David L Carlson
2015-Oct-23 19:25 UTC
[R] elementwise matrix multiplication with a special structure
Don't use html formatted emails. The r-help server strips the html formatting leaving us the the mess you see below. You don't need any loops:> aa <- matrix(1:9, 3, 3) > bb <- aa * 10 > cc <- bb * 10 > x <- as.matrix(expand.grid(k=1:3, j=1:3, c=1:3, r=1:3)) > str(x)int [1:81, 1:4] 1 2 3 1 2 3 1 2 3 1 ... - attr(*, "dimnames")=List of 2 ..$ : NULL ..$ : chr [1:4] "k" "j" "c" "r"> dd <- aa[x[, c("j", "k")]] * bb[x[, c("r", "j")]] * cc[x[, c("k", "c")]] > dd[1] 1000 8000 21000 8000 40000 96000 21000 84000 189000 [10] 4000 20000 42000 32000 100000 192000 84000 210000 378000 [19] 7000 32000 63000 56000 160000 288000 147000 336000 567000 [28] 2000 16000 42000 10000 50000 120000 24000 96000 216000 [37] 8000 40000 84000 40000 125000 240000 96000 240000 432000 [46] 14000 64000 126000 70000 200000 360000 168000 384000 648000 [55] 3000 24000 63000 12000 60000 144000 27000 108000 243000 [64] 12000 60000 126000 48000 150000 288000 108000 270000 486000 [73] 21000 96000 189000 84000 240000 432000 189000 432000 729000 # Or a bit more compactly> dd <- aa[x[, c(2, 1)]] * bb[x[, c(4, 2)]] * cc[x[, c(1, 3)]]------------------------------------- David L Carlson Department of Anthropology Texas A&M University College Station, TX 77840-4352 -----Original Message----- From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of eugen pircalabelu via R-help Sent: Friday, October 23, 2015 12:31 PM To: r-help at r-project.org Subject: [R] elementwise matrix multiplication with a special structure Hello R users, I have the following annoying matrix multiplication and I do not know how to avoid the nested loops, so maybe someone can help me with some ideas. I have searched the forum for past posts but nothing came up.Here it is: ?aa=matrix(1:9,3,3)?bb=matrix(seq(10,90,by=10),3,3)?cc=matrix(seq(100,900, by=100),3,3)?dd=NULL for(r in 1:3){????for(c in 1:3){????????for(j in 1:3){????????????for(k in 1:3){dd=c(dd,aa[j,k]*bb[r,j]*cc[k,c])}}}}dd?[1] ? 1000 ? 8000 ?21000 ? 8000 ?40000 ?96000 ?21000 ?84000 189000 ? 4000[11] ?20000 ?42000 ?32000 100000 192000 ?84000 210000 378000 ? 7000 ?32000[21] ?63000 ?56000 160000 288000 147000 336000 567000 ? 2000 ?16000 ?42000[31] ?10000 ?50000 120000 ?24000 ?96000 216000 ? 8000 ?40000 ?84000 ?40000[41] 125000 240000 ?96000 240000 432000 ?14000 ?64000 126000 ?70000 200000[51] 360000 168000 384000 648000 ? 3000 ?24000 ?63000 ?12000 ?60000 144000[61] ?27000 108000 243000 ?12000 ?60000 126000 ?48000 150000 288000 108000[71] 270000 486000 ?21000 ?96000 189000 ?84000 240000 432000 189000 432000[81] 729000 What I want to obtain is the content of the vector dd in an efficient (fast) and clever way.Thank you very much and have a great day ahead!Eugen [[alternative HTML version deleted]] ______________________________________________ R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.