On Thu, May 03, 2012 at 09:07:39PM -0400, li li wrote:
> Hi all,
> I have a 100 by 100 matrix and I divided this matrix into 100 groups,
> each is a
> 10 by 10 submatrix. I want find out the means of each group.
> I know we can use apply function for mean by margins. Is there a
> function in R for
> means by groups also? Thanks.
Hi.
Try the following with k = 10, n = 100.
k <- 3
n <- 6
A <- matrix(1:(n^2), nrow=n, ncol=n)
B <- matrix(nrow=n, ncol=n)
B[, ] <- paste((row(B)-1) %/% k + 1, (col(B)-1) %/% k + 1, sep=",")
B
[,1] [,2] [,3] [,4] [,5] [,6]
[1,] "1,1" "1,1" "1,1" "1,2" "1,2" "1,2"
[2,] "1,1" "1,1" "1,1" "1,2" "1,2" "1,2"
[3,] "1,1" "1,1" "1,1" "1,2" "1,2" "1,2"
[4,] "2,1" "2,1" "2,1" "2,2" "2,2" "2,2"
[5,] "2,1" "2,1" "2,1" "2,2" "2,2" "2,2"
[6,] "2,1" "2,1" "2,1" "2,2" "2,2" "2,2"
tapply(c(A), c(B), FUN=mean)
1,1 1,2 2,1 2,2
8 26 11 29
For verification, try
mean(A[1:k, 1:k]) # [1] 8
mean(A[1:k, 1:k + k]) # [1] 26
mean(A[1:k + k, 1:k]) # [1] 11
mean(A[1:k + k, 1:k + k]) # [1] 29
Hope this helps.
Petr Savicky.
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