# [R] Extracting selected rows from a matrix

Kjetil Halvorsen kjetilbrinchmannhalvorsen at gmail.com
Tue Nov 30 01:05:36 CET 2010

```see below.

On Mon, Nov 29, 2010 at 5:56 PM, pankaj borah
<pankajborah2k3 at yahoo.co.in> wrote:
> Hi,
>
> I have matrix of 104 columns and 30000 rows (Each Row has rowname).
>
> I have 13 different  list of selected rownames (character) say 1000 each. Now I want to extract the all the columns according to the rownames in each the list.
>

if those are lists, make them vectors by calling unlist().
then:

> How can I do that in R ?
>
> 1 ) For a single list

Look at this example:

> mat <- matrix(1:100, 10, 10)
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    1   11   21   31   41   51   61   71   81    91
[2,]    2   12   22   32   42   52   62   72   82    92
[3,]    3   13   23   33   43   53   63   73   83    93
[4,]    4   14   24   34   44   54   64   74   84    94
[5,]    5   15   25   35   45   55   65   75   85    95
[6,]    6   16   26   36   46   56   66   76   86    96
> rownames(mat)
NULL
> rownames(mat) <- 1:10
> rownames(mat)
[1] "1"  "2"  "3"  "4"  "5"  "6"  "7"  "8"  "9"  "10"
> mat
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
1     1   11   21   31   41   51   61   71   81    91
2     2   12   22   32   42   52   62   72   82    92
3     3   13   23   33   43   53   63   73   83    93
4     4   14   24   34   44   54   64   74   84    94
5     5   15   25   35   45   55   65   75   85    95
6     6   16   26   36   46   56   66   76   86    96
7     7   17   27   37   47   57   67   77   87    97
8     8   18   28   38   48   58   68   78   88    98
9     9   19   29   39   49   59   69   79   89    99
10   10   20   30   40   50   60   70   80   90   100
> mat[rownames(mat)==7, ]
[1]  7 17 27 37 47 57 67 77 87 97
> mat[rownames(mat)%in%c(7, 5,3),  ]
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
3    3   13   23   33   43   53   63   73   83    93
5    5   15   25   35   45   55   65   75   85    95
7    7   17   27   37   47   57   67   77   87    97
>

> 2) For all the 13 list at a time

use a for loop.

Kjetil

>
> Regards,
>
> Pankaj Barah
> Department of Biology,
>  Norwegian University of Science & Technology (NTNU)
>
>
>
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>
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