On Mar 28, 2012, at 4:46 PM, Kehl Dániel wrote:
Dear list-members,
I have a 9-by-9 matrix lets call it A with first row a11, a12,
a13,..., a19 etc.
I also have a vector of length 3 (B).
I want to construct a matrix of size 3x3 in the following way:
- divide matrix A to 9 3x3 blocks
- first is
a11, a12, a13
a21, a22, a23
a31, a32, a33
- I want to get rowSums of this A1 matrix
- Multiply A1*B and get a scalar, the first element of my new 3x3
matrix.
You're going to need some sort of loop, but maybe you can make it
more elegant my noticing that you can access the 3x3 elements more
elegantly using single subscript values rather than ranges:
amat <- matrix(paste("a", rep(1:9, time=9), rep(1:9, each=9),
sep=""), 9,9)
amat
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,] "a11" "a12" "a13" "a14" "a15" "a16" "a17" "a18" "a19"
[2,] "a21" "a22" "a23" "a24" "a25" "a26" "a27" "a28" "a29"
[3,] "a31" "a32" "a33" "a34" "a35" "a36" "a37" "a38" "a39"
[4,] "a41" "a42" "a43" "a44" "a45" "a46" "a47" "a48" "a49"
[5,] "a51" "a52" "a53" "a54" "a55" "a56" "a57" "a58" "a59"
[6,] "a61" "a62" "a63" "a64" "a65" "a66" "a67" "a68" "a69"
[7,] "a71" "a72" "a73" "a74" "a75" "a76" "a77" "a78" "a79"
[8,] "a81" "a82" "a83" "a84" "a85" "a86" "a87" "a88" "a89"
[9,] "a91" "a92" "a93" "a94" "a95" "a96" "a97" "a98" "a99"
> dim(amat) <- c(3,3,3,3)
> amat[,1,,1] # upper/left 3x3
[,1] [,2] [,3]
[1,] "a11" "a12" "a13"
[2,] "a21" "a22" "a23"
[3,] "a31" "a32" "a33"
> amat[,2,,2] # middle/middle 3x3
[,1] [,2] [,3]
[1,] "a44" "a45" "a46"
[2,] "a54" "a55" "a56"
[3,] "a64" "a65" "a66"
If speed is an issue than perhaps you could use `aperm` to rearrange
this 3x3x3x3 array so that the proposed rowSums operation would be
correct (within three row segments.)
> newmat <- aperm(amat, c(2,4,3,1))
> dim(newmat) <- c( 3*9,3)
> newmat[1:6,1:3]
[,1] [,2] [,3]
[1,] "a11" "a21" "a31"
[2,] "a41" "a51" "a61"
[3,] "a71" "a81" "a91"
[4,] "a14" "a24" "a34"
[5,] "a44" "a54" "a64"
[6,] "a74" "a84" "a94"
I could do that with loop. Can you suggest something that is more
elegant and faster?
Thank you
Daniel
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West Hartford, CT
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