate another whole matrix? Simple example: matrix: ffffff aaaaaa zzzzzz bbbbbb vector: 2, 4, 1, 3 "new" matrix: aaaaaa bbbbbb ffffff zzzzzz I suppose I could allocate another matrix and loop through the vector of indecies placing entries into the new matrix. Do I have to worry about deallocating the memory space of the first matrix or will it automagically go away if I assign the new matrix to the old matrix identifier (assuming nothing else is pointing at it)? Thanks, Esmail
Sorting rows in a matrix based on vector of indecies
3 messages · Esmail Bonakdarian, David Winsemius
Since you do not offer reproducible code, I must guess that you were
thinking of this as a matrix with 6 instances of "a" across the top
row. See if this helps:
> mtx<-matrix( rep(letters[1:4], 6), nrow=4)
> mtx
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] "a" "a" "a" "a" "a" "a"
[2,] "b" "b" "b" "b" "b" "b"
[3,] "c" "c" "c" "c" "c" "c"
[4,] "d" "d" "d" "d" "d" "d"
> mtx[c(2, 4, 1, 3), ]
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] "b" "b" "b" "b" "b" "b"
[2,] "d" "d" "d" "d" "d" "d"
[3,] "a" "a" "a" "a" "a" "a"
[4,] "c" "c" "c" "c" "c" "c"
You can do with that result whatever you want:
assign it, m2 <- mtx[c(2, 4, 1, 3), ]
mtx <- mtx[c(2, 4, 1, 3), ]
(unless you assign back to itself you will not have altered mtx.)
subset it, by element index mtx[c(2, 4, 1, 3), ][20]
by row m3 <- mtx[c(2, 4, 1, 3), ][4, ]
or by column. mtx[c(2, 4, 1, 3), ][ , 6]
David Winsemius On Feb 20, 2009, at 6:16 PM, Esmail Bonakdarian wrote: > Hello > > I have a matrix of size rows x cols. > I also have a vector of size rows. > > The vector contains index values that corresponds to rows in the > matrix. > > I would like to re-arrange/sort the contents of the matrix according > to the entries in the vector. Can this be done efficiently in R and > possibly in place? Or would it be better to allocate another whole > matrix? > > Simple example: > > matrix: > ffffff > aaaaaa > zzzzzz > bbbbbb > > > vector: > 2, 4, 1, 3 > > "new" matrix: > aaaaaa > bbbbbb > ffffff > zzzzzz > > > I suppose I could allocate another matrix and loop through the vector > of indecies placing entries into the new matrix. Do I have to worry > about deallocating the memory space of the first matrix or will it > automagically go away if I assign the new matrix to the old matrix > identifier (assuming nothing else is pointing at it)? > > Thanks, > Esmail > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.
Hi David, This was useful, thanks. The example was just that, there are no "a"s, "b",s etc I was just trying to show what I was trying to do. Using m[v,] where m was the matrix in my example and v the vector of index values works great as you suggested Thanks. R is quite a powerful language as I am starting to discover (and folks on the list here are very helpful). Regards, Esmail
David Winsemius wrote:
See if this helps:
> mtx<-matrix( rep(letters[1:4], 6), nrow=4) > mtx
[,1] [,2] [,3] [,4] [,5] [,6] [1,] "a" "a" "a" "a" "a" "a" [2,] "b" "b" "b" "b" "b" "b" [3,] "c" "c" "c" "c" "c" "c" [4,] "d" "d" "d" "d" "d" "d"
> mtx[c(2, 4, 1, 3), ]
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] "b" "b" "b" "b" "b" "b"
[2,] "d" "d" "d" "d" "d" "d"
[3,] "a" "a" "a" "a" "a" "a"
[4,] "c" "c" "c" "c" "c" "c"
You can do with that result whatever you want:
assign it, m2 <- mtx[c(2, 4, 1, 3), ]