R help - Feb 2009 - Sorting rows in a matrix based on vector of indecies

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

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), ]