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
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and provide commented, minimal, self-contained, reproducible code.
