On Wed, Mar 14, 2012 at 03:22:39AM -0400, Lazarus Mramba wrote:
> Dear all,
>
> I have a large matrix with about 2500 variables, and 100 rows.
>
> I would like to calculate the means of the every 10 variables starting from
> 1:2500 and saving the results as a vector or matrix.
> How can I do that?
> Alternatively, How can I create 250 subset matrices in the order of
> variables 1:2500 in groups of 10 from the single matrix which had initially
> 2500 variables ?
> I guess I have to use a loop, but I can't figure out how.
Hi.
Try the following. I will use smaller parameters for simplicity.
# a matrix 3 times 20
a <- matrix(1:60, nrow=3, ncol=20)
a
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12] [,13]
[,14]
[1,] 1 4 7 10 13 16 19 22 25 28 31 34 37
40
[2,] 2 5 8 11 14 17 20 23 26 29 32 35 38
41
[3,] 3 6 9 12 15 18 21 24 27 30 33 36 39
42
[,15] [,16] [,17] [,18] [,19] [,20]
[1,] 43 46 49 52 55 58
[2,] 44 47 50 53 56 59
[3,] 45 48 51 54 57 60
#combine each 5 consecutive columns to a single column
dim(a) <- c(15, 4)
a
[,1] [,2] [,3] [,4]
[1,] 1 16 31 46
[2,] 2 17 32 47
[3,] 3 18 33 48
[4,] 4 19 34 49
[5,] 5 20 35 50
[6,] 6 21 36 51
[7,] 7 22 37 52
[8,] 8 23 38 53
[9,] 9 24 39 54
[10,] 10 25 40 55
[11,] 11 26 41 56
[12,] 12 27 42 57
[13,] 13 28 43 58
[14,] 14 29 44 59
[15,] 15 30 45 60
# compute column means
colMeans(a)
[1] 8 23 38 53
Hope this helps.
Petr Savicky.
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