[R] Copying rows from a matrix using a vector of indices

[Marc Schwartz (via MN)]

On Wed, 2005-08-17 at 12:02 -0700, Martin Lam wrote:
> Hi,
> 
> I am trying to use a vector of indices to select some
> rows from a matrix. But before I can do that I somehow
> need to convert 'combinations' into a list, since
> 'mode(combinations)' says it's 'numerical'. Any idea
> how I can do that?
> 
> library("combinat")
> 
> combinations <- t(combn(8,2))
> 
> indices <- c(sample(1:length(combinations),10))
> 
> # convert
> ???
> 
> subset <- combinations[indices]
> 
> Thanks in advance,
> 
> Martin


Your goal is not entirely clear.  Given your code above, you end up
with:

> library("combinat")
> 
> combinations <- t(combn(8,2))
> combinations
      [,1] [,2]
 [1,]    1    2
 [2,]    1    3
 [3,]    1    4
 [4,]    1    5
 [5,]    1    6
 [6,]    1    7
 [7,]    1    8
 [8,]    2    3
 [9,]    2    4
[10,]    2    5
[11,]    2    6
[12,]    2    7
[13,]    2    8
[14,]    3    4
[15,]    3    5
[16,]    3    6
[17,]    3    7
[18,]    3    8
[19,]    4    5
[20,]    4    6
[21,]    4    7
[22,]    4    8
[23,]    5    6
[24,]    5    7
[25,]    5    8
[26,]    6    7
[27,]    6    8
[28,]    7    8

combinations is a 28 x 2 matrix (or a 1d vector of length 56).

Your use of sample() with '1:length(combinations)' is the same as 1:56.

> length(combinations)
[1] 56

Hence, you end up with:

> set.seed(128)
> indices <- sample(1:length(combinations), 10)

> indices
 [1] 41 53 38 46 50 44 25 11 43  7


I suspect that you really want to use '1:nrow(combinations)', which
yields 1:28.

> nrow(combinations)
[1] 28


Thus,

> set.seed(128)
> indices <- sample(1:nrow(combinations), 10)

> indices
 [1] 21 26 18 22 23 20 11  5 27  3


Now, you can use:

> subset <- combinations[indices, ]
> subset
      [,1] [,2]
 [1,]    4    7
 [2,]    6    7
 [3,]    3    8
 [4,]    4    8
 [5,]    5    6
 [6,]    4    6
 [7,]    2    6
 [8,]    1    6
 [9,]    6    8
[10,]    1    4

which yields the rows from combinations, defined by 'indices'.

If correct, please review "An Introduction to R" and ?"[" for more
information on subsetting objects in R.

HTH,

Marc Schwartz